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Mars to be of some certain density. To fix our ideas on thesepoints I take the case of the present satellite Phobos. Whatamount of stress will he exert upon the crust of Mars when heapproaches within, say, 40 miles of the planet's surface? We knowhis size approximately—he is about 36 miles in diameter. We canguess his density to be between four times that of water andeight times that of water. We may assume the density of Mars'surface to be about the same as that of our Earth's surface, thatis three times as dense as water. We now find that the greateststress tending to rend open the surface crust of Mars will bebetween 4,000 and 8,000 pounds to the square foot according tothe density we assign to Phobos.
Will such a stress actually tear open the crust? We are not ableto answer this question with any certainty. Much will depend uponthe nature and condition of the crust. Thus, suppose that we arehere (Fig. 12) looking down upon the satellite which is movingalong slowly relatively to Mars' surface, in the direction of thearrow. The satellite has just passed over a weak and cracked partof the planet's crust. Here the stress has been sufficient tostart two cracks. Now you know how easy it is to tear a piece ofcloth when you go to the edge of it in order to make a beginning.Here the stress from the satellite has got to the edge of thecrust. It is greatly concentrated just at the extremities of thecracks. It will, unler such circumstances probably carry on the
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tear. If it does not do so this time, remember the satellite willsome hours later be coming over the same place again, and thenagain for, at least, many hundreds of times. Then also we are notlimited to the assumption that the
{Fig. 12}
satellite is as small as Phobos. Suppose we consider the case ofa satellite approaching Mars which has a diameter double that ofPhobos; a diameter still much less than that of the larger classof asteroids. Even at the distance
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of 65 miles the stress will now amount to as much as from 15 to30 tons per square foot. It is almost certain that such a stressrepeated a comparatively few times over the same parts of theplanet's surface would so rend the crust as to set up lines alongwhich plutonic action would find a vent. That is, we might expectalong these lines all the phenomena of upheaval and volcaniceruption which give rise to surface elevations.
The probable effect of a satellite of this dimension travellingslowly relatively to the surface of Mars is, then, to leave avery conspicuous memorial of his presence behind him. You seefrom the diagram that this memorial will consist o: two parallellines of disturbance.
The linear character of the gravitational effects of thesatellite is due entirely to the motion of the satelliterelatively to the surface of the planet. If the satellite stoodstill above the surface the gravitational stress in the crustwould, of course, be exerted radially outwards from the centre ofthe satellite. It would attain at the central point beneath thesatellite its maximum vertical effect, and at some radialdistance measured outwards from this point, which distance we cancalculate, its maximum horizontal tearing effect. When thesatellite moves relatively to the planet's crust, the horizontaltearing force acts differently according to whether it isdirected in the line of motion or at right angles to this line.
In the direction of motion we see that the satellite
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creates as it passes over the crust a wave of rarefaction ortension as at D, followed by compression just beneath thesatellite and by a reversed direction of gravitational pull asthe satellite passes onwards. These stresses rapidly replace oneanother as the satellite travels along. They are resisted by theinertia of the crust, and are taken up by its elasticity. Thenature of this succession of alternate compressions andrarefactions in the crust possess some resemblance to thosearising in an earthquake shock.
If we consider the effects taking place laterally to the line ofmotion we see that there are no such changes in the nature of theforces in the crust. At each passage of the satellite thehorizontal tearing stress increases to a maximum, when it isexerted laterally, along the line passing through the horizontalprojection of the satellite and at right angles to the line ofmotion, and again dies away. It is always a tearing stress,renewed again and again.
This effect is at its maximum along two particular parallel lineswhich are tangents to the circle of maximum horizontal stress andwhich run parallel with the path of the satellite. The distanceseparating these lines depend upon the elevation of the satelliteabove the planet's surface. Such lines mark out the theoreticalaxes of the "double canals" which future crustal movements willmore fully develop.
It is interesting to consider what the effect of such
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conditions would be if they arose at the surface of our ownplanet. We assume a horizontal force in the crust adequate to setup tensile stresses of the order, say, of fifteen tons to thesquare foot and these stresses to be repeated every few hours;our world being also subject to the dynamic effects we recognisein and beneath its crust.
It is easy to see that the areas over which the satellite exertedits gravitational stresses must become the foci —foci of linearform—of tectonic developments or crust movements. The relief ofstresses, from whatever cause arising, in and beneath the crustmust surely take place in these regions of disturbance and alongthese linear areas. Here must become concentrated the foldingmovements, which are under existing conditions brought into thegeosynclines, along with their attendant volcanic phenomena. Inthe case of Mars such a concentration of tectonic events wouldnot, owing to the absence of extensive subaerial denudation andgreat oceans, be complicated by the existence of such synclinalaccumulations as have controlled terrestrial surface development.With the passage of time the linear features would probablydevelop; the energetic substratum continually asserting itsinfluence along such lines of weakness. It is in the highestdegree probable that radioactivity plays no less a part inMartian history than in terrestrial. The fact of radioactiveheating allows us to assume the thin surface crust and continuedsub-crustal energy throughout the entire period of the planet'shistory.
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How far willl these effects resemble the double canals of Mars?In this figure and in the calculations I have given you I havesupposed the satellite engaged in marking the planet's surfacewith two lines separated by about the interval separating thewider double canals of Mars—that is about 220 miles apart. Whatthe distance between the lines will be, as already stated, willdepend upon the height of the satellite above the surface when itcomes upon a part of the crust in a condition to be affected bythe stresses it sets up in it. If the satellite does its work ata point lower down above the surface the canal produced will benarrower. The stresses, too, will then be much greater. I mustalso observe that once the crust has yielded to the pullingstress, there is great probability that in future revolutions ofthe satellite a central fracture will result. For then all thepulling force adds itself to the lifting force and tends to crushthe crust inwards on the central line beneath the satellite. Itis thus quite possible that the passage of a satellite may giverise to triple lines. There is reason to believe that the canalson Mars are in some cases triple.
I have spoken all along of the satellite moving slowly over thesurface of Mars. I have done so as I cannot at all pronounce soreadily on what will happen when the satellite's velocity overthe surface of Mars is very great. To account for all the linesmapped by Lowell some of them must have been produced bysatellities moving relatively to the surface of Mars atvelocities so great
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as three miles a second or even rather more. The stresses set upare, in such cases, very difficult to estimate. It has not yetbeen done. Parallel lines of greatest stress or impulse ought tobe formed as in the other case.
I now ask your attention to another kind of evidence that thelines are due in some way to the motion of satellites passingover the surface of Mars.
I may put the fresh evidence to which I refer, in this way: InLowell's map (P1. XXII, p. 192), and in a less degree inSchiaparelli's map (ante p. 166), we are given the course of thelines as fragments of incomplete curves. Now these curves mighthave been anything at all. We must take them as they are,however, when we apply them as a test of the theory that themotion of a satellite round Mars can strike such lines. If it canbe shown that satellites revolving round Mars might strike justsuch curves then we assume this as an added confirmation of thehypothesis.
We must begin by realising what sort of curves a satellite whichdisturbs the surface of a planet would leave behind it after itsdemise. You might think that the satellite revolving round andround the planet must simply describe a circle upon the sphericalsurface of the planet: a "great circle" as it is called; that isthe greatest circle which can be described upon a sphere. Thisgreat circle can, however, only be struck, as you will see, whenthe planet is not turning upon its axis: a condition not likelyto be realised.
This diagram (PI. XXI) shows the surface of a globe
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covered with the usual imaginary lines of latitude and longitude.The orbit of a supposed satellite is shown by a line crossing thesphere at some assumed angle with the equator. Along this linethe satellite always moves at uniform velocity, passing acrossand round the back of the sphere and again across. If the sphereis not turning on its polar axis then this satellite, which wewill suppose armed with a pencil which draws a line upon thesphere, will strike a great circle right round the sphere. Butthe sphere is rotating. And it is to be expected that atdifferent times in a planet's history the rate of rotation variesvery much indeed. There is reason to believe that our own day wasonce only 2½ hours long, or thereabouts. After a preliminary risein velocity of axial rotation, due to shrinkage attending rapidcooling, a planet as it advances in years rotates slower andslower. This phenomenon is due to tidal influences of the sun orof satellites. On the assumption that satellites fell into Marsthere would in his case be a further action tending to shortenhis day as time went on.
The effect of the rotation of the planet will be, of course, thatas the satellite advances with its pencil it finds the surface ofthe sphere being displaced from under it. The line struck ceasesto be the great circle but wanders off in another curve—which isin fact not a circle at all.
You will readily see how we find this curve. Suppose the sphereto be rotating at such a speed that while the satellite isadvancing the distance _Oa_, the point _b_ on the
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sphere will be carried into the path of the satellite. The pencilwill mark this point. Similarly we find that all the points alongthis full curved line are points which will just find themselvesunder the satellite as it passes with its pencil. This curve isthen the track marked out by the revolving satellite. You see itdotted round the back of the sphere to where it cuts the equatorat a certain point. The course of the curve and the point whereit cuts the equator, before proceeding on its way, entirelydepend upon the rate at which we suppose the sphere to berotating and the satellite to be describing the orbit. We maycall the distance measured round the planet's equator separatingthe starting point of the curve from the point at which it againmeets the equator, the "span" of the curve. The span then dependsentirely upon the rate of rotation of the planet on its axis andof the satellite in its orbit round the planet.
But the nature of events might have been somewhat different. Thesatellite is, in the figure, supposed to be rotating round thesphere in the same direction as that in which the sphere isturning. It might have been that Mars had picked up a satellitetravelling in the opposite direction to that in which he wasturning. With the velocity of planet on its axis and of satellitein its orbit the same as before, a different curve would havebeen described. The span of the curve due to a retrogradesatellite will be greater than that due to a direct satellite.The retrograde satellite will have a span more than half
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way round the planet, the direct satellite will describe a curvewhich will be less than half way round the planet: that is a spandue to a retrograde satellite will be more than 180 degrees,while the span due to a direct satellite will be less than 180degrees upon the planet's equator.
I would draw your attention to the fact that what the span willbe does not depend upon how much the orbit of the satellite isinclined to the equator. This only decides how far the curvemarked out by the satellite will recede from the equator.
We find then, so far, that it is easy to distinguish between thedirect and the retrograde curves. The span of one is less, of theother greater, than 180 degrees. The number of degrees whicheither sort of curve subtends upon the equator entirely dependsupon the velocity of the satellite and the axial velocity of theplanet.
But of these two velocities that of the satellite may be taken assensibly invariable, when close enough to use his pencil. Thisdepends upon the law of centrifugal force, which teaches us thatthe mass of the planet alone decides the velocity of a satellitein its orbit at any fixed distance from the planet's centre. Theother velocity—that of the planet upon its axis—was, as we haveseen, not in the past what it is now. If then Mars, at varioustimes in his past history, picked up satellites, these satelliteswill describe curves round him having different spans which willdepend upon the velocity of axial rotation of Mars at the timeand upon this only.
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In what way now can we apply this knowledge of the curvesdescribed by a satellite as a test of the lunar origin of thelines on Mars?
To do this we must apply to Lowell's map. We pick out preferably,of course, the most complete and definite curves. The chain ofcanals of which Acheron and Erebus are members mark out a fairlydefinite curve. We produce it by eye, preserving the curvature asfar as possible, till it cuts the equator. Reading the span onthe equator we find' it to be 255 degrees. In the first place wesay then that this curve is due to a retrograde satellite. Wealso note on Lowell's map that the greatest rise of the curve isto a point about 32 degrees north of the equator. This gives theinclination of the satellite's orbit to the plane of Mars'equator.
With these data we calculate the velocity which the planet musthave possessed at the time the canal was formed on the hypothesisthat the curve was indeed the work of a satellite. The finalquestion now remains If we determine the curve due to thisvelocity of Mars on its axis, will this curve fit that one whichappears on Lowell's map, and of which we have really availedourselves of only three points? To answer this question we plotupon a sphere, the curve of a satellite, in the manner I havedescribed, assigning to this sphere the velocity derived from thespan of 255 degrees. Having plotted the curve on the sphere itonly remains to transfer it to Lowell's map. This is easilydone.
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This map (Pl. XXII) shows you the result of treating this, aswell as other curves, in the manner just described. You see thatwhether the fragmentary curves are steep and receding far fromthe equator; or whether they are flat and lying close along theequator; whether they span less or more than 180 degrees; thecurves determined on the supposition that they are the work ofsatellites revolving round Mars agree with the mapped curves;following them with wonderful accuracy; possessing theirproperties, and, indeed, in some cases, actually coinciding withthem.
I may add that the inadmissible span of 180 degrees and spansvery near this value, which are not well admissible, are so faras I can find, absent. The curves are not great circles.
You will require of me that I should explain the centres ofradiation so conspicuous here and there on Lowell's map. Themeeting of more than two lines at the oases is a phenomenonpossibly of the same nature and also requiring explanation.
In the first place the curves to which I have but brieflyreferred actually give rise in most cases to nodal, or crossingpoints; sometimes on the equator, sometimes off the equator;through which the path of the satellite returns again and again.These nodal points will not, however, afford a generalexplanation of the many-branched radiants.
It is probable that we should refer such an appearance
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as is shown at the Sinus Titanum to the perturbations of thesatellite's path due to the surface features on Mars. Observethat the principal radiants are situated upon the boundary of thedark regions or at the oases. Higher surface levels may beinvolved in both cases. Some marked difference in topography mustcharacterise both these features. The latter may possiblyoriginate in the destruction of satellites. Or again, they mayarise in crustal disturbance of a volcanic nature, primarilyinduced or localised by the crossing of two canals. Whatever theorigin of these features it is only necessary to assume that theyrepresent elevated features of some magnitude to explain themultiplication of crossing lines. We must here recall whatobservers say of the multiplicity of the canals. According toLowell, "What their number maybe lies quite beyond thepossibility of count at present; for the better our own air, themore of them are visible."
Such innumerable canals are just what the present theoryrequires. An in-falling satellite will, in the course of the last60 or 80 years of its career, circulate some 100,000 times overMars' surface. Now what will determine the more conspicuousdevelopment of a particular canal? The mass of the satellite; thestate of the surface crust; the proximity of the satellite; andthe amount of repetition over the same ground. The after effectsmay be taken as proportional to the primary disturbance.
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It is probable that elevated surface features will influence twoof these conditions: the number of repetitions and the proximityto the surface. A tract 100 miles in diameter and elevated 5,000or 10,000 feet would seriously perturb the orbit of such a body asPhobos. It is to be expected that not only would it be effectivein swaying the orbit of the satellite in the horizontal directionbut also would draw it down closer to the surface. It is even tobe considered if such a mass might not become nodal to thesatellite's orbit, so that this passed through or above thispoint at various inclinations with its primary direction. Ifacting to bring down the orbit then this will quicken the speedand cause the satellite further on its path to attain a somewhathigher elevation above the surface. The lines most conspicuous inthe telescope are, in short, those which have been favoured by acombination of circumstances as reviewed above, among whichcrustal features have, in some cases, played a part.
I must briefly refer to what is one of the most interestingfeatures of the Martian lines: the manner in which they appear tocome and go like visions.
Something going on in Mars determines the phenomenon. On aparticular night a certain line looks single. A few nights latersigns of doubling are perceived, and later still, when the seeingis particularly good, not one but two lines are seen. Thus, as anexample, we may take the case of Phison and Euphrates. Faintglimpses of the dual state were detected in the summer
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and autumn, but not till November did they appear as distinctlydouble. Observe that by this time the Antarctic snows had melted,and there was in addition, sufficient time for the moisture soliberated to become diffused in the planet's atmosphere.
This increase in the definition and conspicuousness of certaindetails on Mars' surface is further brought into connection withthe liberation of the polar snows and the diffusion of this waterthrough the atmosphere, by the fact that the definition appearedprogressively better from the south pole upwards as the snowdisappeared. Lowell thinks this points to vegetation springing upunder the influence of moisture; he considers, however, as wehave seen, that the canals convey the moisture. He has to assumethe construction of triple canals to explain the doubling of thelines.
If we once admit the canals to be elevated ranges—not necessarilyof great height—the difficulty of accounting for increaseddefinition with increase of moisture vanishes. We need notnecessarily even suppose vegetation concerned. With respect tothis last possibility we may remark that the colour observations,upon which the idea of vegetation is based, are likely to beuncertain owing to possible fatigue effects where a dark objectis seen against a reddish background.
However this may be we have to consider what the effects ofmoisture increasing in the atmosphere of Mars will be with regardto the visibility of elevated ranges,
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We assume a serene and rare atmosphere: the nights intenselycold, the days hot with the unveiled solar radiation. On the hilltops the cold of night will be still more intense and so, also,will the solar radiation by day. The result of this state ofthings will be that the moisture will be precipitated mainly onthe mountains during the cold of night—in the form of frost—andduring the day this covering of frost will melt; and, just as wesee a heavy dew-fall darken the ground in summer, so the meltingice will set off the elevated land against the arid plains below.Our valleys are more moist than our mountains only because ourmoisture is so abundant that it drains off the mountains into thevalleys. If moisture was scarce it would distil from the plainsto the colder elevations of the hills. On this view theaccentuation of a canal is the result of meteorological effectssuch as would arise in the Martian climate; effects which must beinfluenced by conditions of mountain elevation, atmosphericcurrents, etc. We, thus, follow Lowell in ascribing theaccentuation of the canals to the circulation of water in Mars;but we assume a simple and natural mode of conveyance and do notpostulate artificial structures of all but impossible magnitude.That vegetation may take part in the darkening of the elevatedtracts is not improbable. Indeed we would expect that in theMartian climate these tracts would be the only fertile parts ofthe surface.
Clouds also there certainly are. More recent observations
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appear to have set this beyond doubt. Their presence obviouslybrings in other possible explanations of the coming and going ofelevated surface features.
Finally, we may ask what about the reliability of the maps? Aboutthis it is to be said that the most recent map—that by Lowell—hasbeen confirmed by numerous drawings by different observers, andthat it is,itself the result of over 900 drawings. It has becomea standard chart of Mars, and while it would be rash to contendfor absence of errors it appears certain that the trend of theprincipal canals may be relied on, as, also, the general featuresof the planet's surface.
The question of the possibility of illusion has frequently beenraised. What I have said above to a great extent answers suchobjections. The close agreement between the drawings of differentobservers ought really to set the matter at rest. Recently,however, photography has left no further room for scepticism.First photographed in 1905, the planet has since beenphotographed many thousands of times from various observatories.A majority of the canals have been so mapped. The doubling of thecanals is stated to have been also so recorded.[1]
The hypothesis which I have ventured to put before you involvesno organic intervention to account for the
[1] E. C. Slipher's paper in _Popular Astronomy_ for March, 1914,gives a good account of the recent work.
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details on Mars' surface. They are physical surface features.Mars presents his history written upon his face in the scars offormer encounters—like the shield of Sir Launcelot. Some of themost interesting inferences of mathematical and physicalastronomy find a confirmation in his history. The slowing down inthe rate of axial rotation of the primary; the final inevitabledestruction of the satellite; the existence in the past of a farlarger number of asteroids than we at present are acquaintedwith; all these great facts are involved in the theory nowadvanced. If justifiably, then is Mars' face a veritablePrincipia.
To fully answer the question which heads these lectures, weshould go out into the populous solitudes (if the term bepermitted) which lie beyond our system. It is well that there isnow no time left to do so; for, in fact, there we can only dreamdreams wherein the limits of the possible and the impossiblebecome lost.
The marvel of the infinite number of stars is not so marvellousas the rationality that fain would comprehend them. In seekingother minds than ours we seek for what is almost infinitelycomplex and coordinated in a material universe relatively simpleand heterogeneous. In our mental attitude towards the greatquestion, this fact must be regarded as fundamental.
I can only fitly close a discourse which has throughout weighedthe question of the living thought against the unthinking laws ofmatter, by a paraphrase of the words
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of a great poet when he, in higher and, perhaps, more philosophiclanguage, also sought to place the one in comparison with theother.[1]
Richter thought that he was—with his human heartunstrengthened—taken by an angel among the universe of stars.Then, as they journeyed, our solar system was sunken like a faintstar in the abyss, and they travelled yet further, on the wingsof thought, through mightier systems: through all the countlessnumbers of our galaxy. But at length these also were left behind,and faded like a mist into the past. But this was not all. Thedawn of other galaxies appeared in the void. Stars more countlessstill with insufferable light emerged. And these also werepassed. And so they went through galaxies without number till atlength they stood in the great Cathedral of the Universe. Endlesswere the starry aisles; endless the starry columns; infinite thearches and the architraves of stars. And the poet saw the mightygalaxies as steps descending to infinity, and as steps going upto infinity.
Then his human heart fainted and he longed for some narrow cell;longed to lie down in the grave that he might hide from infinity.And he said to the angel:
"Angel, I can go with thee no farther. Is there, then, no end tothe universe of stars?"
[1] De Quincy in his _System of the Heavens_ gives a fineparaphrase of "Richter's Dream."
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Then the angel flung up his glorious hands to the heaven ofheavens, saying "End is there none to the universe of God? Lo!also there is no beginning."
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THE LATENT IMAGE[1]
My inclination has led me, in spite of a lively dread ofincurring a charge of presumption, to address you principally onthat profound and most subtle question, the nature and mode offormation of the photographic image. I am impelled to do so, notonly because the subject is full of fascination and hopefulness,but because the wide topics of photographic methods orphotographic applications would be quite unfittingly handled bythe president you have chosen.
I would first direct your attention to Sir James Dewar'sremarkable result that the photographic plate retainsconsiderable power of forming the latent image at temperaturesapproaching the absolute zero—a result which, as I submit,compels us to regard the fundamental effects progressing in thefilm under the stimulus of light undulations as other than thoseof a purely chemical nature. But few, if any, instances ofchemical combination or decomposition are known at so low atemperature. Purely chemical actions cease, indeed, at far highertemperatures, fluorine being among the few bodies which stillshow
[1] Presidential address to the Photographic Convention of theUnited Kingdom, July, 1905. _Nature_, Vol. 72, p. 308.
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chemical activity at the comparatively elevated temperature of-180° C. In short, this result of Sir James Dewar's suggests thatwe must seek for the foundations of photographic action in somephysical or intra-atomic effect which, as in the case ofradioactivity or fluorescence, is not restricted to intervals oftemperature over which active molecular vis viva prevails. Itcompels us to regard with doubt the role of oxidation or otherchemical action as essential, but rather points to the view thatsuch effects must be secondary or subsidiary. We feel, in a word,that we must turn for guidance to some purely photo-physicaleffect.
Here, in the first place, we naturally recall the views of Bose.This physicist would refer the formation of the image to a strainof the bromide of silver molecule under the electric force in thelight wave, converting it into what might be regarded as anallotropic modification of the normal bromide which subsequentlyresponds specially to the attack of the developer. The functionof the sensitiser, according to this view, is to retard therecovery from strain. Bose obtained many suggestive parallelsbetween the strain phenomena he was able to observe in silver andother substances under electromagnetic radiation and thebehaviour of the photographic plate when subjected tolong-continued exposure to light.
This theory, whatever it may have to recommend it, can hardly beregarded as offering a fundamental explanation. In the firstplace, we are left in the dark as to what
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the strain may be. It may mean many and various things. We knownothing as to the inner mechanism of its effects upon subsequentchemical actions—or at least we cannot correlate it with what isknown of the physics of chemical activity. Finally, as will beseen later, it is hardly adequate to account for the varyingdegrees of stability which may apparently characterise the latentimage. Still, there is much in Bose's work deserving of carefulconsideration. He has by no means exhausted the line ofinvestigation he has originated.
Another theory has doubtless been in the minds of many. I havesaid we must seek guidance in some photo-physical phenomenon.There is one such which preeminently connects light and chemicalphenomena through the intermediary of the effects of the formerupon a component part of the atom. I refer to the phenomena ofphoto-electricity.
It was ascertained by Hertz and his immediate successors thatlight has a remarkable power of discharging negativeelectrification from the surface of bodies—especially fromcertain substances. For long no explanation of the cause of thisappeared. But the electron—the ubiquitous electron—is now knownwith considerable certainty to be responsible. The effect of theelectric force in the light wave is to direct or assist theelectrons contained in the substance to escape from the surfaceof the body. Each electron carries away a very small charge ofnegative electrification. If, then, a body is
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originally charged negatively, it will be gradually discharged bythis convective process. If it is not charged to start with, theelectrons will still be liberated at the surface of the body, andthis will acquire a positive charge. If the body is positivelycharged at first, we cannot discharge it by illumination.
It would be superfluous for me to speak here of the nature ofelectrons or of the various modes in which their presence may bedetected. Suffice it to say, in further connection with the Hertzeffect, that when projected among gaseous molecules the electronsoon attaches itself to one of these. In other words, it ionisesa molecule of the gas or confers its electric charge upon it. Thegaseous molecule may even be itself disrupted by impact of theelectron, if this is moving fast enough, and left bereft of anelectron.
We must note that such ionisation may be regarded as conferringpotential chemical properties upon the molecules of the gas andupon the substance whence the electrons are derived. Similarionisation under electric forces enters, as we now believe, intoall the chemical effects progressing in the galvanic cell, and,indeed, generally in ionised solutes.
An experiment will best illustrate the principles I wish toremind you of. A clean aluminium plate, carefully insulated by asulphur support, is faced by a sheet of copper-wire-gauze placeda couple of centimetres away from it. The gauze is maintained ata high positive
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potential by this dry pile. A sensitive gold-leaf electroscope isattached to the aluminium plate, and its image thrown upon thescreen. I now turn the light from this arc lamp upon the wiregauze, through which it in part passes and shines upon thealuminium plate. The electroscope at once charges up rapidly.There is a liberation of negative electrons at the surface of thealuminium; these, under the attraction of the positive body, arerapidly removed as ions, and the electroscope charges uppositively.
Again, if I simply electrify negatively this aluminium plate sothat the leaves of the attached electroscope diverge widely, andnow expose it to the rays from the arc lamp, the charge, as yousee, is very rapidly dissipated. With positive electrification ofthe aluminium there is no effect attendant on the illumination.
Thus from the work of Hertz and his successors we know thatlight, and more particularly what we call actinic light, is aneffective means of setting free electrons from certainsubstances. In short, our photographic agent, light, has thepower of expelling from certain substances the electron which isso potent a factor in most, if not in all, chemical effects. Ihave not time here to refer to the work of Elster and Geitelwhereby they have shown that this action is to be traced to theelectric force in the light wave, but must turn to the probablebearing of this phenomenon on the familiar facts of photography.I assume that the experiment I have shown you is the most
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fundamental photographic experiment which it is now in our powerto make.
We must first ask from what substances can light liberateelectrons. There are many—metals as well as non-metals andliquids. It is a very general phenomenon and must operate widelythroughout nature. But what chiefly concerns the presentconsideration is the fact that the haloid salts of silver arevigorously photo-electric, and, it is suggestive, possess,according to Schmidt, an activity in the descending orderbromide, chloride, iodide. This is, in other words, their orderof activity as ionisers (under the proper conditions) whenexposed to ultra-violet light. Photographers will recognise thatthis is also the order of their photographic sensitiveness.
Another class of bodies also concerns our subject: the specialsensitisers used by the photographer to modify the spectraldistribution of sensibility of the haloid salts, _e.g._ eosine,fuchsine, cyanine. These again are electron-producers under lightstimulus. Now it has been shown by Stoletow, Hallwachs, andElster and Geitel that there is an intimate connection betweenphoto-electric activity and the absorption of light by thesubstance, and, indeed, that the particular wave-lengths absorbedby the substance are those which are effective in liberating theelectrons. Thus we have strong reason for believing that thevigorous photo-electric activity displayed by the specialsensitisers must be dependent upon their colour absorption. Youwill recognise that this is just
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the connection between their photographic effects and theirbehaviour towards light.
There is yet another suggestive parallel. I referred to theobservation of Sir James Dewar as to the continued sensitivenessof the photographic film at the lowest attained extreme oftemperature, and drew the inference that the fundamentalphotographic action must be of intra-atomic nature, and notdependent upon the vis viva of the molecule or atom. In thenseeking the origin of photographic action in photo-electricphenomena we naturally ask, Are these latter phenomena alsotraceable at low temperatures? If they are, we are entitled tolook upon this fact as a qualifying characteristic or as anotherlink in the chain of evidence connecting photographic withphoto-electric activity.
I have quite recently, with the aid of liquid air supplied to mefrom the laboratory of the Royal Dublin Society, tested thephoto-sensibility of aluminium and also of silver bromide down totemperatures approaching that of the liquid air. The mode ofobservation is essentially that of Schmidt—what he terms hisstatic method. The substance undergoing observation is, however,contained at the bottom of a thin copper tube, 5 cm. in diameter,which is immersed to a depth of about 10 cm in liquid air. Thetube is closed above by a paraffin stopper which carries a thinquartz window as well as the sulphur tubes through which theconnections pass. The air within is very carefully dried byphosphorus
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pentoxide before the experiment. The arc light is used as sourceof illumination. It is found that a vigorous photo-electriceffect continues in the case of the clean aluminium. In the caseof the silver bromide a distinct photo-electric effect is stillobserved. I have not had leisure to make, as yet, any trustworthyestimate of the percentage effect at this temperature in the caseof either substance. Nor have I determined the temperatureaccurately. The latter may be taken as roughly about -150° C,
Sir James Dewar's actual measilrements afforded twenty per cent.of the normal photographic effect at -180° C. and ten per cent.at the temperature of -252.5° C.
With this much to go upon, and the important additional fact thatthe electronic discharge—as from the X-ray tube or fromradium—generates the latent image, I think we are fully entitledto suggest, as a legitimate lead to experiment, the hypothesisthat the beginnings of photographic action involve an electronicdischarge from the light-sensitive molecule; in other words thatthe latent image is built up of ionised atoms or molecules theresult of the photo-electric effect on the illuminated silverhaloid, and it is upon these ionised atoms that the chemicaleffects of the developer are subsequently directed. It may bethat the liberated electrons ionise molecules not directlyaffected, or it may be that in their liberation they disruptcomplex molecules built up in the ripening of the
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emulsion. With the amount we have to go upon we cannot venture toparticularise. It will be said that such an action must be inpart of the nature of a chemical effect. This must be admitted,and, in so far as the rearrangement of molecular fabrics isinvolved, the result will doubtless be controlled by temperatureconditions. The facts observed by Sir James Dewar support this.But there is involved a fundamental process—the liberation of theelectron by the electric force in the light wave, which is aphysical effect, and which, upon the hypothesis of its reality asa factor in forming the latent image, appears to explaincompletely the outstanding photographic sensitiveness of the filmat temperatures far below those at which chemical actions ingeneral cease.
Again, we may assume that the electron—producing power of thespecial sensitiser or dye for the particular ray it absorbs isresponsible, or responsible in part, for the specialsensitiveness it confers upon the film. Sir Wm. Abney has shownthat these sensitisers are active even if laid on as a varnish onthe sensitive surface and removed before development. It must beremembered, however, that at temperatures of about -50° thesesensitisers lose much of their influence on the film; as I havepointed out in a paper read before the Photographic Convention of1894.
It. appears to me that on these views the curious phenomenon ofrecurrent reversals does not present a problem hopeless ofexplanation. The process of photo-
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ionisation constituting the latent image, where the ion isprobably not immediately neutralised by chemical combination,presents features akin to the charging of a capacity—say a Leydenjar. There may be a rising potential between the groups of ionsuntil ultimately a point is attained when there is a spontaneousneutralisation. I may observe that the phenomena of reversalappear to indicate that the change in the silver bromidemolecule, whatever be its nature, is one of gradually increasingintensity, and finally attains a maximum when a return to theoriginal condition occurs. The maximum is the point of mostintense developable image. It is probable that the sensitiser—inthis case the gelatin in which the bromide of silver isimmersed—plays a part in the conditions of stability which areinvolved.
Of great interest in all our considerations and theories is therecent work of Wood on photographic reversal. The result of thiswork is—as I take it—to show that the stability of the latentimage may be very various according to the mode of its formation.Thus it appears that the sort of latent effect which is producedby pressure or friction is the least stable of any. This may bereversed or wiped out by the application of any other known formof photographic stimulus. Thus an exposure to X-rays willobliterate it, or a very brief exposure to light. The latentimage arising from X-rays is next in order of increasingstability. Light action will remove this. Third in order is avery brief light-shock or sudden flash. This
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cannot be reversed by any of the foregoing modes of stimulation,but a long-continued undulatory stimulus, as from lamp-light,will reverse it. Last and most stable of all is the graduallybuilt-up configuration due to long-continued light exposure. Thiscan only be reversed by overdoing it according to the known factsof recurrent reversal. Wood takes occasion to remark that thesephenomena are in bad agreement with the strain theory of Bose. Wehave, in fact, but the one resource—the allotropic modificationof the haloid—whereby to explain all these orders of stability.It appears to me that the elasticity of the electronic theory isgreater. The state of the ionised system may be very variousaccording as it arises from continued rhythmic effects or fromunorganised shocks. The ionisation due to X-rays or to frictionwill probably be quite unorganised, that due to light more orless stable according to the gradual and gentle nature of theforces at work. I think we are entitled to conclude that on thewhole there is nothing in Wood's beautiful experiments opposed tothe photo-electric origin of photographic effects, but that theyrather fall in with what might be anticipated according to thattheory.
When we look for further support to the views I have laid beforeyou we are confronted with many difficulties. I have not as yetdetected any electronic discharge from the film under lightstimulus. This may be due to my defective experiments, or to afact noted by Elster and Geitel concerning the photo-electricproperties of gelatin.
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They obtained a vigorous effect from Balmain's luminous paint,but when this was mixed in gelatin there was no external effect.Schmidt's results as to the continuance of photo-electricactivity when bodies in general are dissolved in each other leadus to believe that an actual conservative property of the mediumand not an effect of this on the luminous paint is here involved.This conservative effect of the gelatin may be concerned with itsefficacy as a sensitiser.
In the views I have laid before you I have endeavoured to showthat the recent addition to our knowledge of the electron as anentity taking part in many physical and chemical effects shouldbe kept in sight in seeking an explanation of the mode of originof the latent image.[1]
[1] For a more detailed account of the subject, and someingenious extensions of the views expressed above, see_Photo-Electricity_, by H. Stanley Allen: Longmans, Green & Ca.,1913.
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PLEOCHROIC HALOES[1]
IT is now well established that a helium atom is expelled fromcertain of the radioactive elements at the moment oftransformation. The helium atom or alpha ray leaves thetransforming atom with a velocity which varies in the differentradioactive elements, but which is always very great, attainingas much as 2 x 109 cms. per second; a velocity which, ifunchecked, would carry the atom round the earth in less than twoseconds. The alpha ray carries a positive charge of double theionic amount.
When an alpha ray is discharged from the transforming elementinto a gaseous medium its velocity is rapidly checked and itsenergy absorbed. A certain amount of energy is thus transferredfrom the transforming atom to the gas. We recognise this energyin the gas by the altered properties of the latter; chiefly bythe fact that it becomes a conductor of electricity. Themechanism by which this change is effected is in part known. Theatoms of the gas, which appear to be freely penetrated by thealpha ray, are so far dismembered as to yield charged electronsor ions; the atoms remaining charged with an equal and oppositecharge. Such a medium of
[1] Being the Huxley Lecture, delivered at the University ofBirmingham on October 30th, 1912. Bedrock, Jan., 1913.
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free electric charges becomes a conductor of electricity byconvection when an electromotive force is applied. The gas alsoacquires other properties in virtue of its ionisation. Undercertain conditions it may acquire chemical activity and newcombinations may be formed or existing ones broken up. When itsinitial velocity is expended the helium atom gives up itsproperties as an alpha ray and thenceforth remains possessed ofthe ordinary varying velocity of thermal agitation. Bragg andKleeman and others have investigated the career of the alpha raywhen its path or range lies in a gas at ordinary or obtainableconditions of pressure and temperature. We will review some ofthe facts ascertained.
The range or distance traversed in a gas at ordinary pressures isa few centimetres. The following table, compiled by Geiger, givesthe range in air at the temperature of 15° C.:
cms. cms. cms.Uranium 1 - 2.50 Thorium - 2.72 Radioactinium 4.60Uranium 2 - 2.90 Radiothorium 3.87 Actinium X - 4.40Ionium - 3.00 Thorium X - 4.30 Act Emanation 5.70Radium - 3.30 Th Emanation 5.00 Actinium A - 6.50Ra Emanation 4.16 Thorium A - 5.70 Actinium C - 5.40Radium A - 4.75 Thorium C1 - 4.80Radium C - 6.94 Thorium C2 - 8.60Radium F - 3.77
It will be seen that the ray of greatest range is that proceedingfrom thorium C2, which reaches a distance of 8.6 cms. In theuranium family the fastest ray is
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that of radium C. It attains 6.94 cms. There is thus anappreciable difference between the ultimate distances traversedby the most energetic rays of the two families. The shortestranges are those of uranium 1 and 2.
The ionisation effected by these rays is by no means uniformalong the path of the ray. By examining the conductivity of thegas at different points along the path of the ray, the ionisationat these points may be determined. At the limits of the range theionisation
{Fig. 13}
ceases. In this manner the range is, in fact, determined. Thedotted curve (Fig. 13) depicts the recent investigation of theionisation effected by a sheaf of parallel rays of radium C inair, as determined by Geiger. The range is laid out horizontallyin centimetres. The numbers of ions are laid out vertically. Theremarkable nature of the results will be at once apparent. Weshould have expected that the ray at the beginning of its path,when its velocity and kinetic energy were greatest, would havebeen more effective than towards the end of its range
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when its energy had almost run out. But the curve shows that itis just the other way. The lagging ray, about to resign itsionising properties, becomes a much more efficient ioniser thanit was at first. The maximum efficiency is, however, in the caseof a bundle of parallel rays, not quite at the end of the range,but about half a centimetre from it. The increase to the maximumis rapid, the fall from the maximum to nothing is much morerapid.
It can be shown that the ionisation effected anywhere along thepath of the ray is inversely proportional to the velocity of theray at that point. But this evidently does not apply to the last5 or 10 mms. of the range where the rate of ionisation and of thespeed of the ray change most rapidly. To what are the changingproperties of the rays near the end of their path to be ascribed?It is only recently that this matter has been elucidated.
When the alpha ray has sufficiently slowed down, its power ofpassing right through atoms, without appreciably experiencing anyeffects from them, diminishes. The opposing atoms begin to exertan influence on the path of the ray, deflecting it a little. Theheavier atoms will deflect it most. This effect has been verysuccessfully investigated by Geiger. It is known as "scattering."The angle of scattering increases rapidly with the decrease ofvelocity. Now the effect of the scattering will be to cause someof the rays to complete their ranges
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or, more accurately, to leave their direct line of advance alittle sooner than others. In the beautiful experiments of C. T.R. Wilson we are enabled to obtain ocular demonstration of thescattering. The photograph (Fig. 14.), which I owe to thekindness of Mr. Wilson, shows the deflection of the ray towardsthe end of its path. In
{Fig. 14}
this case the path of the ray has been rendered visible by thecondensation of water particles under the influence of theionisation; the atmosphere in which the ray travels being in astate of supersaturation with water vapour at the instant of thepassage of the ray. It is evident that if we were observing theionisation along a sheaf of parallel rays, all starting withequal velocity,
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the effect of the bending of some of the rays near the end oftheir range must be to cause a decrease in the aggregateionisation near the very end of the ultimate range. For, in fact,some of the rays complete their work of ionising at points in thegas before the end is reached. This is the cause, or at least animportant contributory cause, of the decline in the ionisationnear the end of the range, when the effects of a bundle of raysare being observed. The explanation does not suggest that theionising power of any one ray is actually diminished before itfinally ceases to be an alpha ray.
The full line in Fig. 13 gives the ionisation curve which it maybe expected would be struck out by a single alpha ray. In it theionisation goes on increasing till it abruptly ceases altogether,with the entire loss of the initial kinetic energy of theparticle.
A highly remarkable fact was found out by Bragg. The effect ofthe atom traversed by the ray in checking the velocity of the rayis independent of the physical and chemical condition of theatom. He measured the "stopping power" of a medium by thedistance the ray can penetrate into it compared with the distanceto which it can penetrate in air. The less the ratio the greateris the stopping power. The stopping power of a substance isproportional to the square root of its atomic weight. Thestopping power of an atom is not altered if it is in chemicalunion with another atom. The atomic weight is the one quality ofimportance. The physical
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state, whether the element is in the solid, liquid or gaseousstate, is unimportant. And when we deal with molecules thestopping power is simply proportional to the sum of the squareroots of the atomic weights of the atoms entering into themolecule. This is the "additive law," and it obviously enables usto calculate what the range in any substance of known chemicalcomposition and density will be, compared with the range in air.
This is of special importance in connection with phenomena wehave presently to consider. It means that, knowing the chemicalcomposition and density of any medium whatsoever, solid, liquidor gaseous, we can calculate accurately the distance to which anyparticular alpha ray will penetrate. Nor have the temperature andpressure to which the medium is subjected any influence save inso far as they may affect the proximity of one atom to another.The retardation of the alpha ray in the atom is not affected.
This valuable additive law, however, cannot be applied instrictness to the amount of ionisation attending the ray. Theform of the molecule, or more generally its volume, may have aninfluence upon this. Bragg draws the conclusion, from this factas well as from the notable increase of ionisation with loss ofspeed, that the ionisation is dependent upon the time the rayspends in the molecule. The energy of the ray is, indeed, foundto be less efficient in producing ionisation in the smalleratomm.
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Before leaving our review of the general laws governing thepassage of alpha rays through matter, another point of interestmust be referred to. We have hitherto spoken in general terms ofthe fact that ionisation attends the passage of the ray. We havesaid nothing as to the nature of the ionisation so produced. Butin point of fact the ionisation due to an alpha ray is suigeneris. A glance at one of Wilson's photographs (Fig. 14.)illustrates this. The white streak of water particles marks thepath of the ray. The ions produced are evidently closely crowdedalong the track of the ray. They have been called into existencein a very minute instant of time. Now we know that ions ofopposite sign if left to themselves recombine. The rate ofrecombination depends upon the product of the number of each signpresent in unit volume. Here the numbers are very great and thevolume very small. The ionic density is therefore high, andrecombination very rapidly removes the ions after they areformed. We see here a peculiarity of the ionisation effected byalpha rays. It is linear in distribution and very local. Much ofthe ionisation in gases is again undone by recombination beforediffusion leads to the separation of the ions. This "initialrecombination" is greatest towards the end of the path of the raywhere the ionisation is a maximum. Here it may be so effectivethat the form of the curve is completely lost unless a very largeelectromotive force is used to separate the ions when theionisation is being investigated.
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We have now reviewed recent work at sufficient length tounderstand something of the nature of the most important advanceever made in our knowledge of the atom. Let us glance briefly atwhat we have learned. The radioactive atom in sinking to a loweratomic weight casts out with enormous velocity an atom of helium.It thus loses a definite portion of its mass and of its energy.Helium which is chemically one of the most inert of the elements,is, when possessed of such great kinetic energy, able topenetrate and ionise the atoms which it meets in its path. Itspends its energy in the act of ionising them, coming to rest,when it moves in air, in a few centimetres. Its initial velocitydepends upon the particular radioactive element which has givenrise to it. The length of its path is therefore differentaccording to the radioactive element from which it proceeds. Theretardation which it experiences in its path depends entirelyupon the atomic weight of the atoms which it traverses. As itadvances in its path its effectiveness in ionising the atomrapidly increases and attains a very marked maximum. In a gas theions produced being much crowded together recombine rapidly; sorapidly that the actual ionisation may be quite concealed unlessa sufficiently strong electric force is applied to separate them.Such is a brief summary of the climax of radioactivediscovery:—the birth, life and death of the alpha ray. Its adventinto Science has altered fundamentally our conception of
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matter. It is fraught with momentous bearings upon GeologicalScience. How the work of the alpha ray is sometimes recordedvisibly in the rocks and what we may learn from that record, Ipropose now to bring before you.
In certain minerals, notably the brown variety of mica known asbiotite, the microscope reveals minute circular marks occurringhere and there, quite irregularly. The most usual appearance isthat of a circular area darker in colour than the surroundingmineral. The radii of these little disc-shaped marks when welldefined are found to be remarkably uniform, in some cases fourhundredths of a millimetre and in others three hundredths, about.These are the measurements in biotite. In other minerals themeasurements are not quite the same as in biotite. Such minuteobjects are quite invisible to the naked eye. In some rocks theyare very abundant, indeed they may be crowded together in suchnumbers as to darken the colour of the mineral containing them.They have long been a mystery to petrologists.
Close examination shows that there is always a small speck of aforeign body at the centre of the circle, and it is oftenpossible to identify the nature of this central substance, smallthough it be. Most generally it is found to be the mineralzircon. Now this mineral was shown by Strutt to contain radium inquantities much exceeding those found in ordinary rocksubstances.
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Some other mineral may occasionally form the nucleus, but wenever find any which is not known to be specially likely tocontain a radioactive substance. Another circumstance we notice.The smaller this central nucleus the more perfect in form is thedarkened circular area surrounding it. When the circle is veryperfect and the central mineral clearly defined at its centre wefind by measurement that the radius of the darkened area isgenerally 0.033 mm. It may sometimes be 0.040 mm. These arealways the measurements in biotite. In other minerals the radiiare a little different.
We see in the photograph (Pl. XXIII, lower figure), muchmagnified, a halo contained in biotite. We are looking at aregion in a rock-section, the rock being ground down to such athickness that light freely passes through it. The biotite is inthe centre of the field. Quartz and felspar surround it. The rockis a granite. The biotite is not all one crystal. Two crystals,mutually inclined, are cut across. The halo extends across bothcrystals, but owing to the fact that polarised light is used intaking the photograph it appears darker in one crystal than inthe other. We see the zircon which composes the nucleus. The finestriated appearance of the biotite is due to the cleavage of thatmineral, which is cut across in the section.
The question arises whether the darkened area surrounding thezircon may not be due to the influence of the radioactivesubstances contained in the zircon. The
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extraordinary uniformity of the radial measurements of perfectlyformed haloes (to use the name by which they have long beenknown) suggests that they may be the result of alpha radiation.For in that case, as we have seen, we can at once account for thedefinite radius as simply representing the range of the ray inbiotite. The furthest-reaching ray will define the radius of thehalo. In the case of the uranium family this will be radium C,and in the case of thorium it will be thorium C. Now here wepossess a means of at once confirming or rejecting the view thatthe halo is a radioactive phenomenon and occasioned by alpharadiation; for we can calculate what the range of these rays willbe in biotite, availing ourselves of Bragg's additive law,already referred to. When we make this calculation we find thatradium C just penetrates 0.033 mm. and thorium C 0.040 mm. Theproof is complete that we are dealing with the effects of alpharays. Observe now that not only is the coincidence of measurementand calculation a proof of the view that alpha radiation hasoccasioned the halo, but it is a very complete verification ofthe important fact stated by Bragg, that the stopping powerdepends solely on the atomic weight of the atoms traversed by theray.
We have seen that our examination of the rocks reveals only thetwo sorts of halo: the radium halo and the thorium halo. This isnot without teaching. For why not find an actinium halo? NowRutherford long ago suggested that this element and itsderivatives were
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probably an offspring of the uranium family; a side branch, as itwere, in the formation of which relatively few transforming atomstook part. On Rutherford's theory then, actinium should alwaysaccompany uranium and radium, but in very subordinate amount. Theabsence of actinium haloes clearly supports this view. For ifactinium was an independent element we would be sure to findactinium haloes. The difference in radius should be noticeable.If, on the other hand, actinium
was always associated with uranium and radium, then its effectswould be submerged in those of the much more potent effects ofthe uranium series of elements.
It will have occurred to you already that if the radioactiveorigin of the halo is assured the shape of a halo is not reallycircular, but spherical. This is so. There is no such thing as adisc-shaped halo. The halo is a spherical volume containing theradioactive nucleus at its centre. The true radius of the halomay, therefore, only be measured on sections passing through thenucleus.
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In order to understand the mode of formation of a halo we mayprofitably study on a diagram the events which go on within thehalo-sphere. Such a diagram is seen in Fig. 15. It shows torelatively correct scale the limiting range of all the alpha-rayproducing members of the uranium and thorium families. We knowthat each member of a family will exist in equilibrium amountwithin the nucleus possessing the parent element. Each alpha rayleaving the nucleus will just attain its range and then cease toaffect the mica. Within the halosphere, there must be, therefore,the accumulated effects of the influences of all the rays. Eachhas its own sphere of influence, and the spheres are allconcentric.
The radii in biotite of the several spheres are given in thefollowing table
URANIUM FAMILY.Radium C - 0.0330 mm.Radium A - 0.0224 mm.Ra Emanation - 0.0196 mm.Radium F - 0.0177 mm.Radium - 0.0156 mm.Ionium - 0.0141 mm.Uranium 1 - 0.0137 mm.Uranium 2 - 0.0118 mm.
THORIUM FAMILY.Thorium CE - 0.040 mm.Thorium A - 0.026 mm.Th Emanation - 0.023 mm.Thorium Ci - 0.022 mm.Thorium X - 0.020 mm.Radiothorium - 0.119 mm.Thorium - 0.013 mm.
In the photograph (Pl. XXIV, lower figure), we see a uranium anda thorium halo in the same crystal of mica. The mica is containedin a rock-section and is cut across the cleavage. The effects ofthorium Ca are clearly shown
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as a lighter border surrounding the accumulated inner darkeningdue to the other thorium rays. The uranium halo (to the right)similarly shows the effects of radium C, but less distinctly.
Haloes which are uniformly dark all over as described above are,in point of fact, "over-exposed"; to borrow a familiarphotographic term. Haloes are found which show much beautifulinternal detail. Too vigorous action obscures this detail just asdetail is lost in an over-exposed photograph. We may again have"under-exposed" haloes in which the action of the several rays isincomplete or in which the action of certain of the rays has leftlittle if any trace. Beginning at the most under-exposed haloeswe find circular dark marks having the radius 0.012 or 0.013 mm.These haloes are due to uranium, although their inner darkeningis doubtless aided by the passage of rays which were too few toextend the darkening beyond the vigorous effects of the twouranium rays. Then we find haloes carried out to the radii 0.016,0.018 and 0.019 mm. The last sometimes show very beautiful outerrings having radial dimensions such as would be produced byradium A and radium C. Finally we may have haloes in whichinterior detail is lost so far out as the radius due to emanationor radium A, while outside this floats the ring due to radium C.Certain variations of these effects may occur, marking,apparently, different stages of exposure. Plates XXIII and XXIV(upper figure) illustrate some of these stages;
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the latter photograph being greatly enlarged to show clearly thehalo-sphere of radium A.
In most of the cases mentioned above the structure evidentlyshows the existence of concentric spherical shells of darkenedbiotite. This is a very interesting fact. For it proves that inthe mineral the alpha ray gives rise to the same increasedionisation towards the end of its range, as Bragg determined inthe case of gases. And we must conclude that the halo in everycase grows in this manner. A spherical shell of darkened biotiteis first produced and the inner colouration is only effected asthe more feeble ionisation along the track of the ray in courseof ages gives rise to sufficient alteration of the mineral. Thismore feeble ionisation is, near the nucleus, enhanced in itseffects by the fact that there all the rays combine to increasethe ionisation and, moreover, the several tracks are therecrowded by the convergency to the centre. Hence the mostelementary haloes seldom show definite rings due to uranium,etc., but appear as embryonic disc-like markings. The photographsillustrate many of the phases of halo development.
Rutherford succeeded in making a halo artificially by compressinginto a capillary glass tube a quantity of the emanation ofradium. As the emanation decayed the various derived productscame into existence and all the several alpha rays penetrated theglass, darkening the walls of the capillary out to the limit ofthe range of radium C in glass. Plate XXV shows a magnifiedsection of the
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tube. The dark central part is the capillary. The tubular halosurrounds it. This experiment has, however, been anticipated bysome scores of millions of years, for here is the same effect ina biotite crystal (Pl. XXV). Along what are apparently tubularpassages or cracks in the mica, a solution, rich in radioactivesubstances, has moved; probably during the final consolidation ofthe granite in which the mica occurs. A continuous and veryregular halo has developed along these conduits. A string ofhalo-spheres may lie along such passages. We must infer thatsolutions or gases able to establish the radioactive nuclei movedalong these conduits, and we are entitled to ask if all thehaloes in this biotite are not, in this sense, of secondaryorigin. There is, I may add, much to support such a conclusion.
The widespread distribution of radioactive substances is mostreadily appreciated by examination of sections of rocks cut thinenough for microscopic investigation. It is, indeed, difficult tofind, in the older rocks of granitic type, mica which does notshow haloes, or traces of haloes. Often we find that every one ofthe inclusions in the mica—that is, every one of the earlierformed substances—contain radioactive elements, as indicated bythe presence of darkened borders. As will be seen presently thequantities involved are generally vanishingly small. For exampleit was found by direct determination that in one gram of thehalo-rich mica of Co. Carlow there was rather less than twelvebillionths of a gram of radium, We are