FIG 4Fig. 4.
Fig. 4.
The simple apparatus with which such determinations are made is due to the physicist Kundt. It consists of a glass tube, through one end ofwhich a glass rod passes, so that half the rod is enclosed in the tube, while the other half projects outside it. In the experiments on argon, the rod was sealed into the tube; in other cases, it is better to attach it with indiarubber, or to cause the rod to pass through a cork. The open end of the tube is connected with a supply of the gas, so that, after the tube has been pumped empty of air, the gas, in a pure and dry condition, can be admitted. Some light powder (and for this purpose lycopodium dust—the dried spores of a species of fungus—is best) is placed in the tube, and distributed uniformly throughout it, so that when the latter is in a horizontal position, a streak of the powder lies along it from end to end. The portion of rod outside the tube is rubbed with a rag wetted with alcohol, when it emits a shrill tone or squeak, due to longitudinal vibrations; the pitch of the tone depends, naturally, on the length of the rod, a long rod giving a deeper tone than a short one. The vibrations of the rod set the gas in the tube in motion, and the sound-waves are conveyed from end to end ofthe tube through the gas. As the tube is closed at the end through which the gas was admitted, these waves echo back through it; and a great deal of care must be taken to make the echo strengthen the waves, so that the compressions produced by the back waves are coincident in position with the compressions produced by the forward waves travelling onwards from the rod. The gas, could we see it, would represent portions compressed and portions rarefied at regular intervals along the tube. Where the gas is compressed, it gathers the lycopodium dust together in small heaps, the position of each heap signifying a node of compression. Hence, comparing the distances between the nodes of compression for any gas and for air, we find the relative wave-lengths of sound in the two gases; and, as the velocity of sound in air has been accurately measured, we thus determine the velocity of sound-waves in the gas under experiment.
Such experiments were made by Kundt and by his co-worker Warburg on mercury gas, and they found that in this case the value of γ was 1·67; that is, in the equation
γ =c2d/p
the value 1·67 had to be ascribed to γ, in order to render it equal to the product of the square of the velocity into the density, divided by the pressure.
Similar experiments with argon led to the same result as Kundt and Warburg found for mercury gas; but the calculation becomes more simple if it is allowable to take for granted that the elasticity, or alteration of pressure produced by unit alteration of volume, is identical in the case of argon and air. The full equations are—
and
wherenis the number of vibrations per second,λthe wave-length of sound, andathe coefficient of the expansion of a gas for a rise of 1° in temperature,t, viz. 0·00367. Now if the expressionp(1 +at) can be shown to be identical for argon and for air, the value ofγfor argon can be calculated by the very simple proportion—
λ2dair: λ2dargon:: 1·408 : γargon.
This involved a measurement of the rate of rise of pressure of argon,p, per degree of rise of temperature,t; or, in other words, the verification of Boyle’s and Gay-Lussac’s laws for argon; and this research was successfully carried out by Dr. Randall of the Johns Hopkins University of Baltimore, U.S.A., and Dr. Kuenen, of Leyden, working in Professor Ramsay’s laboratory.[29]They made use of a constant volume thermometer, and measured the rise of pressure corresponding to a definite rise of temperature, comparing the gases argon and helium in this respect with air. The values found between 0° and 100° for air, argon, and helium were—
It may therefore be taken for certain that, within the limits of experimental error, the value of the expressionp(1 +at) is identical for all three gases.
We see, then, that for argon, as for mercury gas, the value ofγ, the ratio between the specific heats at constant volume and at constant pressure, is 1 to 1·66, whereas for air, hydrogen, oxygen, nitrogen, carbon monoxide, and nitric oxide, it is 1 to 1·4.
We have now to consider what conclusion can be drawn from this difference.
On the usually accepted theory of the constitution of matter, it is held that atoms may be regarded as spheres, hard, elastic, smooth, and practically incompressible. True, we really know little or nothing regarding the properties of such particles, if particles there be; but in considering their behaviour it is necessary to make certain suppositions, and to see whether observed facts can be pictured to our minds in accordance with such postulates. If, from the known behaviour of large masses, conclusions can be drawn regarding small masses, and if these conclusions harmonise with what is found to be the behaviour of large numbers of small masses acting at once, the justice of the supposition is, although not proved, at least rendered defensible as one mode of regarding natural phenomena.
Molecules, on this supposition, may consist of single atoms, or they may consist of pairs of such atoms, joined in some fashion like the bulged ends of a dumb-bell; or lastly, they may consist of greater numbers of atoms arranged in some different manner, the arrangementdepending on their relative size and attraction for each other. It must be clearly understood, however, that such mental pictures are not to be taken as actually representing the true constitution of matter, but merely as attempts to picture such forms as will allow of our drawing conclusions regarding their behaviour from known configurations of large masses.
The molecules of gases are imagined to be in a state of continual motion, up and down, backwards and forwards, and from side to side. It is true that they must also move in directions which cannot be described by any of these expressions, but such other directions may be conceived as partaking more or less of motions in the three directions specified;i.e.in being resolvable into these. To these motions have been applied the term “degrees of freedom.” Such motions through space, in which the molecule is transported from one position in space to another, form three of the possible six degrees of freedom which a molecule may possess, and the molecules are said to possess “energy of translation” in virtue of this motion. The other three consist in rotations in three planes at right angles to each other.
Now, it can be shown that the product of pressure and volume of a gas,pv, is equal to ⅔rds of the energy of translation of all molecules of the gas, or
pv= ⅔NR,
where N stands for the number of molecules in unit volume, and R for their energy of translation; inasmuch as a pressure diminishing a volume is of the nature of work, or energy. For one gram of air at O° C. and 76 cms. pressure (normal temperature and pressure), the pressure (p), measured in grams per square centimetre, is 1033, and the volume (v) is 773·3 cubic centimetres; and the raising of the temperature through 1°, as was shown before, requires 2927 gram-centimetres of work. Further, since the product of pressure into volume is equal to ⅔rds of the energy due to motion, or the translational energy of the gas,
NR =3⁄2(pv) =3⁄2× 2927 = 4391 gram-centimetres.
Dividing this number by 42,380, the mechanical equivalent of heat, or the number of gram-centimetres corresponding to one calory, the quotient is 0·1040 calory. If the energy of the air were due to the translational motion of its molecules, we should expect this number, 0·1040, to stand for the specific heat of air at constant volume; but it has been found equal to 0·1683, as already shown.
We have seen that to convert specific heat at constant volume into specific heat at constant pressure 0·0692 must be added. Hence at constant pressure the specific heat of such an ideal gas should be 0·1732. And the relation between specific heat at constant volume and that at constant pressure should be 0·1040 to 0·1732, or 1 to 1⅔. The conclusion to be drawn from these numbers for air, 0·1683 and 0·2375, which bear to each other the ratio of 1: 1·41, is that air cannot be such an ideal gas; that in communicating heat to it some of that heat must be employed in performing some kind of work other than that of raising its temperature. What this work may possibly be we shall consider later.
But Kundt and Warburg found, from their experiments on the ratio between the specific heats of mercury gas, this ideal ratio, 1 to 1⅔; and Professor Ramsay obtained the same ideal ratio, or one very close to it indeed, 1 to 1·659, for argon. He subsequently found this ideal ratio also to hold for helium (1 to 1·652), and it must thereforebe concluded that such gases possess only three degrees of freedom; or, in other words, their molecules, when heated, expend all the energy imparted to them in translational motion through space.
This is the consequence which we should infer from the supposition that such molecules are hard, smooth, elastic spheres. Were they each composed of two atoms, we should have to picture them as dumbbell-like structures; and here we enter on a theoretical conception put forward by Professor Boltzmann, but which has not been accepted universally by physicists.
Fig. 5.
Fig. 6.
Boltzmann imagines that to the three “degrees of freedom” of a single atom molecule there may be added, provided the molecule consists of two atoms, two other degrees of freedom, namely, freedom to rotate about two planes at right angles to each other. The knobs at the end of each imaginary dumb-bell may revolve round a central point in the handle joining them, and it is clear that they may revolve in one horizontal and in one vertical plane, as shown in Fig. 5. Such diatomic molecules are said to possess five “degrees of freedom.” They will not, it is supposed, rotate round the line joining the centres of the spheres,because, as before said, the atoms are pictured as perfectly smooth. But if the molecules are triatomic, as, for example, CO2or N2O, they will have six degrees of freedom, for with the addition of an additional atom they have an additional plane of rotation (see Fig. 6). Boltzmann has attempted to show that the ratio of the specific heats of diatomic molecules should be as 1 to 1·4. In actual fact it approximates to that number. For the commoner gases it is—
In all cases the numbers are too large, and this is a serious difficulty, because any tendency to rotate round the central line would cause the values to be less, not greater than 1·4. For triatomic molecules the calculated value of γ is 1⅓, but in actual fact theratio in the case of triatomic molecules, such as H2O, CO2, N2O, etc., is always less than 1⅓. These speculations stand on a basis very different from the first conception, namely, that all heat must be employed in communicating translational motion to molecules of mercury gas, argon, and helium, and it appears that the atoms of these three elements must necessarily be regarded as having the properties of smooth elastic spheres. The atoms and the molecules must in their several cases be identical. And, inasmuch as the chemical evidence regarding mercury leads to the same conclusion, it appears legitimate to infer that argon and helium must also be monatomic elements.
THE POSITION OF ARGON AMONG THE ELEMENTS
From what has been said in the preceding chapter there can be no doubt that the molecular weight of argon is 39·88. We have now to consider what this conclusion involves. Taken in conjunction with the fact that the ratio between its specific heat at constant volume and that at constant pressure is 1⅔, it follows that energy imparted to it is employed solely in communicating translational motion to its molecules. In the case of mercury gas such behaviour is taken as evidence that the conclusion following from the formulae of its compounds, from the density of its compounds in the gaseous state, and from its own vapour-density, as well as from its specific heat in the liquid state, namely, that its molecules are monatomic, is correct. Is it legitimateto conclude that because argon in the gaseous state has the same ratio of specific heats, therefore it also is a monatomic gas?
The conclusion will depend on our conception of an atom and a molecule, and in the present state of our ignorance regarding these abstract entities no positive answer can be given. It appears certain that, on raising the temperature of argon, very little, if any, energy is absorbed in imparting vibrational motion to its molecules; and our choice lies between our ability or inability to conceive of a molecule so constituted as to be incapable of internal motion. If there be any truth underlying Professor Boltzmann’s conception, a molecule of argon cannot consist of any complex structure of atoms, otherwise it would possess more than three degrees of freedom, and heat would be utilised in causing rotational motions. As we know for a fact that the ratio between the specific heats of gases diminishes with the increasing complexity of their molecules, perhaps the safest conclusion is the one adopted by the discoverers of argon, that the balance of evidence drawn fromthissource is in favour of its monatomic nature.
But this hypothesis raises difficulties which are not lightly to be met. These difficulties arise from a consideration of the position of argon when it is classified with other elements.
In 1863 Mr. John Newlands pointed out in a letter to theChemical Newsthat if the elements be arranged in the order of their atomic weights in a tabular form, they fall naturally into such groups that elements similar to each other in chemical behaviour occur in the same columns. This idea was elaborated farther in 1869 by Professor Mendeléeff of St. Petersburg and by the late Professor Lothar Meyer, and the table may be made to assume the subjoined form (the atomic weights are given with only approximate accuracy):—
The Elements arranged in the Periodic System.Hydrogen1·01Helium4·2Lithium7·0Beryllium9·1Boron11·0Carbon12·0Nitrogen14·0Oxygen16·0Fluorine19·0?Sodium23·0Magnesium24·3Aluminium27·0Silicon28·3Phosphorus31·0Sulphur32·1Chlorine35·5Argon39·9Potassium39·1Calcium40·1Scandium44·1Titanium48·1Arsenic75·1Selenium79·0Bromine80·0?Rubidium85·5Strontium87·5Yttrium89·0Zirconium90·0Antimony120·3Tellurium126·3Iodine126·9?Caesium132·9Barium137·0Lanthanum142·3Cerium140·3Erbium166·0?167·0?169·0??170·0?172·0Ytterbium173·0?177·0Bismuth208·1?214·0?219·0??221·0?225·0?230·0Thorium232·4Hydrogen1·01Helium4·2Lithium7·0Beryllium9·1Boron11·0Carbon12·0Nitrogen14·0Oxygen16·0Fluorine19·0?Sodium23·0Magnesium24·3Aluminium27·0Silicon28·3Phosphorus31·0Sulphur32·1Chlorine35·5Argon39·9{Iron56·0Copper63·4Zinc65·3Gallium69·9Germanium72·3Vanadium51·4Chromium52·3Manganese55·0{Cobalt58·7{Nickel58·6{Ruthenium101·6Silver107·9Cadmium112·1Indium113·7Tin119·1Niobium94·0Molybdenum95·7?100·0{Rhodium103·0{Palladium106·3?156·0?158·0?159·0Terbium162·0Neodymium140·8Praseodymium143·6Samarium150·0?152,153,154{Osmium191·3Gold197·2Mercury200·2Thallium204·2Lead206·9Tantalum182·5Tungsten184·0?190·0{Iridium193·0{Platinum194·3Uranium240·0?244·0
The Elements arranged in the Periodic System.Lithium7·0Beryllium9·1Boron11·0Carbon12·0Sodium23·0Magnesium24·3Aluminium27·0Silicon28·3Potassium39·1Calcium40·1Scandium44·1Titanium48·1Rubidium85·5Strontium87·5Yttrium89·0Zirconium90·0Caesium132·9Barium137·0Lanthanum142·3Cerium140·3?170·0?172·0Ytterbium173·0?177·0?221·0?225·0?230·0Thorium232·4Hydrogen1·01Helium4·2Nitrogen14·0Oxygen16·0Fluorine19·0?Phosphorus31·0Sulphur32·1Chlorine35·5Argon39·9Arsenic75·1Selenium79·0Bromine80·0?Antimony120·3Tellurium126·3Iodine126·9?Erbium166·0?167·0?169·0?Bismuth208·1?214·0?219·0?Lithium7·0Beryllium9·1Boron11·0Carbon12·0Sodium23·0Magnesium24·3Aluminium27·0Silicon28·3Copper63·4Zinc65·3Gallium69·9Germanium72·3Silver107·9Cadmium112·1Indium113·7Tin119·1?156·0?158·0?159·0Terbium162·0Gold197·2Mercury200·2Thallium204·2Lead206·9Hydrogen1·01Helium4·2Nitrogen14·0Oxygen16·0Fluorine19·0?Phosphorus31·0Sulphur32·1Chlorine35·5Argon39·9{Iron56·0Vanadium51·4Chromium52·3Manganese55·0{Cobalt58·7{Nickel58·6{Ruthenium101·6Niobium94·0Molybdenum95·7?100·0{Rhodium103·0{Palladium106·3Neodymium140·8Praseodymium143·6Samarium150·0?152,153,154{Osmium191·3Tantalum182·5Tungsten184·0?190·0{Iridium193·0{Platinum194·3Uranium240·0?244·0
The elements in the first column all agree in that they are white soft substances, with metallic lustre, but tarnish rapidly in air, owing to the action of water-vapour; they are all violently attacked by water, and they are without exception monads, that is, they replace hydrogen in its compounds atom for atom. The elements in column two are also all white metals, attacked by water with more or less ease; but in their case one atom replaces two atoms of hydrogen, whence they are called dyads, or bivalent elements (worth two). And so on with the other columns. All elements in vertical columns exhibit chemical similarity, and, indeed, are often strikingly like in properties.
The subdivision, produced by folding the loose page, is intended to show that the elements represented on it have a double set of resemblances. But there are various anomalous and inexplicable phenomena still attached to this arrangement of elements. For example, copper, although it replaces one atom of hydrogen in some of its compounds, and is thus a monad, forms more numerous and more stable compounds in acting as a dyad and replacing two atoms of hydrogen. Gold, which belongs to the same column, is at once univalent and tervalent; mercury, both univalent and bivalent; thallium, univalent and tervalent; tin and lead, bivalent and quadrivalent, and so on. It is as if some elements had a tendency to enter a column not their own.
Again, on comparing the atomic weights of the elements, it is seen that the differences are far from being regular. As a rule, the difference in the vertical columns between any single element and the one following it is approximately 16, or some multiple of 16. Thus we have lithium, sodium, and potassium; beryllium, magnesium, and calcium;boron, aluminium, and scandium; carbon and silicon; oxygen and sulphur; fluorine and chlorine—all with a difference of 16 approximately. But here we come to a break: silicon and titanium, phosphorus and vanadium, sulphur and chromium, chlorine and manganese, each show a difference of about 20.
Passing on, between the atomic weights of potassium, rubidium, and caesium there is a difference of about 16 × 3; a similar difference between calcium, strontium, and barium; between scandium and yttrium; between titanium, zirconium, and cerium, and so on; but with wider and wider divergence from the supposed constant, 48 = 16 × 3. In short, we have a seeming regularity, but only a very approximate one—a regularity, in fact, in which a vivid imagination must play a conspicuous part in order to detect it.
Now, up to the present, no reason has been suggested to account for the divergence from this irregular regularity, which a little expenditure of time will enable any one to trace through all these numbers. But one thing has been remarked; there is the same seeming regularity betweencertain physical properties of elements and their compounds: their specific volumes, their melting-points, their refractive indices, and other properties vary from member to member of the same column in a manner bearing more or less similarity to the periodic variation of the atomic weights.
It happens that among compounds of carbon we are acquainted with series of compounds which, in variation of molecular weights and gradation of properties, bear a striking resemblance to the elements thus arranged. Thus we have the series:—
and a host of others up to a compound of the formula C30H62; in each case there is a constant difference of 14 between the molecular weight of any one hydrocarbon and that immediately preceding or succeeding it in the column. Such a series is termed a homologous series. The analogy is very tempting; to suppose that a similar constant difference should exist in the relations of the atomic weights of the elements, and that they too are undecomposable compounds of twounknown elements, is an attractive hypothesis, but one for which there exists no proof; indeed, it is rendered improbable by the irregularities just pointed out.
But there is one noticeable feature in the periodic arrangement of the elements. It is, that although the differences are irregular (e.g.between B = 11 and C = 12 the difference is 1, while between O = 16 and F = 19 the difference is 3), yet there is no marked displacement in theorderof arrangements of the elements, inasmuch as no element has an atomic weightlowerthan that preceding it in the horizontal line. It was for some time supposed that tellurium and iodine were thus misplaced; and indeed it is even now not quite established that they are not, but the balance of evidence is in favour of tellurium having a lower atomic weight than iodine.
Argon, however, is a marked exception. With an atomic weight of 39·88, its natural position would lie between those of potassium and calcium; but there is no room for it. And for this reason considerable doubts have been thrown on the validity of the conclusion to be drawn from thefound ratio of its specific heats, 1⅔, viz. that its molecule and its atom are identical. If it were a diatomic gas, like chlorine or hydrogen, its atomic weight would be 19·94, and it would find a fitting position after fluorine and before sodium. And the difference between its atomic weight and that of helium, to which the atomic weight 2·1 would for the same reasons then attach, would be 17·84, one not incomparable with 16. But, as before remarked, it is difficult, if not altogether impossible, to conceive of a diatomic structure to which all energy imparted in the form of heat should result in translational motion, and as a matter of fact none such is known.
There are two methods of escape from this dilemma. If the gases termed argon and helium are not single elements, but mixtures of monatomic elements, then what has been termed their atomic weights will represent the mean of the atomic weights of two or more elements, taken in the proportion in which they occur. For example, supposing that argon is a mixture of an element of atomic weight 37 with one of atomic weight 82, the found atomic weight, nearly 40, would imply a mixture of 93·3 per cent of the lighter, with 6·7 per cent of the heavier element. We musttherefore carefully examine all evidence for or against the supposition that argon is a mixture of elements.
It is well known that elements with high atomic weights have, as a rule, higher boiling-points than those with low atomic weights in the same columns. Perhaps the most striking case is that of the elements fluorine, chlorine, bromine, and iodine. Whereas fluorine has never been liquefied (chiefly owing to difficulties of manipulation, due to its extraordinarily energetic action on almost every element and compound), chlorine boils at -102°, bromine at 59°, and iodine at 184°. And if a mixture of chlorine and bromine gases be cooled, the bromine, if present in sufficient amount, will condense first, in a fairly pure state, little chlorine condensing with it. But in a mixture containing only 7 per cent of bromine with 93 per cent of chlorine (analogous to a mixture of the two supposed constituents of the argon mixture) the pressure of the bromine gas in the mixture would be only7⁄100ths of the normal pressure, or 53·2 millimetres. At this pressure the boiling-point of bromine is about -5°, so that, on cooling to that temperature, bromine would begin to show signs of liquefaction. Thisis, however, still nearly 100° above the boiling-point of chlorine; and there would therefore be no difficulty whatever in detecting such a percentage of bromine in a mixture of chlorine and bromine gases on cooling the mixture to a moderately low temperature.[30]
Argon has been liquefied. A sample of pure argon was sent by Professor Ramsay to Professor Olszewski of Cracow, well known for his accurate researches at low temperatures; and he found the boiling-point of argon at atmospheric pressure to be -186·9°, and its melting-point to be -189·6°. There was no appearance of liquid before the boiling-point was reached, nor was there any alteration of temperature as the argon boiled away, and these are signs of a single substance, not of a mixture; moreover, the melting-point was a definite one; and here again, mixtures never melt suddenly, but always show signs of softening before melting. So far as this evidence goes, therefore, it points to the conclusion that argon is not a mixture of two elements.
Other evidence may be sought for in the spectrum of argon, which was carefully examined by Mr. Crookes. It consists of a great number of lines, extending all through the spectrum, from far down in the red to far beyond the visible violet; the invisible lines were examined by the aid of photography, for ultraviolet light, although invisible to the eye, impresses a photographic plate. The most striking feature of this spectrum is the change which can be produced in it by altering the intensity of the electric discharge which is passed through the tube containing argon at a low pressure. By interposing a Leyden jar between the secondary terminals of the induction-coil from which sparks are taken through the gas, the colour of the light in the tube changes from a brilliant red to an equally brilliant blue. A large number of lines in the red spectrum disappear, on interposing the jar, while many lines in the blue-green, blue, and violet part of the spectrum, invisible before, shine out with great brilliancy. There is no other gas in which a similar alteration of intensity of discharge produces such a marked difference, although in many gases, supposed to be simple substances, similar changes may be produced. So far as we know at present, however,such a change cannot be definitely ascribed to the presence of a mixture of two elements, although it is in itself a very remarkable phenomenon.
On the other hand, Professors Runge and Paschen, in a paper communicated to the Royal Academy of Science of Berlin in July 1895, have adduced reasons for concluding that helium, the gas from clèveite, is a mixture; it appears to show lines belonging to two spectra, each series of lines exhibiting certain regularities. But this, although an important conclusion in itself, has no direct bearing on the question of the simplicity of argon.
One method of separating the constituents of a mixture is by taking advantage of their different solubilities in water, or in some other appropriate solvent. And as argon was found to have the solubility of 4 volumes in 100 of water, while helium is very sparingly soluble, only 0·7 volume per 100, it is not unreasonable to suppose that, if argon consisted of a mixture of elements in argon, one should be more soluble than another. Exhaustive experiments in this direction have still to be carried out; but Lord Rayleigh has made experiments which render it very improbable that any separation into its constituents, if it be amixture, can be thus effected. Wishing to ascertain if there were any helium in the air, he shook up atmospheric argon with water, until a very small fraction remained undissolved. The spectrum of this small residue was identical with that of the original argon, from which it would appear that this method, at least, is incapable of effecting any separation.
A completely decisive proof that argon is not a mixture has just been furnished by experiments carried out by Dr. Collie and Professor Ramsay, in which a large quantity of argon was submitted to fractional diffusion. From what was said onp. 162, it will be seen that if argon consisted of a mixture of two gases of different densities, such a process should separate the mixture more or less completely into its two constituents. After a long series of diffusions, however, the density of that portion of argon passing first through the porous plug, which would have been less had any gas of lower density been present, was found to be identical with that of the last portions of gas. On the other hand, by aid of the same diffusion-apparatus, a fair separation of oxygen (density 16) from carbon dioxide (density 22) was effected, although, as the reader will observe, the densities of these two gasesdo not differ greatly. Hence, if argon consists of two kinds of matter they must have the same density, and hence the same molecular weights, and the difficulty is not removed. But as the spectrum of the first and last portions was the same and was identical with that of argon, this supposition is improbable.
The evidence is therefore distinctly against the supposition that argon is a mixture of two or more elements.
There is, however, another possible method of accounting for the high atomic weight of argon, which, if it could be reduced by a few units, would fall into its place after chlorine and before potassium. It is that argon consists of a mixture of many monatomic, with comparatively few diatomic, molecules. If there were only about 500 molecules of diatomic argon in every 10,000 molecules of the gas, its density, supposing it to consist entirely of monatomic molecules, would be 19, and its atomic and molecular weights 38, a number which would fit between the atomic weight of chlorine, 35·5, and that of potassium, 39·1. Several instances of this kind are known. Chlorine itself, whenheated to high temperatures, changes from diatomic to monatomic molecules, and the density decreases with the change. For example, at 1000° the found density of chlorine is 27, implying a molecular weight of 54; now 54 is neither the weight of a monatomic molecule of chlorine, viz. 35·5, nor of a diatomic molecule, which is 71; but it corresponds to that of a mixture of monatomic and diatomic molecules. Here fall of temperature causes combination of monatomic molecules with each other to form diatomic molecules; and rise of temperature increases the number of monatomic molecules, at the expense of the diatomic molecules. Is there no sign of similar behaviour with argon?
It has already been mentioned that the rise of pressure of argon with rise of temperature has been carefully measured by Drs. Randall and Kuenen, and that it is quite normal; no sign of splitting has been observed. But the range of temperature was not great (it was only from 0° to 280°), and it is quite possible that the change, if there was one, was so minute as to have escaped detection. Again, a more delicate method of detecting such a change is in the measurement of the ratio ofthe specific heats. The most trustworthy number obtained was 1·659 for the ratio, instead of 1·667, the theoretical figure. A mixture of 5 per cent of diatomic molecules should have reduced this ratio to 1·648. Here the evidence is, however, inconclusive. But on the whole, the presumption is against the hypothesis that argon is a mixture of monatomic with diatomic molecules.
It still remains for us, therefore, to account for the fact that in the periodic table there is no place for argon, provided it be insisted on that the elements must follow each other in the numerical order of their atomic weights. If the numbers in the table actually showed regular intervals, or if there were any regularity to be detected in their differences, argon might be regarded as of wholly exceptional behaviour. But this is not so. Argon is an extreme instance of divergence, but similar divergences, though not of equal magnitude, are common.
In attempting to offer an explanation of such anomalies, it must be remembered that the question is in itself a far-reaching one; and that although argon has served to direct attention anew to the anomalies of the periodic table, yet these anomalies existed before argon wasdiscovered. It is necessary above all things to be clear as to what is under discussion. We speak of “atomic weights,” or “atomic masses.” What is meant precisely by these expressions?
By mass, we understand that property of a body, in virtue of which, when acted on by a certain force for a certain time, it acquires a certain velocity. The product of the mass into half the velocity squared, or ½(MV)2(where M and V stand respectively for mass and for velocity), is what is termed kinetic energy. If the mass chosen be 1 gram, and the velocity 1 centimetre per second, the unit of energy is the product; it is termed an erg. The same unit of energy, the erg, is derived by the action of unit force, termed 1 dyne, through unit length, 1 centimetre. We have thus two equations, where F and L represent force and length, Kinetic Energy = ½MV2and Linear Energy = FL.
Now we choose mass as a unit of measure of thequantity of material; and we are justified in doing so, because experiments have shown that material, confined in a closed space, does not appreciably alter its mass. The mass is proportional to theweight, generally measured asthe force exerted at some definite latitude on the earth’s surface, tending to pull the body towards the centre of the earth. This force is equal to about 981 dynes at London. There is no known reason why mass and weight should be proportional to each other; for the cause of the attraction of the earth has never been satisfactorily elucidated. We therefore use weight, or the attraction of the earth, as a convenient means of determining the relative masses of two objects. Hence it appears rational to prefer the expression “atomic weight” to “atomic mass,” seeing that the former represents the actual result of experiment, and also because we are dealing in atomic weights with relative numbers. But this is really a matter of choice.
The atomic weights therefore represent the relative masses in which the elements generally unite. They often, however, unite in multiples of these weights, as formulated by Dalton’s second law. The weights, arranged in numerical order in columns, give us the periodic table.
Now energy can be measured in other units besides those of force and mass. Heat is one form of energy, and it is measured by an interval oftemperature, and by a property which we term specific heat. It happens that the latter property varies, not with the mass of the substance heated, but with its atom, so that all elements have approximately the same atomic heat; that is, quantities of elements proportional in mass to their atomic weights require approximately equal increments of heat to raise their temperature through an equal interval, say 1°. This is the formulation of Dulong and Petit’s law previously alluded to. But here we meet with irregularities, which have up till now defied classification. The heat imparted to an aggregation of atoms is not expended solely in raising their temperature; other work is done also, as is generally supposed, in the nature of expansion, or separation of parts, as in overcoming the attraction between the atoms in the molecule, or in imparting special motion to the atoms; such work, however, involves an expenditure of energy which is either very small in proportion to the total energy imparted as heat, or is nearly the same for all elements. At present we cannot decide between these alternatives, owing to the lack of knowledge of the nature of liquids and solids. The main fact, however, is incontestable: that the heatenergy required to raise different elements through the same interval of temperature is the same, not for equal masses or weights of the substances, but for theiratomicmasses.
Again, many compounds when dissolved in water conduct an electric current, while they themselves are decomposed; and the different ingredients of the compound are often deposited at the points where the electric current enters or leaves the liquid. Where they are not so deposited, it is usually because of their action on the solvent water. Now Faraday found that when elements are deposited, equal quantities of electricity are conveyed either by equal numbers of atoms or by some simple fraction of these numbers. To this fraction we apply the termvalency. Thus we say that an atom of oxygen is bivalent, or a dyad, because it conveys twice as much electricity through the liquid in which it is present as an atom of hydrogen, which is termed univalent, or a monad. Here we see a direct connection between the conveyance of an electric charge and the atomic weight. The electric unit of quantity is in fact defined as that which can be conveyed by a certain weight of hydrogen, by 8 times that weight of oxygen, by 108 times that weight ofsilver, and so on. Therefore the electrical unit is connected, not with unit of mass, or with the gravitational unit, but with the atomic unit.
We have therefore a number of systems, each capable of being equated to a unit of energy, but of which the terms are in some as yet unknown way related to each other, and often more directly than they can be related to mass and weight. This relation is only an approximate one in the case of specific heats; it appears to be an absolute one in the conveyance of electrical energy. The arrangement of the elements in the periodic table must therefore be considered by the light of such general views.
I would venture to suggest, as a tentative method of solving this problem, that it be considered whether mass or weight are such invariable properties of matter as have generally been taken for granted. That the relative weights or masses in which elements combine always retain their invariable proportion is true, so far as we can determine; but it must be remembered that we cannot cause them to combine except under a very limited series of conditions. For example,the act of combination equalises the temperature of two combining atoms; it also, in all probability, equalises their electrical charges. It is a legitimate speculation whether, could we maintain a difference in their temperature or in their electric potential, their atomic weights might not also change. Indeed, we are ignorant whether mass is changed by alteration of temperature. Experiments made by Sir John Airy, and interpreted by Professor Hicks, appear to show that variation of temperature is not without some influence on gravitational attraction. Others by Professor Landolt point in the same direction.
It therefore appears to me not impossible that the mass of the atoms may be affected by the various and different properties which they possess, some to a greater, some to a lesser extent. It must be admitted that atoms differ from each other in the readiness with which they combine with those of the same kind to form molecules; and that molecules of different elements differ from each other in their capacity to form molecular aggregates. Take, for example, such cases as caesium and fluorine, each intensely active, but towards different objects: caesium the most electro-positive of the metals, and fluorinethe most electro-negative of elements. Surely their activity must be due to some cause which cannot but exert influence on their other properties, such as their mass and their gravitational attraction, as it doubtless has influence on their specific heats, and on many of their other physical properties. And contrast these instances with helium and with argon, the most indifferent of substances, the atoms of which are unwilling, and apparently unable, to pair even with themselves; it is hardly conceivable that these peculiarities should leave their other, and, as we are in the habit of thinking, invariable, properties unaffected. I venture to suggest that these powers of combination, due to some configuration or to some attractive force, tend to lessen the gravitational attraction by which we measure their atomic weights; that helium and argon, which possess little, if any, of such power to combine, show what may be termed the normal atomic weights, inasmuch as their gravitational attraction is subject to no deduction attributable to their reacting powers.
I cannot but think that, when some numerical values are assigned to this combinational power, it will be found that they will so increase their atomic weights as to display that regularity which, so far as we can see at present, is conspicuous by its absence.
I am aware that these suggestions are of a wholly speculative character; and yet I venture to put them forward in the firm conviction that no true progress in knowledge has ever been made without such speculations. It was the speculative phlogistic theory which combined phenomena apparently so distinct as the burning of a candle and the rusting of iron. It is true that that theory is now a phantom of the past, yet it served its purpose in directing attention to phenomena of a similar character. It would be easy to multiply instances of the kind; in almost every case some useful object has been served by speculation preceding exact knowledge. The object of science, as indeed of inquiry in all departments of human interest, is to reconcile the world of man with the world of nature, and to endeavour to know in part that of which we hope one day to attain to a perfect knowledge.
THE END
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