Reproduced by the permission of Proprietors of “Knowledge.â€General view, of the Great Paris Telescope, showing the eye-end. The tube is 180 feet long, and 59 inches in diameter. It weighs 21 tons.
Reproduced by the permission of Proprietors of “Knowledge.â€General view, of the Great Paris Telescope, showing the eye-end. The tube is 180 feet long, and 59 inches in diameter. It weighs 21 tons.
The object-glass of the great telescope was cast by M. Mantois, famous as the manufacturer of large lenses. The glass used was boiled and reboiled many times to get rid of all bubbles. Then it was run into a mould and allowed to cool very gradually. A whole month elapsed before the breaking of a mould, when the lens often proved to be cracked on the surface, owing to the exterior having cooled faster than the interior and parted company with it. At last, however, a perfect cast resulted.
M. Despret undertook the even more formidable task of casting the mirror at his works at Jeumont, North France. A special furnace and oven, capable of containing over fifteen tons of molten glass, had to be constructed. The mirror, 6-1/2 feet in diameter and eleven inches thick, absorbed 3-3/4 tons of liquid glass; and so great was the difficulty of cooling it gradually, that out of the twenty casts eighteen were failures.
The rough lenses and mirror having been ground to approximate correctness in the ordinary way, there arose the question of polishing, which is generally done by one of the most sensitive and perfect instruments existing-the human hand. In this case, owing to the enormous size of the objects to be treated, hand work would not do. The mere hot touch of a workman would raise on the glass a tiny protuberance, which would be worn level with therest of the surface by the polisher, and on the cooling of the part would leave a depression, only 1-75,000 of an inch deep, perhaps, but sufficient to produce distortion, and require that the lens should be ground down again, and the whole surface polished afresh.
M. Gautier therefore polished by machinery. It proved a very difficult process altogether, on account of frictional heating, the rise of temperature in the polishing room, and the presence of dust. To insure success it was found necessary to warm all the polishing machinery, and to keep it at a fixed temperature.
At the end of almost a year the polishing was finished, after the lenses and mirror had been subjected to the most searching tests, able to detect irregularities not exceeding 1-250,000 of an inch. M. Gautier applied to the mirror M. Foucault’s test, which is worth mentioning. A point of light thrown by the mirror is focused through a telescope. The eyepiece is then moved inwards and outwards so as to throw the point out of focus. If the point becomes a luminous circle surrounded by concentric rings, the surface throwing the light point is perfectly plane or smooth. If, however, a pushing-in shows a vertical flattening of the point, and a pulling-out a horizontal flattening, that part is concave; if the reverse happens, convexity is the cause.
For the removal of the mirror from Jeumont to Paris a special train was engaged, and precautions were taken rivalling those by which travelling Royalty is guarded. The train ran at night without stopping, and at a constant pace, so that the vibration of theglass atoms might not vary. On arriving at Paris, the mirror was transferred to a ponderous waggon, and escorted by a body of men to the Exposition buildings. The huge object-lens received equally careful treatment.
The telescope was housed at the Exhibition in a long gallery pointing due north and south, the siderostat at the north end. At the other, the eyepiece, end, a large amphitheatre accommodated the public assembled to watch the projection of stellar or lunar images on to a screen thirty feet high, while a lecturer explained what was visible from time to time. The images of the sun and moon as they appeared at the primary focus in the eyepiece measured from twenty-one to twenty-two inches in diameter, and the screen projections were magnified from these about thirty times superficially.
The eyepiece section consisted of a short tube, of the same breadth as the main tube, resting on four wheels that travelled along rails. Special gearing moved this truck-like construction backwards and forwards to bring a sharp focus into the eyepiece or on to a photographic plate. Focusing was thus easy enough when once the desired object came in view; but the observer being unable to control the siderostat, 250 feet distant, had to telephone directions to an assistant stationed near the mirror whenever he wished to examine an object not in the field of vision.
By the courtesy of the proprietors of theStrandMagazinewe are allowed to quote M. Deloncle’s own words describing his emotions on his first view through the giant telescope:—
“As is invariably the case, whenever an innovation that sets at nought old-established theories is brought forward, the prophecies of failure were many and loud, and I had more than a suspicion that my success would cause less satisfaction to others than to myself. Better than any one else I myself was cognisant of the unpropitious conditions in which my instrument had to work. The proximity of the river, the dust raised by hundreds of thousands of trampling feet, the trepidation of the soil, the working of the machinery, the changes of temperature, the glare from the thousands of electric lamps in close proximity—each of these circumstances, and many others of a more technical nature, which it would be tedious to enumerate, but which were no less important, would have been more than sufficient to make any astronomer despair of success even in observatories where all the surroundings are chosen with the utmost care.
“In regions pure of calm and serene air large new instruments take months, more often years, to regulate properly.
“In spite of everything, however, I still felt confident. Our calculations had been gone over again and again, and I could see nothing that in my opinion warranted the worst apprehensions of my kind critics.
“It was with ill-restrained impatience that I waitedfor the first night when the moon should show herself in a suitable position for being observed; but the night arrived in due course.
“Everything was in readiness. The movable portion of the roof of the building had been slid back, and the mirror of the siderostat stood bared to the sky.
“In the dark, square chamber at the other end of the instrument, 200 feet away, into which the eyepiece of the instrument opened, I had taken my station with two or three friends. An attendant at the telephone stood waiting at my elbow to transmit my orders to his colleague in charge of the levers that regulated the siderostat and its mirror.
“The moon had risen now, and her silvery glory shone and sparkled in the mirror.
“‘A right declension,’ I ordered.
“The telephone bell rang in reply. ‘Slowly, still slower; now to the left—enough; again a right declension—slower; stop now—very, very slowly.’
“On the ground-glass before our eyes the moon’s image crept up from one corner until it had overspread the glass completely. And there we stood in the centre of Paris, examining the surface of our satellite with all its craters and valleys and bleak desolation.
“I had won the day.â€
Most of us are able to recognise when we see them shadowgraphs taken by the aid of the now famous X-rays. They generally represent some part of the structure of men, beasts, birds, or fishes. Very dark patches show the position of the bones, large and small; lighter patches the more solid muscles clinging to the bony framework; and outside these again are shadowy tracts corresponding to the thinnest and most transparent portions of the fleshy envelope.
In an age fruitful as this in scientific marvels, it often takes some considerable time for the public to grasp the full importance of a fresh discovery. But when, in 1896, it was announced that Professor Röntgen of Würzburg had actually taken photographs of the internal organs of still living creatures, and penetrated metal and other opaque substances with a new kind of ray, great interest was manifested throughout the civilised world. On the one hand the “new photography†seemed to upset popular ideas of opacity; on the other it savoured strongly of the black art, and, by its easy excursions through the human body, seemed likely to revolutionise medical and surgical methods. At first many strange ideasabout the X-rays got afloat, attributing to them powers which would have surprised even their modest discoverer. It was also thought that the records were made in a camera after the ordinary manner of photography, but as a matter of fact Röntgen used neither lens nor camera, the operation being similar to that of casting a shadow on a wall by means of a lamp. In X-radiography a specially constructed electrically-lit glass tube takes the place of the lamp, and for the wall is substituted a sensitised plate. The object to be radiographed is merely inserted between them, its various parts offering varying resistance to the rays, so that the plate is affected unequally, and after exposure may be developed and printed from it the usual way. Photographs obtained by using X-rays are therefore properly called shadowgraphs or skiagraphs.
The discovery that has made Professor Röntgen famous is, like many great discoveries, based upon the labours of other men in the same field. Geissler, whose vacuum tubes are so well known for their striking colour effects, had already noticed that electric discharges sent through very much rarefied air or gases produced beautiful glows. Sir William Crookes, following the same line of research, and reducing with a Sprengel air-pump the internal pressure of the tubes to 1/100000 of an atmosphere, found that a luminous glow streamed from the cathode, or negative pole, in a straight line, heating and rendering phosphorescent anything that it met.Crookes regarded the glow as composed of “radiant matter,†and explained its existence as follows. The airy particles inside the tube, being few in number, are able to move about with far greater freedom than in the tightly packed atmosphere outside the tube. A particle, on reaching the cathode, is repelled violently by it in a straight line, to “bombard†another particle, the walls of the tube, or any object set up in its path, the sudden arrest of motion being converted into light and heat.
By means of special tubes he proved that the “radiant matter†could turn little vanes, and that the flow continued even when the terminals of the shocking-coil wereoutsidethe glass, thus meeting the contention of Puluj that the radiant matter was nothing more than small particles of platinum torn from the terminals. He also showed that, when intercepted, radiant matter cast a shadow, the intercepting object receiving the energy of the bombardment; but that when the obstruction was removed the hitherto sheltered part of the glass wall of the tube glowed with a brighter phosphorescence than the part which had become “tired†by prolonged bombardment. Experiments further revealed the fact that the shaft of “Cathode rays†could be deflected by a magnet from their course, and that they affected an ordinary photographic plate exposed to them.
In 1894 Lenard, a Hungarian, and pupil of the famous Hertz, fitted a Crookes’ tube with a “window†of aluminium in its side replacing a part of the glass,and saw that the course of the rays could be traced through the outside air. From this it was evident that something else than matter must be present in the shaft of energy sent from the negative terminal of the tube, as there was no direct communication between the interior and the exterior of the tube to account for the external phosphorescence. Whatever was the nature of the rays he succeeded in making them penetrate and impress themselves on a sensitised plate enclosed in a metal box.
Then in 1896 came Röntgen’s great discovery that the rays from a Crookes’ tube, after traversing theglass, could pierce opaque matter. He covered the tube with thick cardboard, but found that it would still cast the shadows of books, cards, wood, metals, the human hand, &c., on to a photographic plate even at the distance of some feet. The rays would also pass through the wood, metal, or bones in course of time; but certain bodies, notably metals, offered a much greater resistance than others, such as wood, leather, and paper. Professor Röntgen crowned his efforts by showing that a skeleton could be “shadow-graphed†while its owner was still alive.
Naturally everybody wished to know not only what the rays could do, but what they were. Röntgen, not being able to identify them with any known rays, took refuge in the algebraical symbol of the unknown quantity and dubbed them X-rays. He discovered this much, however, that they were invisible to the eye under ordinary conditions;that they travelled in straight lines only, passing through a prism, water, or other refracting bodies without turning aside from their path; and that a magnet exerted no power over them. This last fact was sufficient of itself to prevent their confusion with the radiant matter “cathode rays†of the tube. Röntgen thought, nevertheless, that they might be the cathode rays transmuted in some manner by their passage through the glass, so as to resemble in their motion sound-waves,i.e.moving straight forward and not swaying from side to side in a series of zig-zags. The existence of such ether waves had for some time before been suspected by Lord Kelvin.
Other authorities have other theories. We may mention the view that X represents the ultra-violet rays of the spectrum, caused by vibrations of such extreme rapidity as to be imperceptible to the human eye, just as sounds of extremely high pitch are inaudible to the ear. This theory is to a certain extent upheld by the behaviour of the photographic plate, which is least affected by the colours of the spectrum at the red end and most by those at the violet end. A photographer is able to use red or orange light in his dark room because his plates cannot “see†them, though he can; whereas the reverse would be the case with X-rays. This ultra-violet theory claims for X-rays a rate of ether vibration of trillions of waves per second.
An alternative theory is to relegate the rays to the gap in the scale of ether-waves between heatwaves and light-waves. But this does not explain any more satisfactorily than the other the peculiar phenomenon of non-refraction.
The apparatus employed in X-photography consists of a Crookes’ tube of a special type, a powerful shocking or induction coil, a fluorescent screen and photographic plates and appliances for developing, &c., besides a supply of high-pressure electricity derived from the main, a small dynamo or batteries.
A Crookes’ tube is four to five inches in diameter, globular in its middle portion, but tapering away towards each end. Through one extremity is led a platinum wire, terminating in a saucer-shaped platinum plate an inch or so across. At the focus of this, the negative terminal, is fixed a platinum plate at an angle to the path of the rays so as to deflect them through the side of the tube. The positive terminal penetrates the glass at one side. The tube contains, as we have seen, a very tiny residue of air. If this were entirely exhausted the action of the tube would cease; so that some tubes are so arranged that when rarefaction becomes too high the passage of an electrical current through small bars of chemicals, whose ends project through the sides of the tube, liberates gas from the bars in sufficient quantity to render the tube active again.
When the Ruhmkorff induction coil is joined to the electric circuit a series of violent discharges of great rapidity occur between the tube terminals, resembling in their power the discharge of a Leyden jar, though for want of a dense atmosphere the brilliant spark has been replaced by a glow and brush-light in the tube. The coil is of large dimensions, capable of passing a spark across an air-gap of ten to twelve inches. It will perhaps increase the reader’s respect for X-rays to learn that a coil of proper size contains upwards of thirteen miles of wire; though indeed this quantity is nothing in comparison with the 150 miles wound on the huge inductorium formerly exhibited at the London Polytechnic.
If we were invited to an X-ray demonstration we should find the operator and his apparatus in a darkened room. He turns on the current and the darkness is broken by a velvety glow surrounding the negative terminal, which gradually extends until the whole tube becomes clothed in a green phosphorescence. A sharply-defined line athwart the tube separates the shadowed part behind the receiving plate at the negative focus—now intensely hot—from that on which the reflected rays fall directly.
One of us is now invited to extend a hand close to the tube. The operator then holds on the near side of the hand his fluorescent screen, which is nothing more than a framework supportinga paper smeared on one side with platino-cyanide of barium, a chemical that, in common with several others, was discovered by Salvioni of Perugia to be sensitive to the rays and able to make them visible to the human eye. The value of the screen to the X-radiographer is that of the ground-glass plate to the ordinary photographer, as it allows him to see exactly what things are before the sensitised plate is brought into position, and in fact largely obviates the necessity for making a permanent record.
The screen shows clearly and in full detail all the bones of the hand—so clearly that one is almost irresistibly drawn to peep behind to see if a real hand is there. One of us now extends an arm and the screen shows us theulnaand theradiusworking round each other, now both visible, now one obscuring the other. On presenting the body to the course of the rays a remarkable shadow is cast on to the screen. The spinal column and the ribs; the action of the heart and lungs are seen quite distinctly. A deep breath causes the movement of a dark mass—the liver. There is no privacy in presence of the rays. The enlarged heart, the diseased lung, the ulcerated liver betrays itself at once. In a second of time the phosphorescent screen reveals what might baulk medical examination for months.
If a photographic slide containing a dry-plate be substituted for the focusing-screen, the rayssoon penetrate any covering in which the plate may be wrapped to protect it from ordinary light rays. The process of taking a shadowgraph may therefore be conducted in broad daylight, which is under certain conditions a great advantage, though the sensitiveness of plates exposed to Röntgen rays entails special care being taken of them when they are not in use. In the early days of X-radiography an exposure of some minutes was necessary to secure a negative, but now, thanks to the improvements in the tubes, a few seconds is often sufficient.
The discovery of the X-rays is a great discovery, because it has done much to promote the noblest possible cause, the alleviation of human suffering. Not everybody will appreciate a more rapid mode of telegraphy, or a new method of spinning yarn, but the dullest intellect will give due credit to a scientific process that helps to save life and limb. Who among us is not liable to break an arm or leg, or suffer from internal injuries invisible to the eye? Who among us therefore should not be thankful on reflecting that, in event of such a mishap, the X-rays will be at hand to show just what the trouble is, how to deal with it, and how far the healing advances day by day? The X-ray apparatus is now as necessary for the proper equipment of a hospital as a camera for that of a photographic studio.
It is especially welcome in the hospitals which accompany an army into the field. Since May 1896many a wounded soldier has had reason to bless the patient work that led to the discovery at Würzburg. The Greek war, the war in Cuba, the Tirah campaign, the Egyptian campaign, and the war in South Africa, have given a quick succession of fine opportunities for putting the new photography to the test. There is now small excuse for the useless and agonising probings that once added to the dangers and horrors of the military hospital. Even if the X-ray equipment, by reason of its weight, cannot conveniently be kept at the front of a rapidly moving army, it can be set up in the “advanced†or “base†hospitals, whither the wounded are sent after a first rough dressing of their injuries. The medical staff there subject their patients to the searching rays, are able to record the exact position of a bullet or shell-fragment, and the damage it has done; and by promptly removing the intruder to greatly lessen its power to harm.
The Röntgen ray has added to the surgeon’s armoury a powerful weapon. Its possibilities are not yet fully known, but there can be no doubt that it marks a new epoch in surgical work. And for this reason Professor Röntgen deserves to rank with Harvey, the discoverer of the blood’s circulation; with Jenner, the father of vaccination; and with Sir James Young Simpson, the first doctor to use chloroform as an anæsthetic.
Strange as it seems to take photographs with invisible rays, it is still stranger to be able to affect sensitised plates without apparently the presence of any kind of rays.
Professor W. J. Russell, Vice-President of the Royal Society of London, has discovered that many substances have the power of impressing their outlines automatically on a sensitive film, if the substance be placed in a dark cupboard in contact with, or very close to a dry-plate.
After some hours, or it may be days, development of the plate will reveal a distinct impression of the body in question. Dr. Russell experimented with wood, metal, leaves, drawings, printed matter, lace. Zinc proved to be an unusually active agent. A plate of the metal, highly polished and then ruled with patterns, had at the end of a few days imparted a record of every scratch and mark to the plate. And not only will zinc impress itself, but it affects substances which are not themselves active, throwing shadowgraphs on to the plate. This was demonstrated with samples of lace, laid between a plate and a small sheet of bright zinc; also with a skeleton leaf. It is curious that while the interposition of thin films of celluloid, gutta-percha, vegetable parchment, and gold-beater’s skin—all inactive—between the zinc and the plate has no obstructive effect, aplate of thin glass counteracts the action of the zinc. Besides zinc, nickel, aluminium, pewter, lead, and tin among the metals influence a sensitised plate. Another totally different substance, printer’s ink, has a similar power; or at least some printer’s ink, for Professor Russell found that different samples varied greatly in their effects. What is especially curious, the printed matter onboth sidesof a piece of newspaper appeared on the plate, and that the effect proceeded from the ink and not from any rays passing from beyond it is proved by the fact that the type came outdarkin the development, whereas if it had been a case of shadowgraphy, the ink by intercepting rays would have producedwhiteletters. Professor Russell has also shown that modern writing ink is incapable of producing an impression unaided, but that on the other hand paper written on a hundred years ago or a printed book centuries old will, with the help of zinc, yield a picture in which even faded and uncertain characters appear quite distinctly. This opens the way to a practical use of the discovery, in the deciphering of old and partly obliterated manuscripts.
A very interesting experiment may be made with that useful possession—a five-pound note. Place the note printed side next to the plate, and the printing appears dark; but insert the note between a zinc sheet and the plate, its back being this time towards the sensitised surface, and the printing appearswhite; and the zinc, after contact with the printed side, willitself yield a picture of the inscription as though it had absorbed some virtue from the note!
As explanation of this paradoxical dark photography—or whatever it is—two theories may be advanced. The one—favoured by Professor Russell—is that all “active†substances give offvapoursable to act on a photographic plate. In support of this may be urged the fact that the interposition of glass prevents the making of dark pictures. But on the other hand it must be remembered that celluloid and sheet-gelatine, also air-tight substances, are able to store up light and to give it out again. It is well known among photographers that to allow sunlight to fall on the inside of a camera is apt to have a “fogging†effect on a plate that is exposed in the camera afterwards, though the greatest care be taken to keep all external light from the plate. But here the glass again presents a difficulty, for if this were a case of reflected light, glass would evidently belessobstructive than opaque vegetable parchment or gutta-percha.
One day George Stephenson and a friend stood watching a train drawn by one of his locomotives.
“What moves that train?†asked Stephenson.
“The engine,†replied his friend.
“And what moves the engine?â€
“The steam.â€
“And what produces the steam?â€
“Coal.â€
“And what produces coal?â€
This last query nonplussed his friend, and Stephenson himself replied, “The sun.â€
The “bottled sunshine†that drove the locomotive was stored up millions of years ago in the dense forests then covering the face of the globe. Every day vegetation was built by the sunbeams, and in the course of ages this growth was crushed into fossil form by the pressure of high-piled rock and débris. To-day we cast “black diamonds†into our grates and furnaces, to call out the warmth and power that is a legacy from a period long prior to the advent of fire-loving man, often forgetful of its real source.
We see the influence of the sun more directly in the motions of wind and water. Had not thesun’s action deposited snow and rain on the uplands of the world, there would be no roaring waterfall, no rushing torrent, no smooth-flowing stream. But for the sun heating the atmosphere unequally, there would not be that rushing of cool air to replace hot which we know as wind.
We press Sol into our service when we burn fuel; our wind-mills and water-mills make him our slave. Of late years many prophets have arisen to warn us that we must not be too lavish of our coal; that the time is not so far distant, reckoning by centuries, when the coal-seams of the world will be worked out and leave our descendants destitute of what plays so important a part in modern life. Now, though waste is unpardonable, and the care for posterity praiseworthy, there really seems to be no good reason why we should alarm ourselves about the welfare of the people of the far future. Even if coal fails, the winds and the rivers will be there, and the huge unharnessed energy of the tides, and the sun himself is ready to answer appeals for help, if rightly shaped. He does not demand the prayers of Persian fire-worshippers, but rather the scientific gathering of his good gifts.
Place your hand on a roof lying square to the summer sun, and you will find it too hot for the touch. Concentrate a beam of sunshine through a small burning-glass. How fierce is the small glowing focal spot that makes us draw our hands suddenly away! Suppose now a large glass many feet acrossbending several square yards of sun rays to a point, and at that point a boiler. The boiler would develop steam, and the steam might be led into cylinders and forced to drudge for us.
Do many of us realise the enormous energy of a hot summer’s day? The heat falling in the tropics on a single square foot of the earth’s surface has been estimated as the equivalent of one-third of a horse-power. The force of Niagara itself would on this basis be matched by the sunshine streaming on to a square mile or so. A steamship might be propelled by the heat that scorches its decks.
For many centuries inventors have tried to utilise this huge waste power. We all know how, according to the story, Archimedes burnt up the Roman ships besieging his native town, Syracuse, by concentrating on them the sun heat cast from hundreds of mirrors. This story is less probable than interesting as a proof that the ancients were aware of the sun’s power. The first genuine solar machine was the work of Ericsson, the builder of theMonitor. He focused sun heat on a boiler, which gave the equivalent of one horse-power for every hundred square feet of mirrors employed. This was not what engineers would call a “high efficiency,†a great deal of heat being wasted, but it led the way to further improvements.
In America, especially in the dry, arid regions, where fuel is scarce and the sun shines pitilessly day after day, all the year round, sun-catchers of various types have been erected and worked successfully.Dr. William Calver, of Washington, has built in the barren wastes of Arizona huge frames of mirrors, travelling on circular rails, so that they may be brought to face the sun at all hours between sunrise and sunset. Dr. Calver employs no less than 1600 mirrors. As each of these mirrors develops 10-15 degrees of heat it is obvious, after an appeal to simple arithmetic, that the united efforts of these reflectors should produce the tremendous temperature 16,000-24,000 degrees, which, expressed comparatively, means the paltry 90 degrees in the shade beneath which we grow restive multiplied hundreds of times. Hitherto the greatest known heat had been that of the arc of the electric lamp, in which the incandescent particles between pole and pole attain 6000 degrees Fahrenheit.
The combined effect of the burning mirrors is irresistible. They can, we are told, in a few moments reduce Russian iron to the consistency of warmed wax, though it mocks the heat of many blast-furnaces. They will bake bricks twenty times as rapidly as any kiln, and the bricks produced are not the friable blocks which a mason chips easily with his trowel, but bodies so hard as to scratch case-hardened steel.
There are at work in California sun-motors of another design. The reader must imagine a huge conical lamp-shade turned over on to its smaller end, its inner surface lined with nearly 1800 mirrors 2 feet long and 3 inches broad, the whole supported on a light iron framework, and he will have a goodidea of the apparatus used on the Pasadena ostrich farm. The machine is arrangedin meridian, that is, at right angles to the path of the sun, which it follows all day long by the agency of clockwork. In the focus of the mirrors is a boiler, 13 feet 6 inches long, coated with black, heat-absorbing substances. This boiler holds over 100 gallons of water, and being fed automatically will raise steam untended all the day through. The steam is led by pipes to an engine working a pump, capable of delivering 1400 gallons per minute.
The cheapness of the apparatus in proportion to its utility is so marked that, in regions where sunshine is almost perpetual, the solar motor will in time become as common as are windmills and factory chimneys elsewhere. If the heat falling on a few square yards of mirror lifts nearly 100,000 gallons of water an hour, there is indeed hope for the Sahara, the Persian Desert, Arabia, Mongolia, Mexico, Australia. That is to say, if the water under the earth be in these parts as plentiful as the sunshine above it. The effect of water on the most unpromising soil is marvellous. Already in Algeria the French have reclaimed thousands of square miles by scientific irrigation. In Australia huge artesian wells have made habitable for man and beast millions of acres that were before desert.
It is only a just retribution that the sun should be harnessed and compelled to draw water for tracts to which he has so long denied it. The sun-motoris only just entering on its useful career, and at present we can but dream of the great effects it may have on future civilisation. Yet its principle is so simple, so scientific, and so obvious, that it is easy to imagine it at no far distant date a dangerous rival to King Coal himself. To quarry coal from the bowels of the earth and transform it into heat, is to traverse two sides of a triangle, the third being to use the sunshine of the passing hour.
Among common phenomena few are more interesting than the changes undergone by the substance called water. Its usual form is a liquid. Under the influence of frost it becomes hard as iron, brittle as glass. At the touch of fire it passes into unsubstantial vapour.
This transformation illustrates the great principle that the form of every substance in the universe is a question of heat. A metal transported from the earth to the sun would first melt and then vaporise; while what we here know only as vapours would in the moon turn into liquids.
We notice that, as regards bulk, the most striking change is from liquid to gaseous form. In steam the atoms and molecules of water are endowed with enormous repulsive vigour. Each atom suddenly shows a huge distaste for the company of its neighbours, drives them off, and endeavours to occupy the largest possible amount of private space.
Now, though we are accustomed to see water-atoms thus stirred into an activity which gives us the giant steam as servant, it has probably fallen to the lot of but few of us to encounter certain gaseous substances so utterly deprived of their self-assertivenessas to collapse into a liquid mass, in which shape they are quite strangers to us. What gaseous body do we know better than the air we breathe? and what should we less expect to be reducible to the consistency of water? Yet science has lately brought prominently into notice that strange child of pressure and cold, Liquid Air; of which great things are prophesied, and about which many strange facts may be told.
Very likely our readers have sometimes noticed a porter uncoupling the air-tube between two railway carriages. He first turns off the tap at each end of the tube, and then by a twist disconnects a joint in the centre. At the moment of disconnection what appears to be a small cloud of steam issues from the joint. This is, however, the result of cold, not heat, the tube being full of highly-compressed air, which by its sudden expansion develops cold sufficient to freeze any particles of moisture in the surrounding air.
Keep this in mind, and also what happens when you inflate your cycle-tyre. The air-pump grows hotter and hotter as inflation proceeds: until at last, if of metal, it becomes uncomfortably warm. The heat is caused by the forcing together of air-molecules, and inasmuch as all force produces heat, your strength is transformed into warmth.
In these two operations, compression and expansion, we have the key to the creation of liquid air—the great power, as some say, of to-morrow.
By kind permission of The Liquid Air Co.A view of the Liquid Air Co.’s factory at Pimlico. On the left are the three compressors, squeezing the air at pressures of 90, 500 and 2,200 lbs. to the square inch respectively. On the right is the reservoir in which the liquid is stored.
By kind permission of The Liquid Air Co.A view of the Liquid Air Co.’s factory at Pimlico. On the left are the three compressors, squeezing the air at pressures of 90, 500 and 2,200 lbs. to the square inch respectively. On the right is the reservoir in which the liquid is stored.
Suppose we take a volume of air and squeeze it into 1/100 of its original space. The combativeness of the air-atoms is immensely increased. They pound each other frantically, and become very hot in the process. Now, by cooling the vessel in which they are, we rob them of their energy. They become quiet, but they are much closer than before. Then imagine that all of a sudden we let them loose again. The life is gone out of them, their heat has departed, and on separating they shiver grievously. In other words, the heat contained by the 1/100 volume is suddenly compelled to “spread itself thin†over the whole volume: result—intense cold. And if this air be brought to bear upon a second vessel filled likewise with compressed air, the cold will be even more intense, until at last the air-atoms lose all their strength and collapse into a liquid.
Liquid air is no new thing. Who first made it is uncertain. The credit has been claimed for several people, among them Olzewski, a Pole, and Pictet, a Swiss. As a mere laboratory experiment the manufacture of liquid air in small quantities has been known for twenty years or more. The earlier process was one of terrific compression alone, actually forcing the air molecules by sheer strength into such close contact that their antagonism to one another was temporarily overcome. So expensive was the process that the first ounce of liquid air is estimated to have cost over £600!
In order to make liquid air an article of commercethe most important condition was a wholesale decrease in cost of production. In 1857 C. W. Siemens took out a patent for making the liquid on what is known as the regenerative principle, whereby the compressed air is chilled by expanding a part of it. Professor Dewar—a scientist well known for his researches in the field of liquid gases—had in 1892 produced liquid air by a modification of the principle at comparatively small cost; and other inventors have since then still further reduced the expense, until at the present day there appears to be a prospect of liquid air becoming cheap enough to prove a dangerous rival to steam and electricity.
A company, known as the Liquid Air, Power and Automobile Company, has established large plants in America and England for the manufacture of the liquid on a commercial scale. The writer paid a visit to their depot in Gillingham Street, London, where he was shown the process by Mr. Hans Knudsen, the inventor of much of the machinery there used. The reader will doubtless like to learn the “plain, unvarnished truth†about the creation of this peculiar liquid, and to hear of the freaks in which it indulges—if indeed those may be called freaks which are but obedience to the unchanging laws of Nature.
On entering the factory the first thing that strikes the eye and ear is the monstrous fifty horse-power gas-engine, pounding away with an energy that shakes the whole building. From its ponderousflywheels great leather belts pass to the compressors, three in number, by which the air, drawn from outside the building through special purifiers, is subjected to an increasing pressure. Three dials on the wall show exactly what is going on inside the compressors. The first stands at 90 lbs. to the square inch, the second at 500, and the third at 2200, or rather less than a ton pressure on the area of a penny! The pistons of the low-pressure compressor is ten inches in diameter, but that of the high pressure only two inches, or 1/25 of the area, so great is the resistance to be overcome in the last stage of compression.
Now, if the cycle-pump heats our hands, it will be easily understood that the temperature of the compressors is very high. They are water-jacketed like the cylinders of a gas-engine, so that a circulating stream of cold water may absorb some of the heat. The compressed air is passed through spiral tubes winding through large tanks of water which fairly boils from the fierceness of the heat of compression.
When the air has been sufficiently cooled it is allowed to pass into a small chamber, expanding as it goes, and from the small into a larger chamber, where the cold of expansion becomes so acute that the air-molecules collapse into liquid, which collects in a special receptacle. Arrangements are made whereby any vapour rising from the liquid passes through a space outside the expansionchambers, so that it helps to cool the incoming air and is not wasted.
The liquid-air tank is inside a great wooden case, carefully protected from the heat of the atmosphere by non-conducting substances. A tap being turned, a rush of vapour shoots out, soon followed by a clear, bluish liquid, which is the air we breathe in a fresh guise.
A quantity of it is collected in a saucepan. It simmers at first, and presently boils like water on a fire. The air-heat isby comparisonso great that the liquid cannot resist it, and strives to regain its former condition.
You may dip your finger into the saucepan—if you withdraw it again quickly—without hurt. The cushion of air that your finger takes in with it protects you against harm—for a moment. But if you held it in the liquid for a couple of seconds you would be minus a digit. Pour a little over your coat sleeve. It flows harmlessly to the ground, where it suddenly expands into a cloud of chilly vapour.
Put some in a test tube and cork it up. The cork soon flies out with a report—the pressure of the boiling air drives it. Now watch the boiling process. The nitrogen being more volatile—as it boils at a lower temperature than oxygen—passes off first, leaving the pure, blue oxygen. The temperature of this liquid is over 312 degrees below zero (as far below the temperature of theair we breathe as the temperature of molten lead is above it!). A tumbler of liquid oxygen dipped into water is soon covered with a coating of ice, which can be detached from the tumbler and itself used as a cup to hold the liquid. If a bit of steel wire be now twisted round a lighted match and the whole dipped into the cup, the steel flares fiercely and fuses into small pellets; which means that an operation requiring 3000 degrees Fahrenheit has been accomplished in a liquid 300 degrees below zero!
Liquid air has curious effects upon certain substances. It makes iron so brittle that a ladle immersed for a few moments may be crushed in the hands; but, curiously enough, it has a toughening effect on copper and brass. Meat, eggs, fruit, and all bodies containing water become hard as steel and as breakable as glass. Mercury is by it congealed to the consistency of iron; even alcohol, that can brave the utmost Arctic cold, succumbs to it. The writer was present when some thermometers, manufactured by Messrs. Negretti and Zambra, were tested with liquid air. The spirit in the tubes rapidly descended to 250 degrees below zero, then sank slowly, and at about 260 degrees froze and burst the bulb. The measuring of such extreme temperatures is a very difficult matter in consequence of the inability of spirit to withstand them, and special apparatus, registering cold by the shrinkage of metal, mustbe used for testing some liquid gases, notably liquid hydrogen, which is so much colder than liquid air that it actually freezes it into a solid ice form!
For handling and transporting liquid gases glass receptacles with a double skin from which all air has been exhausted are employed. The surrounding vacuum is so perfect an insulator that a “Dewar bulb†full of liquid air scarcely cools the hand, though the intervening space is less than an inch. This fact is hard to square with the assertion of scientific men that our atmosphere extends but a hundred or two miles from the earth’s surface, and that the recesses of space are a vacuum. If it were so, how would heat reach us from the sun, ninety-two millions of miles away?
One use at least for liquid air is sufficiently obvious. As a refrigerating agent it is unequalled. Bulk for bulk its effect is of course far greater than that of ice; and it has this advantage over other freezing compounds, that whereas slow freezing has a destructive effect upon the tissues of meat and fruit, the instantaneous action of liquid air has no bad results when the thing frozen is thawed out again. The Liquid Air Company therefore proposes erecting depôts at large ports for supplying ships, to preserve the food, cool the cabins in the tropics, and, we hope, to alleviate some of the horrors of the stokehold.
Liquid air is already used in medical and surgical science. In surgery it is substituted for anæsthetics, deadening any part of the body on which an operation has to be performed. In fever hospitals, too, its cooling influence will be welcomed; and liquid oxygen takes the places of compressed oxygen for reviving the flickering flame of life. It will also prove invaluable for divers and submarine boats.
In combination with oil and charcoal liquid air, under the name of “oxyliquit,†becomes a powerful blasting agent. Cartridges of paper filled with the oil and charcoal are provided with a firing primer. When everything is ready for the blasting the cartridges are dropped into a vessel full of liquid air, saturated, placed in position, and exploded. Mr. Knudsen assured the writer that oxyliquit is twice as powerful as nitro-glycerine, and its cost but one-third of that of the other explosive. It is also safer to handle, for in case of a misfire the cartridge becomes harmless in a few minutes, after the liquid air has evaporated.
But the greatest use will be found for liquid air when it exerts its force less violently. It is the result of power; its condition is abnormal; and its return to its ordinary state is accompanied by a great development of energy. If it be placed in a closed vessel it is capable of exerting a pressure of 12,000 lbs. to the square inch. Its return to atmospheric condition may be regulated by exposing it more or less to the heat of the atmosphere. So longas it remains liquid it represents so muchstored force, like the electricity stored in accumulators. The Liquid Air Company have at their Gillingham Street depôt a neat little motor car worked by liquid air. A copper reservoir, carefully protected, is filled with the liquid, which is by mechanical means squirted into coils, in which it rapidly expands, and from them passes to the cylinders. A charge of eighteen gallons will move the car forty miles at an average pace of twelve miles an hour, without any of the noise, dirt, smell, or vapour inseparable from the employment of steam or petroleum. The speed of the car is regulated by the amount of liquid injected into the expansion coils.
We now come to the question of cost—the unromantic balance in which new discoveries are weighed and many found wanting. The storage of liquid air is feasible for long periods. (A large vacuum bulb filled and exposed to the atmosphere had some of the liquid still unevaporated at the end of twenty-two days.) But will it be too costly for ordinary practical purposes now served by steam and electricity? The managers of the Liquid Air Company, while deprecating extravagant prophecies about the future of their commodity, are nevertheless confident that it has “come to stay.†With the small 50 horse-power plant its production costs upwards of one shilling a gallon, but with much larger plant of 1000 horse-power they calculate that the expenses will be covered and a profit left if they retail it atbut one penny the gallon. This great reduction in cost arises from the economising of “waste energy.†In the first place the power of expansion previous to the liquefaction of the compressed air will be utilised to work motors. Secondly, the heat of the cooling tanks will be turned to account, and even the “exhaust†of a motor would be cold enough for ordinary refrigerating. It is, of course, impossible to get more out of a thing than has been put into it; and liquid air will therefore not develop even as much power as was required to form it. But its handiness and cleanliness strongly recommend it for many purposes, as we have seen; and as soon as it is turned out in large quantities new uses will be found for it. Perhaps the day will come when liquid-air motors will replace the petrol car, and in every village we shall see hung out the sign, “Liquid air sold here.†As the French say, “Qui vivra verra.â€
A body of enterprising Manchester merchants, in the year 1754, put on the road a “flying coach,†which, according to their special advertisement, would, “however incredible it may appear, actually, barring accidents, arrive in London in four and a half days after leaving Manchester.†According to the Lord Chancellor of the time such swift travelling was considered dangerous as well as wonderful—the condition of the roads might well make it so—and also injurious to health. “I was gravely advised,†he says, “to stay a day in York on my journey between Edinburgh and London, as several passengers who had gone through without stopping had died of apoplexy from the rapidity of the motion.â€
As the coach took a fortnight to pass from the Scotch to the English capital, at an average pace of between three and four miles an hour, it is probable that the Chancellor’s advisers would be very seriously indisposed by the mere sight of a motor-car whirling along in its attendant cloud of dust, could they be resuscitated for the purpose. And we, on the other hand, should prefer to get out and walk to “flying†at the safe speed of their mail coaches.