Part 13
It is especially welcome in the hospitals which accompany an army into the field. Since May 1896 many 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.
PHOTOGRAPHY IN THE DARK.
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, a plate 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 on _both sides_ of 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 out _dark_ in the development, whereas if it had been a case of shadowgraphy, the ink by intercepting rays would have produced _white_ letters. 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 appears _white_; and the zinc, after contact with the printed side, will itself 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 off _vapours_ able 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 be _less_ obstructive than opaque vegetable parchment or gutta-percha.
SOLAR MOTORS.
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?”
“The steam.”
“And what produces the steam?”
“Coal.”
“And what produces coal?”
This last query nonplussed his friend, and Stephenson himself replied, “The sun.”
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 the sun’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 across bending 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 the _Monitor_. 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 good idea of the apparatus used on the Pasadena ostrich farm. The machine is arranged _in 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-motor is 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.
LIQUID AIR.
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-assertiveness as 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.
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 commerce the 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 ponderous flywheels 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 expansion chambers, 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 is _by comparison_ so 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 the air 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, must be 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?