Part 15 (1/2)
This transformation ill.u.s.trates 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 s.p.a.ce.
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-a.s.sertiveness as to collapse into a liquid ma.s.s, 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.
[Ill.u.s.tration: _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 s.p.a.ce. 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 s.h.i.+ver 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 quant.i.ties 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 pa.s.s 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 pa.s.sed 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 pa.s.s 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 pa.s.ses through a s.p.a.ce 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 quant.i.ty 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 cus.h.i.+on 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--pa.s.ses 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 bra.s.s. Meat, eggs, fruit, and all bodies containing water become hard as steel and as breakable as gla.s.s. Mercury is by it congealed to the consistency of iron; even alcohol, that can brave the utmost Arctic cold, succ.u.mbs 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 gla.s.s 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 s.p.a.ce is less than an inch. This fact is hard to square with the a.s.sertion of scientific men that our atmosphere extends but a hundred or two miles from the earth's surface, and that the recesses of s.p.a.ce are a vacuum.