Of course air so highly compressed as that is hot, but after it has passed down the coil and has escaped from the valve which liberates it at the bottom it is much cooler. But that is only the beginning of the operation. The expanded, and therefore cooled, air finds its way upward through the turns of the coil down which the following air is coming. That, expanding in its turn, is colder still, because of the cooling action of the first air, and so the process goes on.
This is perhaps easier to understand if we imagine that the air comes through the coil in gusts and we notice what happens to each succeeding gust. The first comes down, expands, cools and ascends, thereby cooling the second gust as it comes down. The second then, after expansion, will be cooler than the first was. That in its turn will cool the third, and so the third after expansion will be cooler than the second. And that will go on, each succeeding gust being cooler than the one before. And although the flow of air is continuous, and not in gusts, the result is just the same: it goes on getting cooler and cooler until at last the air comes out in its liquid form. This liquid collects in a little chamber formed at the bottom of the vessel which contains the coil and can be drawn off when desired.
Air in its liquid state looks very much like water. In fact it is difficult to get chance observers to believe that it is not water. It boils at a temperature far below the freezing-point of water, so that liquid air if placed in a cup made of ice will boil furiously. Ice is so much the hotter that it behaves towards liquid air as a very hot fire does to water.
The feature of the above machine, it will be noticed, is that no cooling water is required, as in the refrigerating machine, although the principle of the two is the same. The coil is the "condenser" and the vessel in which it is enclosed is the "evaporator," and so the cold air produced by the process in the evaporator cools the coil of the condenser. Thus it is "self-intensive," as the makers call it.
Hydrogen can be liquefied in a similar machine, except that it needs a little preliminary cooling with liquid air. Liquid hydrogen is the coolest thing known approaching the region of absolute zero.
And now we can turn to the wonderful discoveries which have followed upon the manufacture of liquid air.
To make the story complete we need to go back to the time of Priestly and Cavendish, early in last century. They investigated the atmosphere and showed that it consisted of oxygen and nitrogen in certain invariable proportions, with under certain conditions a small proportion of carbonic acid. These facts were so well authenticated, and they seemed to explain everything so satisfactorily, that it was quite thought almost up to the end of the nineteenth century that there was nothing more to learn about the atmosphere.
Nevertheless there was an idea in the minds of some scientists that there must be another group of elements somewhere, the existence of which was then undiscovered, but it was never dreamed that these were in the air.
Soon after the weights of the atoms had been found a medical student named Prout in an anonymous essay called attention to the fact that there were curious numerical relationships between them. Speculation on the subject went on for many years, until in 1865 the great Russian chemist Mendeléeff published his conclusions. He had arranged the elements in the form of a table in the order of their atomic weights. The table consisted of twelve rows of names forming eight vertical columns, and the remarkable thing was that all those elements which fell into any particular column, although their atomic weights were very widely different, had similar properties. This enabled him to predict the discovery of certain new elements, for the table contained a number of blank spaces. Three elements have been found since, and their atomic weights and properties are just such as to fill three of the blank spaces. One blank space, it is thought, may be filled some day by the gas coronium, which like helium has been discovered in the sun, but unlike it has not yet been detected here. When it is, there is the place in the table which it may fill. The table then commenced with what is still called Group 1, but for reasons too complicated to explain here it appeared as if there must be a group before that, a group the chief characteristic of which would be the inactivity of the elements included in it. These were expected to be of various atomic weights, but these weights, it was anticipated, would so occur in the intervals between the others that they would all fall into a new column to the left of "Group 1."
In the year 1892 Lord Rayleigh was investigating the question of the density of a number of different gases, including, so it happened, nitrogen. Now there are several ways of procuring nitrogen. One is to get it from the atmosphere by ridding it of the oxygen with which it is normally mixed. Another way is to split up some compound, such as ammonia, of which it forms a part, in such a way as to catch the nitrogen and leave the other elements with which it was combined elsewhere.