But although the "CO_{2} process" described above is the simplest illustration of the principle, there are other systems. In one very popular form ammonia gas is the "working fluid." This is liquefied by pressure and cooling with water, being subsequently expanded just as described above. Another much-used system is the "ammonia-absorption" process, in which the ammonia is not liquefied, but when under pressure is absorbed by water, returning to gas again when the pressure is released. But the degree of cold attained in these commercial machines is as nothing to the extremely intense cold generated on the same principles in the liquid-air machine, which is found in every well-equipped physical laboratory. Briefly, this consists of a coil of many turns of small tube enclosed in a small double vessel, the space between the inner and outer skins of which is packed with insulating material. A compressor pumps air in at the top of the coil at a pressure of from 150 to 200 atmospheres. An "atmosphere," it may be remarked, is a unit often used in scientific matters, meaning the normal pressure of the atmosphere, which is, roughly speaking, 15 lb. per square inch. Hence 200 atmospheres is about 3000 lb. per square inch. 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. [Illustration: _By permission of Messrs. J. and E. Hall, Ltd., London and Dartford_ MACHINE-MADE ICE Here we see a huge block of ice being lifted (it may be on a hot summer day) from the mould in which it has been made] 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.