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.