Page:Popular Science Monthly Volume 55.djvu/52

42 square inch. Again it is cooled in the same way as before by means of water which circulates around the coil B$2$. Once more the air is compressed, this time in the cylinder A$3$, in which it is subjected to a pressure of two thousand to twenty-five hundred pounds to the square inch; and then this compressed air is brought down to the ordinary temperature in the cooler B$3$. The air under this great pressure is now passed through the purifier C, where it is freed from particles of dust and to a great extent from moisture. From C the air passes into the inner bent tube, about thirty feet in length, until it reaches D. This may be called the critical point of the apparatus. Here is situated a needle valve from which the air is allowed to escape. It, of course, expands enormously, and is correspondingly cooled. This very cold air passes into the space between the inner and outer tubes, and finally escapes at F. The result of this is that the compressed air in the inner tube is soon cooled down so far that a considerable part of the air that escapes at D appears in the liquid form. This collects in the lower part of the jacket, and on opening the stopcock at E the liquid escapes in a stream the size of one's finger.

In Mr. Tripler's laboratory the liquid is collected in the cans already referred to. Although for the reasons mentioned the evaporation of the liquid is comparatively slow, it is constantly going on, and as the gas formed occupies a very much larger volume under the pressure of the atmosphere than the liquid from which it is formed, it is necessary to leave the cans loosely covered. Otherwise the pressure would increase to such an extent as to burst any but the strongest vessels. One cubic foot of liquid air gives at atmospheric pressure eight hundred cubic feet of gaseous air.

Liquid air obtained as described is a turbid, colorless liquid. The turbidity is due to the presence of solid water and solid carbonic acid. By passing the liquid through a paper filter the solids are removed, and a transparent liquid is thus obtained. This, as already stated, consists mostly of nitrogen and oxygen in the proportion of about four fifths of the former to one fifth of the latter. Though it should not be forgotten that this liquid contains argon in small quantity, besides three or four other substances in still smaller quantities, as has recently been shown by Professor Ramsay, we may disregard everything except the nitrogen and oxygen. Liquid air is a mixture of these two substances. They are not chemically combined as hydrogen and oxygen are, for example, in water. This mixture boils at -191° C. (-312° F.), which is the temperature of the liquid as it is in the cans. As the nitrogen boils at a lower temperature (-194° C. or 318° F.) than oxygen (-183° C. or 297° F.), more nitrogen is converted into gas in a given time than