42 POPULAR SCIENCE MONTHLY. 



square inch. Again it is cooled in the same way as before by means 

 of water which circulates around the coil B^. Once more the air 

 is compressed, this time in the cylinder A^, 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^. 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 corre- 

 spondingly 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 evap- 

 oration 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 atmos- 

 pheric 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 quan- 

 tities, 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 com- 

 bined 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 



