36 POPULAR SCIENCE MONTHLY. 



ample, in the formation of rain, while the other constituents are 

 liquefied with difficulty. The name " liquid air " is applied to the 

 substance that is obtained by converting the air as a whole into a 

 liquid; but in this process the water and the carbonic acid become 

 solid and can be filtered from the liquid so that the latter consists 

 almost wholly of oxygen and nitrogen. A few years ago this liquid 

 was obtainable in only very small quantities. To-day, thanks espe- 

 cially to the efforts of Mr. Charles E. Tripler, of New York, it can 

 be produced in any desired quantity, and at moderate cost. In con- 

 sequence of this, it has come to be talked about in a familiar way, 

 and many persons have had the privilege of seeing and feeling it, 

 and of learning something about its wonderful properties. The 

 object of this article is to explain the method employed in the pro- 

 duction of liquid air, to give an account of some of its properties, 

 and to indicate some of the uses to which it may possibly be put. 



In the older text-books of physics and of chemistry certain gases 

 were classed as " permanent," under the impression that these c'ould 

 not be liquefied, and this impression was based upon the fact that 

 all efforts to liquefy them had failed. A brief account of these efforts 

 will be helpful. 



Among the so-called permanent gases was chlorine. An Eng- 

 lish chemist, JSTorthmore, first succeeded, early in this century, in 

 liquefying chlorine. His work was, however, lost sight of, and in 

 1823 Faraday at the Royal Institution showed independently that 

 this transformation of gaseous chlorine into the liquid can be effected 

 comparatively easily. The method used by him is this: When 

 chlorine gas is passed into cold water it forms with the water a solid 

 product known as chlorine hydrate. If kept well cooled this hydrate 

 can be dried. If then its temperature is raised even to the ordinary 

 temperature of the room, the solid hydrate is decomposed into liquid 

 water and gaseous chlorine. Faraday put some of the solid hydrate 

 into a stout glass tube sealed at one end and bent at the middle. The 

 other end of the tube was then closed. The tube was then suspended 

 so that the two ends were turned downward. On gently warming the 

 end in which was the solid hydrate this was decomposed into chlorine 

 and water. But the gas given off would under ordinary conditions 

 have occupied a much larger space than the solid hydrate. Being 

 prevented from expanding by the tube in which it was inclosed, it 

 was under very considerable pressure. The end of the tube that was 

 not warmed was cooled, and in this end, in consequence of the pres- 

 sure and the comparatively low temperature, chlorine, which is gase- 

 ous under the ordinary pressure of the air, appeared as a liquid. The 

 general method made use of by Faraday in th5s classical experiment 

 is that which is always made use of for the purpose of liquefying 



