LIQUID AIR. 39 



are used it is only necessary to allow them to pass rapidly into the 

 gaseous state, when more or less heat is absorbed. This is the basis 

 for the use of liquid ammonia in the manufacture of ice. A vessel 

 containing the liquid ammonia is placed in another containing water. 

 The inner vessel being opened, the liquid ammonia is rapidly con- 

 verted into the gas; heat is absorbed from the water; it freezes. 

 When a vessel containing liquid carbonic acid is opened so that the 

 gas that is formed escapes through a small valve, so much heat is 

 absorbed that a part of the liquid carbonic acid is itself frozen. 

 In this case the substance is present in all three states of aggregation 

 the solid, the liquid, and the gaseous. The use of a mixture of 

 ether and solid carbonic acid as a freezing mixture has already been 

 referred to. Its value depends, of course, principally upon the fact 

 that solid carbonic acid is liquefied, and the liquid then converted into 

 gas, both of which operations involve absorption of heat. 



We are now prepared to understand the important experiments 

 of Cailletet and of Pictet, the results of which were published in 

 1877. It should be said that they worked independently of each 

 other Cailletet in Paris and Pictet in Geneva. Pictet liquefied 

 carbonic acid and sulphur dioxide by pressure. The liquid carbonic 

 acid was passed through a tube that was surrounded by liquid sulphur 

 dioxide boiling in a partial vacuum. The liquid carbonic acid thus 

 cooled was then boiled under diminished pressure in a jacket sur- 

 rounding a tube in which the gas to be liquefied was contained under 

 high pressure. When this gas was allowed to escape from a small 

 opening its temperature was so reduced by the expansion that a part 

 of it was liquefied in the tube and passed off as a liquid. Cailletet 

 worked in essentially the same way, but on a smaller scale. Neither 

 of these experimenters liquefied oxygen or nitrogen on the large 

 scale, but they pointed out the way that must be followed in order 

 that success may be attained. They destroyed the belief in " per- 

 manent " gases. 



Later experimenters in this field are Wroblewski, Olszewski, and 

 Dewar, who have been interested mainly in the purely scientific side 

 of the problem, while Linde in Germany, Hampson in England, and 

 Tripler in the United States have their minds on the practical side. 

 Notwithstanding the low temperatures involved in the experiments, 

 a number of heated discussions have been carried on in the scientific 

 journals touching the question of priority. To the unprejudiced 

 observer it appears that all of those named above are entitled to 

 credit. They have all helped the cause along, but just how to ap- 

 portion the credit no one knows. In a general way, however, some 

 of the results obtained by each in turn should be given. Wroblewski 

 and Olszewski have carried on the work begun by Cailletet and Pictet, 



