2 ATTAINMENT OF VERY LOW TEMPERATURES. 



hydrogen at temperatures on the scales of the two thermometers, 

 corresponding to pressures below 800 millimeters of mercury. The 

 liquid hydrogen employed in the researches was obtained by means 

 of the apparatus already referred to. For this purpose, however, it 

 was modified in some details so as to render the production of liquid 

 hydrogen a matter of certainty. Indeed, eight out of nine experiments 

 carried out during the summer of 1902 were successful ; the one 

 failure being due to the breaking down of the compressor. The re- 

 sults of these investigations have been published in the Philosophical 

 Transactions of the Royal Society. 



On concluding this work I turned my attention to the improve- 

 ment of the apparatus for liquefying hydrogen, and to the theoretical 

 investigation of the processes involved. An account of this work is 

 contained in the following pages. 



II. Historical. 



The researches of numerous investigators, notably those of Faraday 

 and Thilorier, carried out during the early part of the last century, 

 had resulted in the liquefaction of all the gases then known, with the 

 exception of hydrogen, nitrogen, oxygen, carbon monoxide, nitric 

 oxide, and methane, which hence received the name of permanent 

 gases. Though Faraday had probably some conception of the explana- 

 tion of their behavior, it was undoubtedly a matter of general opinion 

 that the gases possessed in themselves a permanent character, or that 

 they would ultimately liquefy were the pressure on them sufficient. It 

 v/as not until 1869, when Andrews proved there is for each substance 

 a critical temperature above which the liquid and vapor cannot be 

 differentiated, that existence of permanent gases was explained. 



No further advance towards the liquefaction of the permanent 

 gases was made till 1877 when, on December 24, communications were 

 received by the French Academy independently from M. Raoul Pictet 

 of Geneva and from M. Louis Cailletet of Chatillon-sur-Seine, announc- 

 ing the liquefaction of oxygen. A week later M. Cailletet described to 

 the academy some experiments in which, by the process of com- 

 pression and subsequent expansion of hydrogen, he had succeeded in 

 producing in it a mist, presumably of liquid. 



The following is transcribed from the paper which appeared in the 

 Comptes Rendus (1877, 85, 1270) : 



"Dans mes premiers essais, je n'avais rien reconnu de particulier ; 

 mais, comme il arrive souvent dans les sciences experimentales, I'habi- 

 tude d'observer les phenomenes finit par en faire reconnaitre les signes 

 dans les conditions ou ils avaient d'abord passe inapergus. 



" C'est ce qui arrive pour I'hydrogene. En repetant aujord'hui 

 meme, en presence de MM. Berthelot, H. Sainte-Claire Deville et 



