for the year 1878. xxvii 



to bringing refractory gaseous bodies within the boundaries 

 of the assumed law. In 1823, Michael Faraday, at the 

 suggestion of Sir Humphrey Davy, heated hydrate of chlo- 

 rine in an hermetically sealed glass tube, and made the 

 discovery of liquefied chlorine gas. Faraday made the 

 discovery, and, unaided, puzzled out the proper interprC' 

 tation of the result of the experiment; but that Davy 

 had a penetrative insight into the nature of the 

 chemico-physical problem involved in it, seems obvious from 

 his own words. '' One of three things," he says, " might be 

 expected to happen as the result of the experiment — either 

 that the solid and crystalline hydrate of chlorine would 

 become a fluids or that a decomposition of water with for- 

 mation of euchlorine would take place, or that the chlorine 

 would separate in a condensed state." He goes on to point 

 out how much more is to be effected in future liquefaction 

 experiments from pressure obtained in sealed vessels than 

 from refrigeration, and further how the agency of pressure 

 may be assisted by artificial cold in cases where gases 

 approach the state of vapour. Faraday, in the course of his 

 labours, reduced many gases, and Thilorier in 1834 contrived 

 an apparatus for liquefying carbonic acid in quantity, and 

 reducing it to the state of snow, which, as a means of 

 attaining very low temperatures, greatly assisted the course 

 of subsequent experiment, and indeed is now largely used in 

 physical investigation and in the arts. In 1845, by the com- 

 bination of pressure and refrigeration, Faraday succeeded in 

 adding to the list of gases susceptible of assuming the Hquid 

 and solid states ; but still oxygen, nitrogen, and hydrogen 

 held out against all experimental coercion, and in that sense 

 remained still in the category of permanent gases. This is 

 how the case has stood until the experiments of M. Calletet, 

 and more especially those of M. Pictet, have been crowned 

 with the success of breaking down the dividing wall between 



