136 



PRINCIPLES OF CHEMISTRY 



great pressures. 30 This conclusion was verified (1879) by tire ex- 

 periments of Pictet and Cailletet. 31 They compressed gases at a 



30 This conclusion was arrived at by me in 1870 (Ann. Phys. Chem. 141, 023). 



31 Pictet, in his researches, effected the direct liquefaction of many gases which up to 

 that time had not been liquefied. He employed the apparatus used for the manufacture 

 of ice on a large scale, employing the vaporisation of liquid sulphurous anhydride 

 which may be liquefied by pressure alone. This anhydride is a gas which is transformed 

 into a liquid at the ordinary temperature under a pressure of several atmosphere- 

 Note 27), and boils at 10 at the ordinary atmospheric pressure. This liquid, like all 

 others, boils at a lower temperature under a diminished pressure, and by continually 

 pumping out the gas which comes off by means of a powerful air-pump its boiling point 

 falls as low as 75. Consequently, if we on the one hand force liquid sulphurous 

 anhydride into a vessel, and on the other hand pump out the gas from the same vessel 

 by powerful air-pumps, then the liquefied gas will boil in the vessel, and cause the tempera- 

 ture in it to fall to 7S 3 . If a second vessel is placed inside this vessel, then another 

 gas may be easily liquefied in it at the low temperature produced by the boiling liquid 

 sulphurous anhydride. Pictet in this manner easily liquefied carbonic anhydride, COo 

 (at 60 under a pressure of from four to six atmospheres). This gas is more refractory 

 to liquefaction than sulphurous anhydride, but for this reason it gives on evaporating a 

 still lower temperature than can be attained by the evaporation of sulphurous anhydride. 

 A temperature of 80 may be obtained by the evaporation of liquid carbonic anhydride at 

 a pressure of 760 mm., and in an atmosphere rarefied by a powerful pump the temperature 

 falls to 140. By employing such low temperatures, it was possible, with the aid of 

 .pressure, to liquefy the majority of the other gases. It is evident that special pumps 

 which are capable of rarefying gases are necessary to reduce the pressure in the 

 chambers in which the sulphurous and carbonic anhydride boil ; and that, in order to 

 re-condense the resultant gases into liquids, special force pumps are required for pumping 

 the liquid anhydrides into the refrigerating chamber. Thus, in Pictet's apparatus 

 (fig. 24), the carbonic anhydride was liquefied by the aid of the pumps E F, which com- 



FIG. 24. General arrangement of the apparatus employed by Pictet for liquefying gases. 



