1884.] 



on Liquefied Gases. 



151 



critical pressure is proportional to the molecular volume, so that the 

 determination of the critical temperature and pressure of a substance 

 gives us a perfectly independent measure of the molecular volumes. 

 Prof. Dewar pointed out the great advantage of employing a liquid of 

 low critical temperature and pressure such as liquid marsh-gas for 

 producing exceedingly low temperature. He hoped to be able to 

 approach the absolute zero by the evaporation of liquefied marsh-gas 

 whose critical temperature was less than — 100° C, and whose 

 critical pressure was only 39 atmospheres." 



I ought to mention that the marsh-gas used in my experiments was 

 made by the action of water on zinc methyl, and was therefore very 

 pure, and that the observed critical pressure was not 39 atmospheres, 

 but 47*6. The following table gives the values of the ratio of the 

 absolute critical temperature to the critical pressure in the case of a 

 number of substances. The values for ammonia, sulphuretted hydrogen, 

 cyanogen, marsh-gas, and hydride of ethyl are new. 



Chlorine Clg 



Hydrochloric acid HCl 



Oxygen Oj 



Water H2O 



Nitrogen Nj 



Hydrogen sulphide .. .. H2S 



Ammonia .. " .. .. H3N 



Diethylaraine (C2H5)2HN 



Nitrous oxide NgO 



Sulphurous acid SO., 



Marsh-gas CH, 



Acetylene C2H2 



Ethylene CjH^ 



Ethyl hydride C.Hg 



Amylene ^sHio 



Benzol OgHg 



Chloroform CHCI3 



Carbon chloride CCI4 



Carbonic acid CO2 



Bisulphide of carbon . . . . CSg 



Cyanogen C2N2 



A glance at the last column of the table shows that a large number 

 of substances have at their respective critical temperatures simple 

 volume relations. Thus hydrochloric acid, water, ammonia, and 

 marsh-gas, the four chemical substances from which the great majority 

 of chemical compounds may be derived by processes of substitr.tion, 

 have nearly the same volume ; while the more complex derivatives 

 show an increased volume which bears a simple ratio to that of the 

 typical body. As the critical pressures are not known with any great 



