[CLARK] AIR AND THE LAW OF CORRESPONDLNG STATES 



49 



an advantage if the temperatures are not too far from the critical 

 temperature, where the curves fall too close together. 



On account of the fact that air behaves like a simple substance 

 when not too near the critical region, Professor Kamerlingh Onnes 

 suggested that we determine the critical date for air by comparison 

 with carbon dioxide and other gases. It is interesting to note that 

 the method gives the correct values of critical temperature and pres- 

 sure with a higher degree of accuracy than that of most previous 

 work. 



Figure 1 



We have excellent, though not extensive, data for air in the 

 observations of Witkowski,^ and for carbon dioxide and ethylene in 



Pv 



the papers of Amagat.^ The air isotherms in terms of log -=- and 



log P were drawn on a large scale on heavy paper. The isotherms 

 of the other substances were drawn to the same scale on thin trans- 

 parent paper and placed on the air curves. Then, by shifting the 

 carbon dioxide or ethylene curves on the air curves they were brought 

 into corresponding states. Taking the critical pressure and tem- 

 perature of carbon dioxide as 31-1°C. and 75 atmospheres respec- 

 tively, and of ethylene 10-1°C. and 51 atmospheres respectively, the 

 mean of a number of trials gives the critical temperature of air, 



iWitkowski: Philosophical Magazine (5) 41, 288, 1896. 



^ Amagat: See Barus, The Laws of Gases, for a collection of papers by Amagat. 



Sec. Ill.Sig. 4 



