4IO TRANS. ST. LOUIS ACAD. SCIENCE. 



or 



^■=^- • • ■ (■3> 



For air this pressure is about 3.3 millimeters of mercury, and 

 for other gases it is proportional to the volumes of a unit mass, at 

 a standard temperature and pressure. 



The thermodynamic surfaces of various gases will lie the one 

 above the other, those having the largest value of R being upper- 

 most. If vv^e now substitute the value of ^' of (13) in the origi- 

 nal equation of the surface, we have 



V = ^JZi T, ... (14) 



which is independent of J?. Hence, for all gases which follow 

 the law represented in (1) the lines on their respective surfaces 

 where the isentropic lines coincide with the direction of maximum 

 slope (13), will all lie in a common plane passing through the 

 axis of R and at right angles to the plane of v T, its trace upon 

 the latter plane being represented by (14). 



If the gases have a common temperature while in this condi- 

 tion, (14) shows that they will also have a common density, 

 which when Z'is 273° will be o.^Q^Qo^g grammes to the cubic 

 centimeter. 



It will be observed that for air, the pressure indicated in (13) 

 is practically the same as that at which Maxwell's law for vis- 

 cosity begins to fail. This, however, is a mere coincidence, as 

 the two phenomena have nothing in common, as is evident both 

 from theoretical considerations and from experimental results. 



Washington University, March 30, 1SS2. 



