90 VISCOSITY OF GASES 235 



perfect gas, and therefore conforms to Boyle's law. In 

 this case the density D must be assumed to be proportional 

 to the pressure, so that we may put 



A = ap, B = bp%, 



where a and b are independent of the pressure, and there- 

 fore can be variable with the temperature only. 



The assumption that the gas obeys Boyle's law 

 appears, however, as very improbable if we remember that 

 the temperature in the experiments lay between 25 and 

 41, and was, therefore, on neither side very different from 

 the critical temperature 31, at which, under certain cir- 

 cumstances, the distinction between liquid and gaseous 

 carbonic acid ceases. Instead of Boyle's law, therefore, we 

 might with equal justice assume that the density of the 

 carbonic acid vapour varies but very little with the pressure, 

 just like that of any liquid. That this assumption is for 

 the most part really satisfied under the circumstances of 

 Warburg and von Babo's experiments is seen from the 

 observations of Andrews on the continuity of the gaseous 

 and liquid states, being especially perceptible from the curves 

 which are given in his memoir. 1 These show the volume of 

 the partly gaseous and partly liquid carbonic acid to be 

 nearly constant at temperatures which are very near to those 

 employed by Warburg and von Babo, and under simi- 

 larly high pressures ; whence it follows that the density of 

 the carbonic acid vapour is nearly equal to that of liquid 

 carbonic acid, and that therefore, under the circumstances 

 now coming into consideration, they are both independent of 

 the pressure. 



I have therefore calculated the results of the observa- 

 tions communicated by Warburg and von Babo 2 under 

 both assumptions, supposing in the first place that Boyle's 

 law is obeyed and in the second that the density D is 

 invariable. 



The results of the calculations gave that at 25 f l, the 



1 Phil. Trans. 1869, clix. p. 575 ; Pogg. Ann. Erg.-Bd. v. p. 64, 1871. 



2 Table xii. of their complete memoir ; p. 512 of the extract. 



