90 VISCOSITY OF GASES 239 



We must therefore conclude that carbonic acid at 

 temperatures between 30 and 40 obeys the laws of perfect 

 gases with sufficient accuracy so long as the pressure remains 

 below a limit of about 70 atmospheres, which corresponds 

 nearly to the critical pressure. But if the pressure exceeds 

 this limit, carbonic acid behaves, at least approximately, 

 like a liquid the density of which is scarcely altered by 

 pressure. 



Since this behaviour is confirmed also by observations of 

 another kind, we may look on the result of our calculations 

 as a sign that the theory of the viscosity of partially 

 dissociated gases developed in 89 is substantially founded 

 on truth. We shall have to assume that the formula 



really represents the coefficient of viscosity of a partially 

 dissociated gas of density 8, and A and B are to be looked 

 upon as constants if the pressure is sufficiently high, but to 

 be put 



A = ap, B = bp% 



for smaller values of the pressure p, a and b being constants. 

 I might probably have found a general formula appli- 

 cable to all values of the pressure if I had attempted to use 

 as basis of my calculations one of the general laws which 

 have been proposed by van der Waals, Clausius, and 

 others to represent the connection between the pressure and 

 the density. I have had to abandon doing this, as I wished 

 to delay the appearance of this book no longer, 



91. Viscosity of the Perfectly -dissociated Gas 

 and of the Non- dissociated Gas 



I have, on the other hand, sought for a more compre- 

 hensive proof of the theoretical formula by returning to the 

 theoretical meaning of the magnitudes A and B, the values 

 of which I have obtained from the observations, and investi- 

 gating the conclusions of another kind to which they lead. 

 According to 90, 



A = D2 - g) (1 - ), B = 



