242 PHENOMENA DEPENDENT ON MOLECULAR PATHS 91 



A comparison of these values with those obtained for the 

 viscosity of carbonic acid under ordinary pressures is ob- 

 viously the next thing. As was mentioned before, in 79, 

 Graham's experiments on carbonic acid at gave 

 77 = 0-000145 ; Puluj found the nearly equal value ij = 

 0-000143, while von Obermayer and Schumann agree 

 in finding the rather smaller value ?? = 0-000138 ; [and the 

 mean of all these is 



rj = 0-000141]. 



Since this is only slightly greater than the mean value 

 17! = 0-000130 



now found for perfectly dissociated carbonic acid, the 

 assumption that carbonic acid under ordinary circumstances 

 consists almost entirely of simple molecules C0 2 , seems to 

 be justified. 



Nearly the same results are deducible also from the 

 formulae which contain constant values of A and B, if we 

 make an assumption regarding the variation of the density 

 with the pressure such as after our former explanations 

 cannot be taken as entirely improbable. I assume that the 

 density of gaseous carbonic acid obeys Boyle's law up to 

 the critical pressure, which is 77 atmospheres according to 

 Andrews 1 ; from this limit onward, however, I assume the 

 density not to be variable with the pressure. In reality 

 there will certainly be a continuous change from the one 

 state to the other, but I think that we may take this 

 assumption to be allowable as an approximation to the true 

 behaviour. 



I therefore put for the density of the intensely com- 

 pressed gas which has become independent of the pressure 

 the value 



D = 77 x 0-0019653/(1 + aS) = 0-15133/(1 + aty, 



wherein $ is the temperature and a the coefficient of expan- 

 sion 0-00367. On putting this value in the formula 



A = 



1 Phil Trans. 1876, clxvi. p. 421. 



