136 
Transactions of the Boyal Society of South Africa. 
Both curves (Fig. 3) exhibit the same general features as those 
previously obtained — for greater than a certain critical value, C, is 
negative for all values of ^ ; for smaller values of , we have becoming 
negative, zero, positive, zero, and again negative as ^ increases towards 
unity ; the two inversion temperatures are further apart the lower the 
value of , and they coalesce at the critical . But this critical value 
is somewhat smaller than expected, and, if the values previously quoted 
for 7^ are correct, would seem to exclude the possibility of inversion 
occurring with Acetone, Benzene, and Chloroform, for which it has actually 
been observed. But it is quite possible that the values of y which were 
then used were obtained under conditions not sufficiently far removed 
from the state of saturation to allow of their being regarded as identical 
with the state of a perfect gas. The Wiedemann y for Benzene, for 
instance, was 1-129, while a more recent determination by Stevens''' 
gives (at + 100° C.) 1*105 ; according to the present curves the former 
value would render always negative, while the latter would give in- 
version at ^ = 0-7d and ^ = 0-88. For isopentane, for which y^ is 
probably 1-08, the experimental curve would give inversion at ^ = 0-61 
and ^ = 0*74, values which are, as one would expect, almost identical 
with those deduced by Dieteeici from his own experiments in con- 
junction with Young's isothermals. But the paucity of experimental 
data for both y^ and renders it impossible to test the quantitative 
value of the foregoing conclusions. 
October, 1913. 
* E. H. Stevens: Ann. d. Phys. (4), 7, 285, 1902. 
