590 
MESSRS. T. E. THORPE AND J. W. RODGER ON THE RELATIONS 
But at the boiling-point the vapour pressures of different substances are the same, 
and hence if the boiling-point be approximately a corresponding temperature, the 
critical pressures of substances ought to be approximately the same. This conclusion, 
liowever, is not supported by facts, and hence, having regard to the doctrine of corre¬ 
sponding pressures, Guldberg’s view, that the boiling-point may be regarded as a 
corresponding temperature, receives no support from van der Waals’ theory. 
Hitherto the boiling-point has been mainly of value as a comparable temperature 
in dealing with properties which, like density or surface energy, vary but slowly with 
the temperature. 
It was a matter of interest, therefore, to determine if it led to good results in the 
case of a property like viscosity, which alters rapidly with the temperature. No 
doubt physico-chemical relationships, even in the case of these slowly changing 
properties, are not so definite as might be desired, and the want of precision may in 
part be due to the fact that at the boiling-point the substances are not in really com¬ 
parable conditions. Evidence on this point seemed most likely to be gained by the 
study of a property which, like viscosity, varies so largely with temperature. 
In deciding upon the particular boiling-points to be adopted, we have made a 
careful critical examination of all existing data, and have selected the mean value of 
what seemed to be the best authenticated determinations, including our own. 
In the following tables the values of r) have been read from the curves, and are 
multiplied by 10® in order to avoid the use of decimals. 
Coefficients of Viscosity at the Boiling-point. 
[y] in dynes per sq. centim. X 10®.) 
Homologues. 
'/• 
Difference. 
Pentane . 
200 
1 
4 
Hexane. 
204 
- 5 
Heptane. 
199 
- 1 
Octane. 
198 
Isopentane . 
203 
2 
Isoliexane. 
205 
— 7 
Isoheptane . 
t—* 
00 
Isopi’ene. 
188 
4 
Diallyl. 
192 
j 
