151 
fulfilment of the condition that the radius is great with respect to 
the free path or to binding of the dissolved substance with the 
solvent. 
It may finally be remarked that Herzoe calculates the molecular 
weight from the specifie volume of the dissolved substance in solid 
state and the diffusion constant by the aid of equation 3, and that 
he finds it in agreement with the known value as far as the order of 
magnitude is concerned. He derives from this a method to determine 
molecular weights of large molecules, for which osmotic methods 
do not give results '). 
3. for the same substance in the same solvent the influence of 
the temperature can be examined. No data are to be found in the 
literature about this. The reason for this is that the determination 
of diffusion constants, which is attended with great experimental 
difficulties even under ordinary circumstances, becomes still more 
difficult with higher temperature. Of late we have been occupied 
with testing equation 3 at different temperatures; what follows gives 
the description of our experiments and the results yielded by this 
investigation. 
2. Method of investigation. The great 
difficulty which attends diffusion experiments 
at higher temperature is the keeping cou- 
stant of the temperature; in the ways of 
research followed up to now a constant 
temperature is very difficult to attain on 
account of the large dimensions of the appa- 
ratus. We have tried to obtain satisfactory 
results by the application of a micromethod. 
GRAHAM's first method which appeared to 
be suitable for this purpose is based on what 
follows. A diffusion vessel is filled with 
solution up to a certain height, then pure 
water is added till it is full, and it is placed 
Fig. 1. in a vessel with pure water. If in fig. 1 
the X-axis is laid vertieally downward, the origin at the level 
of the upper section, then the differential equation for the diffusion : 
de de 
D 
Ot 0a? 
yields on integration with the initial conditions : 
1) Herzoa. Zeitschr. f. Elektrochem. 16. 1008 (1910). 
