149 
ance which a solid sphere encounters when moving in a medium in 
which the free path is small with respect to the radius of the sphere. 
Einstein has namely demonstrated that for the cause of the diffusion 
we may substitute a force acting on the diffusing particles, which 
is equal to the osmotic pressure *). Like 1 the expression 3 holds, 
therefore, only for particles which are great with regard to the 
free path. 
When we now examine what experimental confirmations for the 
expressions 1 and 3 are to be found in the literature, it appears 
that for particles with a diameter of the order 10-4 and 10-5 em. 
chiefly equation 1 has been tested. Generally the procedure of testing 
is carried out in this way that the mean square of deviation is 
calculated from the observed deviations and then N is determined 
from equation 1. Prrrin’s experiments, carried out with particles the 
radius of which varied between 2.10 and 5.10~4 em., yielded 
values for MN oscillating between 5.5 and 8.0 10°. Prrrin’s most 
accurate determinations carried out with particles of equal size, 
yielded NM —6.9 10**.*) Of late values have been found for N which 
are lower’) and have got closer to Miraikan’s 6.06 10° *), which 
value is pretty generally considered as the most reliable one. 
With regard to the diffusion it is noteworthy that the diffusion 
constant of these particles is very difficult to determine on account 
of the slight velocity at the ordinary temperature. Only by a very 
particular -mode of procedure Prrrin has succeeded in finding a 
value for the diffusion constant. In his determinations the property 
was made use of that gamboge particles, moving in glycerine, adhere 
to a glass wall when colliding with it. So the quantity of 
particles adhering to the wall continually increases, when the sus- 
pension is brought into a vessel, and the diffusion constant can be 
calculated from the number that is found on the wall at different 
times. In this way Briniouin found the value NM = 6.9 10° in 
Prrrin’s laboratory *). Accordingly the expressions 1 and 3 give 
satisfactory results for particles of the order 10 + and 10~° em. 
Likewise experiments have been made with colloidal solutions as 
a test of the equations 1 and 3. The Brownian movement has been 
1) EINSTEIN. Ann. d. Phys. (4) 1%. 549. (1905). 
2) PERRIN. Compt. rend. 146 seq. A summary of these experiments is found in 
DE HAAs—LoRENTz. Die Brownsche Bewegung und einige verwandte Erscheinungen. 
Die Wissenschaft. Band 52. (1 13). 
5) NorpLunp. Zeitschr. f. physik. Chemie 87. 40. (1914). 
4) MILLIKAN. Phys. Zeitschr. 14. 796. (1913). 
5) BRILLOUIN. Ann. chim. et phys. (8) 27 412 (1913). Cf. however WESTGREN. 
Zeitschr.-f. physik. Chem. 89. 63. (1914). 
