COLLOIDS 233 



this process very suitably, though other resins such as mastic 

 or even common resin will also answer the purpose. A number 

 of these emulsions and other imperfect solutions have been the 

 subject of numerous experiments during the last twenty years 

 or more. These liquids contain granules in suspension so small 

 that they commonly pass through ordinary filter paper, 

 and the liquid can only be clarified by resort to special processes. 

 When allowed to rest undisturbed for a long time the upper 

 layers of the liquid gradually become clear, while the floating 

 particles accumulate in regularly increasing proportion in the 

 lower layers. When such a liquid is examined under the micro- 

 scope the granules are seen to be all spheroidal in form and to 

 vary considerably in size, though after the deposition of the 

 coarser particles no solid can be discovered by the unaided eye. 

 They can be most conveniently sorted by submitting the liquid 

 to the action of a centrifuge, the larger masses passing first toward 

 the periphery. This method was introduced by Professor Jean 

 Perrin of the Sorbonne, who has given much attention to the 

 size of particles which exhibit the Brownian movement. A 

 method which has been used for several similar purposes is 

 based on the formula given by the late Professor Sir George 

 Stokes for the velocity of a small sphere of known density falling 

 through a medium of which the viscosity is known. With an 

 emulsion holding in suspension grains of uniform size the rate 

 of fall can be easily measured by observing the time occupied 

 while the top layer of known depth becomes clear. 



Another process is to count under the microscope the number 

 of granules in a very small but known volume of an emulsion 

 containing grains of uniform size, whereof the total mass is 

 known. 



In the case of gamboge the diameter of the granules was found, 

 by the first method, to range from -90 //, to 42 /m, and by the second 

 from '92 p. to -42 fj.. The last result involved the counting of about 

 11,000 grains. 



The main result arrived at by Perrin is the conclusion that the 

 Brownian movement of colloidal particles in suspension and the 

 movement of molecules are of the same type and are due to the 

 action of the same forces. These forces appear to be the trans- 

 latory forces of the moving molecules of surrounding water ; in 

 other words the granules in suspension are pushed about by the 

 molecules surrounding them. In order that movement may re- 



