276 COLLOIDS 



the metals are frequently highly coloured ; thus, gold may give 

 a pink, red, or purple colour in water, and a blue colour in 

 organic solvents, and, similarly, silver yields brownish-red or 

 blue solutions.* This difference in colour given by the same 

 metal depends upon the conditions under which the solutions 

 are formed, and on the consequent size of the particles. 



These facts, more especially those relating to opalescence 

 and the polarization of light, indicate that such solutions are 

 heterogeneous, i.e. they are not true solutions, but are more of 

 the nature of suspensions of very minute particles. This con- 

 clusion is confirmed by the evidence afforded by the ultra- 

 microscope, by the use of which the liquid can be seen to 

 contain particles exhibiting different colours and Brownian 

 movements. The size of the particles is very minute and 

 varies greatly ; some are visible under the high power of the 

 ordinary microscope, whilst others are only just discernible 

 under the ultramicroscope. 



This may explain the fact that the particles of some col- 

 loidal solutions are able to pass through the pores of an ordi- 

 nary filter paper, whereas they are stopped by the smaller 

 pores of a parchment membrane ; consequently, if the pores 

 be relatively large slow diffusion could take place, but if the 

 pores be so small as to allow only the molecules of a crystal- 

 loid, say of sodium chloride, to pass through, then the diffusion 

 of colloids, e.g. starch paste, would be inhibited since the size 

 of the particles is much greater than that of the molecules of 

 salt. 



It must, however, be remarked that such a passage of sub- 

 stances through membranes is understood imperfectly, and that 

 many of the observed facts are incompatible with the filter 

 theory expressed above. 



The solution theory maintains that the membrane dis- 

 solves the diffusing substance which passes through and is 

 given off on the other side. 



The chemical theory holds that the passage of a substance 

 through a membrane is due to the latter playing an active 

 chemical part in the process. 



* Red, or so-called ruby, glass is also a colloidal solution of metallic gold in 

 glass, and the naturally occurring blue rock salt is probably a colloidal solution 

 of sodium in the crystalloid sodium chloride. 



