OSMOTIC PRESSURE 275 



point of the solvent in which they are dissolved. From these 

 observations it must be concluded either that the substances 

 concerned have a very high molecular weight, or else that they 

 exert a very small osmotic pressure. 



Attempts to determine the molecular weights of substances 

 by these means have given widely different and inconcordant 

 results, the figures for egg albumen varying roughly from 

 6,000 to 15,000 according to different authors; moreover, 

 while the depression produced by gallic acid in water corre- 

 sponds to a very high molecular weight, similar measure- 

 ments in glacial acetic acid give a molecular weight 

 corresponding to that calculated from the formula. The con- 

 clusion to be drawn from these observations is that gallic acid 

 gives a colloidal solution in water but not in glacial acetic 

 acid. 



In view of these facts the question arises whether the 

 small depressions observed in the case of colloidal solutions 

 are not really due to traces of impurities, such as salts, which 

 it is almost impossible to remove completely. 



On the other hand, the low osmotic pressure may be ac- 

 counted for by assuming that colloidal solutions are in reality 

 not true solutions, but are what may be described as two- 

 phase systems ; this implies that they are more of the nature 

 of suspensions of one form of matter in another, such, for ex- 

 ample, as a solid in a liquid ; the suspended substance is 

 known as the disperse phase, and the medium in which it is 

 suspended as the continuous phase, a system of nomenclature 

 which indicates that the particles of the liquid are in contact 

 with each other, while those of the disperse phase are 

 separate. 



The above views are supported by much that is known 

 regarding the optical properties of colloidal solutions. 



B. Optical Properties. 



The optical properties of colloidal solutions vary with the 

 nature of the colloid ; thus, for example, while a colloidal solu- 

 tion of silicic acid appears, to the unaided eye, homogeneous 

 and clear, an aqueous solution of soap is more or less opales- 

 cent and polarizes light ; on the other hand, while the colloidal 

 solutions of organic substances are usually colourless, those of 

 18 * 



