PROPERTIES 387 



colloidal solutions are, therefore, no longer regarded as being 

 due to the intrinsic properties of the substances dissolved, but 

 rather to the state of aggregation of the substances concerned. 

 Only on this assumption is it possible to understand how one 

 and the same substance can at one time produce a colloidal 

 solution, and at another an ordinary crystalloidal solution as 

 is, for example, the case with gallic acid, which gives a col- 

 loidal solution in water, but not in glacial acetic acid. 



Graham, moreover, found that many substances which 

 were insoluble in water in the ordinary way could, neverthe- 

 less, be made to produce colloidal solutions exhibiting the 

 characteristic reluctance to diffuse. Since Graham's time, 

 almost all the metals and their insoluble oxides, sulphides, 

 carbonates, sulphates, etc., have been obtained in so-called 

 colloidal solution, including even such insoluble substances 

 as lead and barium sulphates. 



It would appear, therefore, that the properties of a col- 

 loidal solution are not so much due to the substance itself as 

 to the peculiar nature of the solution, or, in other words, the 

 state of aggregation of the dissolved substance. 



The evidence in support of this view is partly optical 

 (Tyndall phenomenon, ultramicroscope, etc.) and partly 

 direct, since it has been shown that many of the substances 

 which are known to us as insoluble can, by a sufficient degree 

 of disintegration, be made to yield colloidal solutions. 



Thus many metals are obtained in colloidal aqueous solu- 

 tion by passing a powerful electric discharge between two 

 poles of the metal held under water, and, again, a number of 

 insoluble crystalloids, such as silica, molybdenum oxide, and 

 vanadium oxide, have been made to yield colloidal solutions 

 by merely finely powdering, or grinding these substances under 

 water.* 



Finally, the whole question has been shown to be amen- 

 able to mathematical treatment by Von Weimarn,f who has 

 worked out the conditions which determine whether a given 

 substance will assume the crystalloid or the colloidal state. 



* Wegelin : " KoUoid Zeitschr.," 1913, 14, 65. 



t For an account of this, see Taylor's " The Chemistry of Colloids," 

 London, 1915. 



25 * 



