A SHORT REVIEW OF COLLOID THEORY 



By H. D. MURRAY. 

 Exhibitioner of Christ Church, Oxford. 



To Thomas Graham is due the credit of first studying, in 

 1862, the properties of the colloid state. Although he drew 

 a sharp distinction between solutions of colloids and crystal- 

 loids, it has, since that time, become increasingly more difficult 

 to do so. Possibly the simplest definition of colloidal solu- 

 tions, which we can at present give, is that they differ from 

 true solutions in not being homogeneous, and they contain, 

 therefore, more than one phase. This is due to the aggrega- 

 tion of the " dissolved " substance into masses which are large 

 compared with molecular magnitudes. Of the physical pro- 

 perties which indicate such heterogeneity, that which gives 

 rise to what is known as the Tyndall effect is probably most 

 reliable. If a lateral beam of intense light is thrown across 

 any solution, when viewed under a microscope, the presence 

 of particles above a certain size is shown by moving specks 

 of light. These are due to diffraction of the light by the par- 

 ticles which, by transmitted light, would be invisible. With 

 such an arrangement the transition between true and colloidal 

 solutions may readily be seen, as, for example, with cane 

 sugar. Dilute solutions of this substance show no turbidity, 

 but, on gradually increasing the concentration, specks begin 

 to appear, until eventually they fill the field of view. 



Graham, as we have said, drew a sharp distinction between 

 colloids and crystalloids, based upon their rate of diffusion 

 in solution through membranes. It now seems certain that 

 such a distinction is purely artificial, and that we ought rather 

 to speak of the colloid state than of colloid substances. This 

 recognition of the universality of the colloid state is one of 

 the great advances of recent years which has made possible 

 the formulation of a general theory of colloids ; but such a 

 theory has still to be brought forward. Wo. Ostwald ' and 

 Von Weimarn,* amongst others, have done much to emphasise 

 the fact that we must now admit the possibility of preparing 



1 Grundriss der Kolloidchemie. 



* Grundzuge der Dispersoidchemie, 1910. 



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