290 Wells <& Foote — One Hundred Years of Chemistry. 



Colloidal Solutions. — Graham, an English chemist, in 

 1861 was the first to make a distinction between sub- 

 stances forming true solutions, which he called crystal- 

 loids, and those of a gummy nature resembling glue, 

 which in solution do not diffuse readily through parch- 

 ment membranes, as crystalloids do, and which he called 

 colloids. The separation of colloids by means of parch- 

 ment was called dialysis, and this process has come into 

 extensive use in preparing pure colloidal solutions. 

 Slow diffusion is now regarded as characteristic of col- 

 loids rather than their gummy condition. 



Colloidal solutions occupy an intermediate position 

 between true solutions and suspensions, resembling one 

 or the other according to the kind of colloid and the fine- 

 ness of division. By preparing filters with pores of 

 varying degrees of fineness, Bechold has been able to 

 separate colloids from each other in accordance with the 

 size of their particles. It has also been possible to pre- 

 pare different solutions of a colloid varying gradually 

 from one in which the particles were undoubtedly in sus- 

 pension to one which had many of the properties of a 

 true solution. 



Beginning in 1889, Carey Lea described in the Journal 

 (37, 476, 1889 et seq.) a variety of methods for preparing 

 colloidal solutions of the metals, consisting in general of 

 treating solutions of metallic salts with mild reducing 

 agents. His work on colloidal silver was particularly 

 extensive and interesting. Solutions of this kind have 

 recently yielded some extremely interesting results by 

 means of the ultra-microscope, an apparatus devised by 

 Zsigmondy and Siedentopf. A very intense beam of 

 light is passed through the solution and observed at right 

 angles with a powerful microscope. Under these condi- 

 tions, particles much too small to be seen by other means, 

 reveal their presence by reflected light. It has been pos- 

 sible in a very dilute solution of known strength to count 

 the particles and thus to calculate their size. The small- 

 est colloidal particles measured in this way were of gold 

 and were shown to have approximately ten times the 

 diameter, or 1000 times the volume, attributed to ordi- 

 nary molecules. It is of interest that the particles 

 appear in rapid motion corresponding to the well-known 

 Brownian movement. 



The chemistry of colloids has now assumed such 



