ONE HUNDRED YEARS OF CHEMISTRY 321 



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 

 importance that it may be considered as a separate 

 branch of the science. It has its own technical journal 

 and deals largely with the chemistry of organic products. 

 All living matter is built up of colloids, and haemoglobin, 

 starch, proteins, rubber and milk are examples of col- 

 loidal substances or solutions. Among inorganic sub- 

 stances, many sulphides, silicic acid, and the amorphous 

 hydroxides, like ferric hydroxide, frequently act as 

 colloids. 



Law of Mass-Action. Berthollet about the beginning 

 of the last century was the first chemist to study the 

 effect of mass, or more correctly, the concentration of 

 substances on chemical action. His views summarized 

 by himself are as follows : * * The chemical activity of a 



