CHEMICAL PHENOMENA IN LIFE 



suspension colloids. Such precipitations are not 

 reversible. When, on the other hand, albumin is 

 precipitated by sodium chloride, it is possible to 

 again dissolve the precipitation by diluting it 

 with water. This process is reversible. Generally 

 in albumin all precipitations with the salts of the 

 light alkaline metals and of magnesium are 

 reversible. But they are not reversible when 

 precipitated by copper salts, iron salts, or any other 

 salt of heavy metals. Precipitations with calcium 

 or strontium salts are inclined to be quite insoluble 

 in water. It is noteworthy that the working of the 

 salt depends upon the acid contained in it. Francis 

 Hofmeister, of Strassburg, was the first to show 

 that alkaline metal salts of different acids have a 

 certain graduated effect on colloid solutions. 

 They may be arranged in the following way, 

 beginning with the acid which precipitates most 

 quickly : 



Citrate, Tartrate, Sulphate, Acetate, Chloride, 

 Nitrate, Chlorate. 



This law became of the greatest importance in 

 the chemistry of colloids. It is not only applic- 

 able to the transition of colloid solutions into solid 

 colloids, but even to the chemical and physical 

 states of solid colloids themselves. 



Graham named solid colloids Gels, the name 



corresponding to that of Sols or liquid colloids. 



The physical condition of certain gels is very 



different. Glue itself, when quite dry, forms a 



4 8 



