10 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



diameter, a particle of colloidal gold to the same magnification 

 would be barely visible as a minute dot, but yet it can be 

 distinguished by means of the ultramicroscope. 



(2) Colloids can readily be precipitated from solution, 

 usually by acidification, by the addition of solutions of various 

 salts or by the introduction of other colloids. 



Certain colloids when once so precipitated are only brought 

 into solution again with difficulty, others readily pass into 

 solution if the precipitating agent is removed, e.g., by dialysis, 

 or if its effect is diminished by dilution. These two classes of 

 colloids are distinguished as irreversible and reversible respec- 

 tively. The difference in behaviour is probably mainly due 

 to differences in the sizes of the precipitated particles. 



A characteristic example of an irreversible colloid is the 

 soluble silica already referred to. On addition of hydro- 

 chloric acid or salt solution to the aqueous solution of silica 

 the latter gelatinises and cannot readily be brought into 

 solution again. 



A colloid when in solution in water, as in the case of the 

 unprecipitated silica, is frequently referred to as a hydrosol ; 

 when precipitated in a gelatinous or anhydrous form it is 

 known as a hydrogel. 



Many enzymes are typical reversible colloids. They 

 can be precipitated from their aqueous solutions by means of 

 alcohol, but redissolve in water if the alcohol is removed by 

 filtration. 



True colloids conduct electricity very slightly, if at all ; in 

 fact, under the influence of the electric current, they move as 

 a whole towards one pole or the other. 



The precipitation of one colloid by another has been shown 

 to be connected with the electrical condition of the respective 

 substances. An electro-positive colloid will precipitate an 

 electro-negative colloid, and vice versa. 



The precipitation of organic colloids by gelatinous mineral 

 hydroxides which is made use of in the clarification of sewage 



