482 PROTOPLASM 



of certain solutions; so also does an alteration in the dielectric 

 constant of the medium through the addition of alcohol, acetone, 

 etc. Chemical change in the environment of the particle, due 

 to reactions between the substance added and the substance 

 adsorbed on the surface of the particle, may be responsible. 

 Silver ions in the stabilizing envelope of silver particles will 

 react with added sulphides; this would strip the metallic colloidal 

 particles of their charge. A chemical reaction is thus the primary 

 cause; reduction in charge, the ultimate one. 



The precipitation of smoke and of fog is again a phenomenon 

 the mechanism of which appears to differ from coagulation. 



Dehydration is a cause of coagulation which seemingly func- 

 tions wholly apart from electric charge. Proteins may be kept 

 in suspension by a water envelope (page 147). Kruyt does not 

 believe that it alone is sufficient; however, if it is removed, 

 coagulation may result. 



The use of salts for precipitating or salting out proteins and 

 other organic compounds is one of the best means of purification. 

 All proteins are coagulated on complete saturation of the solution 

 with ammonium sulphate. Other substances employed as 

 coagulants are sodium chloride, sodium sulphate, zinc sulphate, 

 magnesium sulphate, and organic liquids such as alcohol, ether, 

 and acetone. Proteins differ widely in the relative ease with 

 which they are coagulated by salts. Ease of coagulation of 

 proteins appears to be correlated with molecular weight, as shown 

 by the behavior of globulin and albumin from blood serum; the 

 former of high molecular weight is more easily precipitated than 

 the latter of lower weight. (Any correlation between coagulation 

 and molecular weight is probably due to other factors of which 

 molecular weight is only an index.) 



The relative effectiveness of salts as precipitants brings us 

 again to the question of Hardy's valency rule and the Hofmeister 

 series. When colloidal suspensions of metals or their salts are 

 coagulated by ions, an almost strict valence effect is obtained 

 (page 172); but in proteins, where hydration may play a greater 

 role than charge in determining stability, the ions become 

 arranged in a typical Hofmeister, or lyotropic, series, as Hof- 

 meister showed in the original series (page 445). 



The discussion of coagulation is continued in relation to living 

 systems (page 492). 



