230 SOAPS AND PROTEINS 



metal displaces another explain much of what is ordinarily 

 described under the head of the " peptization," " precipitation " 

 or " coagulation " of the protein colloids. 



When potassium hydroxid, for example, is added to a sodium 

 caseinate/ water system it is " peptized " and becomes more 

 liquid; while through the addition of magnesium, calcium or iron 

 salts precipitation or " coagulation " is produced. We prefer to 

 say that in the first instance materials are formed (potassium 

 caseinate) which are more soluble in water, wherefore the whole 

 system tends in the direction of the less viscid true solution; 

 while in the second, materials are produced which are less soluble 

 in water and are possessed of a lower hydration capacity. Hence 

 their separation from the dispersion medium. 



As reversion from a state of low hydration, low solubility and 

 precipitation to a state of higher hydration and " solution " is most 

 easily obtained in the case of the soaps when the alkali metals are 

 involved, is more difficult when those of the alkaline earths are 

 considered and is generally said to be impossible in the case of the 

 heavy metal soaps, so also in the case of the proteins are the similar 

 reversions accomplished with increasing difficulty and more and 

 more slowly as we pass from the alkalies through the alkaline 

 earths to the heavy metals. The heavy metal salts are for this 

 reason regularly listed as " coagulants " of the proteins, while 

 those of the alkaline earths occupy an ambiguous middle ground. 

 The light metal salts act merely as " precipitants " for the proteins. 

 As previously emphasized 1 and to be touched upon again 2 these 

 facts are of importance not only for the understanding of the 

 nature of certain coagulations but embody the principles which 

 must be employed when such coagulations appear in living matter 

 in consequence of heavy metal poisoning. 



3 



It is necessary now to discuss the effects of temperature upon 

 the proteins, associated with which is the question of their heat 

 coagulation in order to see where this set of phenomena has its 

 analog in the colloid-chemistry of the soaps. 



1 MARTIN H. FISCHER and MARIAN O. HOOKER: Science, 43, 469 (1918). 



2 See page 240. 



