226 SOAPS AND PROTEINS 



regarding the fundamental nature of these group reactions, can 

 not be discovered. RotVivm-e is made to their " peptization " 

 and to their " coagulation " under various circumstances and 

 to the colloid-chemical equivalents in types of change encountered 

 in the physiology and pathology of protoplasm under the terms 

 liquefaction, coagulation and necrosis. 



The term " peptization " may be taken for our purposes as 

 the antonym of " coagulation." The latter term has been applied 

 to what represents at least several different types of change in 

 protein/water systems. What these have in common, however, 

 is a change in state from one in which the protein (or soap) is in 

 solution or suspension, to one in which it is aggregated, separated- 

 out or precipitated. In the terms of WOLFGANG OSTWALD the 

 changes characteristic of coagulation are essentially those of 

 decrease in degree of dispersion, in other words changes in the 

 direction of coarser division of the materials. Associated with 

 such a change may be one in water-holding power, in optical 

 properties, in viscosity, etc. There are those who would restrict 

 the term " coagulation " to such changes as prove irreversible. 

 An albumin would therefore be said to be coagulable through 

 heat or a mercury salt (since lowering of temperature or dilution 

 of the mercury salt does not bring back the albumin to its former 

 " dissolved " state); it would not be coagulable, however, through 

 saturated magnesium sulphate solution (for this on dilution allows 

 the " albumin " to resume its former state). In the latter instance 

 the change is often designated as a precipitation or " salting-out " 

 of the " albumin." It does not matter, for our purposes, how 

 the terms are used, for colloid-chemistry needs to consider them 

 all. The distinctions are, moreover, arbitrary for, as long known, 

 even heat coagulations are not completely irreversible if the high 

 temperature is not maintained too long; and we shall see later 

 that, just as in the case of the soaps, the heavy metal coagula- 

 tions of the proteins can also be "redissolved." What light do 

 the observations on soaps and the soap-like protein compounds 

 already described cast upon the nature of these coagulative 

 changes? 



In order to illustrate how we think the views developed in 

 the preceding pages should be applied for a better understanding 

 of the promises of " peptization " and " coagulation " in protein 

 systems, we shall cite examples illustrating protein change (1) 



