IN PROTEIN SOLUTIONS 17 



tions. The process is termed reversible when a simple reversal 

 of the procedure which led to coagulation causes redispersion. 

 For instance, a jelly which has set on cooling melts again when 

 heat is applied ; whereas a protein which has been coagulated 

 by heating does not redissolve on cooling, and is therefore said 

 to have undergone an irreversible coagulation. 



If, when we cause a coagulation to pass again into the sus- 

 pension which originally existed, we proceed in this reversal by 

 the same path as that traversed in the coagulation process, only 

 in the opposite direction, so that the same state is produced by 

 the procedure, independently of the order of the procedure, such 

 a process is called a homodrome reversible change. If, however, 



c 



FIG. i. Homodrome and heterodrome changes of state. 



the reversal must go by a path different from the original one, 

 so that similar procedure gives rise to various states of existence 

 in the colloid, the process is heterodrome reversible. Homodrome 

 reversibility is only realised in practice in a very restricted 

 range and in a gradual course of reversion of a colloid.* 



Another property of colloids which was noticed even by 

 Graham is connected with their reversible coagulation ; that 

 is, their tendency to alter with age, which gives them the 

 qualities of a more or less unfinished product. The reactions 

 and alterations incidental to the preparation of a colloid do not, 

 in general, terminate with the appearance of the colloidal 

 character, but gradually proceed further after this point is 

 reached. Lyophobe colloids, in particular, show these changes, 



* Homodrome reversible changes, in the strict sense, do not occur in 

 nature, but merely form an ideal case. Nevertheless, the above classi- 

 fication has its practical uses. See Wo. Pauli, Naturw. Rdsch., 1902, 17, 

 Nos. 25, 26, 27, 



