THE COLLOIDAL STATE. 61 



One can at ^any time imagine either O or H 2 O written 

 into equation I instead of m, and so obtain the picture 

 of a simple reversible reaction an oxidation and a reduc- 

 tion, or a hydration and a loss of water, following the 

 type of a simple reaction which may take place in either 

 direction; or in equation II a condensation or a hydro- 

 lytic cleavage. General biochemistry has until now 

 taken notice of only this type of antagonistic reaction, 

 that is to say, reactions which counteract each other in 

 the sense of positive and negative values. 



There exists, however, among the reaction chains in 

 the body a second apparently very common type of 

 antagonistic reaction, the nature of which can be best 

 illustrated by certain changes in physical state that 

 colloids are capable of suffering. 



It is a well-known fact that with the customary methods 

 of investigation it is found that the melting-point and 

 the solidification- point of the crystalloids coincide. It 

 is different, however, in the case of the colloids, in which 

 these two points may lie some distance apart, even when 

 the changes in temperature are brought about most care- 

 fully. In consequence of the indolence with which 

 changes take place in colloids, superheating and under- 

 cooling are the rule. A gelatine the temperature of 

 which lies between the melting-point and the gelation- 

 point shows in consequence a peculiar behavior. If such 

 a gelatine is cooled to beyond the gelation-point and is 

 then carefully warmed back to the original temperature, 

 the gelatine remains solid; if, however, the gelatine is 

 heated to beyond the melting-point and is then carefully 

 cooled down to the starting temperature, it remains 

 liquid. A change in state, therefore, impresses itself upon 



