PROTEINS 481 



then distinguish between irreversible and reversible coagulation. 

 A sharp distinction is to be made between the coagulation of 

 metal sols by added electrolytes and that of proteins by heat. 



Coalescence is the fusing of (usually) liquid droplets to form a 

 continuous mass of the liquid substance. It takes place in the 

 breaking of emulsions, as in the churning of cream to form butter. 



Flocculation is a form of coagulation involving the clumping 

 together of ultramicroscopic particles into outwardly visible and 

 usually slowly falling discrete particles. The word distinguishes 

 only a superficial appearance. 



Agglutination is a term applied to the aggregation of living 

 cells such as bacteria, which come together when a salt or immune 

 serum is added to them and form a reversible mass. As the 

 bacteria can be shaken apart and redispersed, the clumping 

 together is a form of reversible coagulation. 



Salting out differs from certain other forms of coagulation in 

 that it involves dehydration rather than decharging. It takes 

 place only at high salt concentrations. (The term is used in 

 soap manufacture.) 



We may, then, distinguish at least four main types of phe- 

 nomena which cause molecules or particles in suspension to come 

 together and fall, or form a solid mass, viz., precipitation (in the 

 strict chemical sense), the coagulation of metal sols (by electro- 

 lytes), the coagulation of proteins (by heat, salts, enzymes, etc.), 

 and the gelatinization (setting) of jellies. 



The Mechanism of Coagulation. — The coagulation of a posi- 

 tively charged colloidal suspension is accomplished by the addi- 

 tion of a negatively charged suspension. That the charges, by 

 neutralizing each other, are actually responsible for the precipita- 

 tion can be shown by adding a colloidal suspension of the same 

 charge as to sign and by contrasting the effects of electrolytes 

 with those of nonelectrolytes. While there can be no doubt that 

 certain precipitation and coagulation phenomena are electrical 

 in nature, in that they involve a reduction in surface charge, there 

 are some interesting cases where reduction in charge is not the 

 initial but the final step in the process. Thus, suspensions of 

 like charge are not indifferent to each other and may lower 

 stability when combined. This is true of sulphur and arsenous 

 sulphide solutions — both negative — and of sulphur and silver 

 solutions — also both negative. Radiation lowers the stability 



