36 THE VEGETABLE PROTEINS 



other seed globulins when dialysed yield no precipitates, but slightly 

 acid solutions yield such abundantly. 



C. Precipitation by Acids. 



Precipitation by acids may be due either to the acid uniting with a 

 base, already combined with the protein to form a soluble compound, 

 or by the formation of a salt of the protein insoluble in water. The 

 formation of a precipitate on neutralisation, therefore, depends on the 

 nature of the protein. Edestin dissolved in dilute potassium or sodium 

 hydrate is precipitated by adding enough acid to combine with all of 

 the alkali that is present, for the free edestin is insoluble in water. A 

 solution of legumin is not thus precipitated, for the free legumin is sol- 

 uble, but a very little more acid added to the solution precipitates an 

 insoluble salt of legumin. A further addition of acid beyond the 

 amount required to form this salt dissolves the legumin, the quantity 

 necessary depending upon the proportion of mineral salts contained in 

 the solution, for the soluble salts of most seed proteins are insoluble in 

 the presence of small amounts of inorganic salts. Thus edestin dis- 

 solved in the least possible quantity of hydrochloric acid necessary for 

 its solution is precipitated by traces of most mineral salts, but a slight 

 excess of acid requires the addition of more salt for precipitation. The 

 precipitation of most seed proteins by acids, therefore, depends largely 

 on the presence of mineral salts in their solutions. An excess of acid 

 appears to act in much the same way, although a larger molecular con- 

 centration of acid than of salt is necessary to effect precipitation. 



