30 THE ALBUMINS. 



Coagulation. One of the most characteristic properties of the 

 albumins is the physical instability of their solutions and their 

 marked tendency to revert to a solid or semisolid state. This 

 enables them to play the important part which they take in the 

 construction of the various tissues, and no doubt renders possible 

 the manifold and chemically often antagonistic reactions which 

 may simultaneously occur within the bodies of the individual cells. 

 This change may be effected by apparently trivial factors, such as 

 evaporation, contact with porous substances, etc. In this respect 

 also the albumins behave very much like the inorganic colloids. 

 When a solution of sodium silicate is thus added to large excess of 

 dilute hydrochloric acid, the silicic acid which is formed is appar- 

 ently held in solution. If then the excess of hydrochloric acid, 

 together with the sodium chloride formed during the reaction, is 

 removed by dialysis, a clear solution of silicic acid remains 

 in the dialyzer. This is transformed at once into a thick, 

 gelatinous material when a small amount of carbon dioxide is 

 passed through the solution. Some of the albumins, such as the 

 globulins, behave in a similar way. In undergoing such changes 

 the albumins may retain their original properties for a while at 

 least, but after a variable period they become insoluble, and are 

 then said to be coagulated. This change can be brought about at 

 once by the application of heat, and it is important to note that 

 all true native albumins can be coagulated in this manner. After 

 coagulation their solution can only be effected by influences which 

 lead to their more or less extensive destruction. They have lost 

 those physical properties which characterized them individually as 

 albumins ; they are permanently denaturized, as Neumeister ex- 

 presses it. 



The temperature of coagulation differs with the different albu- 

 mins, and is fairly constant for the individual bodies, providing 

 that the reaction of the solution is neutral, or still better very 

 faintly acid. If the reaction is alkaline, coagulation is not com- 

 plete, and in the presence of more than traces of free alkali or an 

 alkaline carbonate it may not occur at all. A markedly acid reac- 

 tion interferes in a like manner. Cohnheim explains these peculiar- 

 ities as follows : On heating the albuminous solution the albumin is 

 denaturized, no matter what the reaction may be or whether salts 

 are present or absent. In the presence of alkalies the denaturized 

 albumins form akaline albuminates, which are readily soluble in the 

 case of the alkalies and with difficulty so in the case of the alkaline 

 earths. With an acid reaction, on the other hand, acid albumin is 

 formed, viz., the hydrochlorate, or acetate, of the denaturized albu- 

 mins, as the case may be ; this by itself is soluble in water, but 

 insoluble in the presence of salts. 



Equally important is the presence of a certain amount of salt. 

 An albuminous solution that has been freed of salt by dialysis does 

 not coagulate on heating ; if, however, salt is subsequently added 

 coagulation occurs. 



