PRECIPITATION 313 



take's place is characteristic for each substance, and varies 

 from 56 C. in the case of fibrinogen to 70-80 C. for serum 

 albumin.* The reaction of the solution as well as the presence 

 of dissolved salts are factors which exercise a powerful in- 

 fluence, a slightly acid solution being most favourable for the 

 phenomenon, whereas an alkaline reaction may prevent co- 

 agulation entirely. According to Blum, formaldehyde is also 

 able to prevent heat coagulation. 



Heat coagulation is best effected as follows. The solution 

 is first boiled, and from 1-3 drops of dilute acetic acid are 

 added for each 10 c.c. of liquid, according to the amount of 

 protein present, the liquid being boiled each time before the 

 addition of each drop. 



If the amount of salts present be small, a little I per cent 

 sodium chloride should first be added, as the precipitation of 

 small quantities of protein cannot otherwise be guaranteed. 



(fr) Some proteins are rendered insoluble by the action of 

 certain ferments, e.g., the precipitation of casein from milk by 

 the action of rennet on caseinogen. 



(c) The addition of absolute alcohol to a neutral or faintly 

 acid solution of a native protein will precipitate it from solution 

 unchanged. If, however, it be left in contact with the alcohol 

 for some time, the protein is rendered insoluble and is co- 

 agulated. 



The solution must not be alkaline, and must contain a 

 small quantity of neutral salts. 



3. Optical activity. 



The solutions of all proteins are laevo-rotatory, the amount 

 varying from - 33*5 in the case of egg albumen to - 80 in 

 the case of casein. 



4. Precipitation without change. 



Certain salts, such as sodium chloride and the sulphates 

 of sodium, magnesium and ammonium, etc., have the property 

 of throwing proteins, except peptones, out of solution. This 

 is, however, purely a physical phenomenon, and must be dis- 

 tinguished from the chemical precipitation described below, 



* The coagulation temperature is not sufficiently well denned to be employed 

 as a means of identification. 



