THE PROTEIN SUBSTANCES. 83 



different composition. The water present amounts to about 

 50 per cent, the proteins to 16 per cent, the fat to 30 per cent, 

 or more, while the ash is about i per cent. The fat contains 

 a notable quantity of lecithin. 



GLOBULINS. 



The proteins of this group differ from the albumins mainly 

 with respect to solubility in water. In pure water they are 

 practically insoluble, but they dissolve in moderately dilute salt 

 solutions. On diluting a globulin solution of this kind precip- 

 itation follows. Globulin solutions coagulate by heat in much 

 the same manner as observed with the albumins, but in general 

 they become permanently insoluble even more readily than do 

 the albumins. 



The preparation of pure globulin is even more difficult than 

 the preparation of pure albumin. The globulin must first be 

 separated by precipitation with some salt; as the salt is later 

 removed by dialysis the globulin remaining becomes insoluble, 

 which makes further treatment difficult. Globulins are not 

 known in crystalline condition. 



SERUM GLOBULIN. This substance makes up a large frac- 

 tion of the protein in blood serum, amounting to nearly as 

 much as the serum albumin. For a long time it was con- 

 founded with the latter, and it was only after a lengthy series 

 of investigations by different chemists that its true nature was 

 recognized. This globulin may be discovered easily in the 

 serum when the latter body is diluted with water, but the sepa- 

 ration is never quite complete by the water treatment alone, as 

 a portion always remains in solution. By salting out with 

 ammonium sulphate to half saturation or with magnesium sul- 

 phate completely the desired end is reached. 



The coagulation temperature of serum globulin is given as 

 75 3 and the specific rotation as [a]^ = 48, but these num- 

 bers are somewhat uncertain, especially the latter. 



Other Globulins. Several other bodies are described as 

 globulins. The most important of these is the so-called cell 



