424 



THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



are rendered insoluble when their solutions are heated. The tempera- 

 ture of heat coagulation differs in different proteins ; thus myosinogen 

 and fibrinogen coagulate at 56 C., serum albumin and serum globulin 

 at about 75 C. 



The proteins which are coagulated by heat come mainly under two 

 the albumins and the globulins. These differ in solubility ; 

 the albumins are soluble in distilled water, the true globulins require 

 salts to hold them in solution. 



Indiffusibility. The proteins (peptones excepted) belong to the 

 class of substances called colloids by Thomas Graham ; that is, they 



pass with difficulty, or not at all, through 

 animal membranes. In the construction of 

 dialysers, vegetable parchment is largely 

 used. 



Proteins may thus be separated from 

 diffusible (crystalloid) substances such as 

 salts, but the process is a tedious one. If 

 some serum or white of egg is placed in a 

 dialyser (fig. 314) and distilled water out- 

 side, the greater amount of the salts passes 

 into the water through the membrane and 

 is replaced by water; the two proteins 

 albumin and globulin remain inside; the 

 globulin is, however, precipitated, as the 

 salts which previously kept it in solution 

 are removed. 



Crystallisation. Haemoglobin, the red 

 pigment of the blood, is a protein substance 

 and is crystallisable (for further details, 

 see The Blood, Chapter XXIX.). Like 

 other proteins it has an enormously large 

 molecule ; though crystalline, it is not 

 crystalloid in Graham's sense of that term. Blood pigment, however, 

 is not the only crystallisable protein. Long ago crystals of protein 

 (globulin or vitellin) were observed in the aleurone grains of many 

 seeds, and in the somewhat similar granules occurring in the egg-yolk 

 of some fishes and amphibians. By appropriate methods these have 

 been separated and recrystallised. Further, egg albumin itself has 

 been crystallised. If a solution of white of egg is diluted with an 

 equal volume of saturated solution of ammonium sulphate, the globulin 

 present is precipitated and is removed by filtration. The filtrate is 

 now allowed to remain some days at the temperature of the air, and 

 as it becomes more concentrated from evaporation, minute spheroidal 

 globules and finally minute needles, either aggregated or separate, 

 make their appearance (Hofmeister). Crystallisation is more rapid if 



FIG. 314. Dialyser made of a tube 

 of parchment paper, suspended 

 in a vessel through which water 

 is kept flowing. 



