394 THE CHEMICAL COMPOSITION OF THE BODY. [CH. xxv. 



Heat Coagulation. Most native proteids, like white of egg, 

 are rendered insoluble when their solutions are heated. The 

 temperature of heat coagulation differs in different proteids ; thus 

 myosinogen and fibrinogen coagulate at 5 6 C., serum albumin and 

 serum globulin at about 75 C. 



The proteids which are coagulated by heat come under two 

 classes : the albumins and the globulins. These differ in solubility ; 

 the albumins are soluble in distilled water, the globulins require 

 salts to hold them in solution. 



Indiffusibility. The proteids (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. 



Proteids may thus be separated from 

 diffusible (crystalloid) substances like 

 salts, but the process is a tedious 

 one. If some serum or white of egg 

 is placed in a dialyser (fig. 337) and 

 distilled water outside, the greater 

 amount of the salts passes into the 

 water through the membrane and is 

 replaced by water ; the two proteids 

 albumin and globulin remain inside ; 

 the globulin is, however, precipitated, 

 as the salts which previously kept it 

 in solution are removed. 



The terms diffusion and dialysis should 

 be distinguished from each other. 



If water is carefully poured on the surface 

 of a solution of any substance, this substance 



gradually spreads through the water, and the composition of the mixture 

 becomes uniform in time. The time occupied is short for substances like 

 sodium chloride, and long for substances like albumin. The phenomenon 

 is called diffusion. If the solutions are separated by a membrane the term 

 dialysis is employed. The term osmosis is employed when semi- permeable 

 membranes are used (see further, under Work of the Kidney). 



Crystallisation. Haemoglobin, the red pigment of the blood, 

 is a proteid substance and is crystallisable (for further details, see 

 The Blood, Chapter XXVI.). Like other proteids it has an 

 enormously large molecule ; though crystalline, it is not, however, 

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

 not the only crystallisable proteid. Long ago crystals of proteid 

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



Fig. 337. Dialyser made of a tube 

 of parchment paper, suspended 

 in a vessel through which water 

 is kept flowing. 



