38 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



water : the true globulins are not, but require salts to hold them in 

 solution. 



Indiffusibility. The proteins (peptones excepted) belong to the 

 class of substances called colloids by Thomas Graham : they pass 

 with difficulty, or not at all, through animal membranes. In the 

 construction of dialysers, vegetable parchment is very largely used 

 (see figs. 7 and 8). 



Proteins may thus be separated from diffusible (crystalloid) sub- 

 stances like salts, but the process is a somewhat tedious one. If 

 some serum or white of egg is placed in a dialyser, and distilled 

 water outside, the greater amount of the salts passes into the water 

 through the membrane ; the two proteins, albumin and globulin, 

 remain inside. The globulin is, however, precipitated, as the salts 

 which previously kept it in solution have been removed. 



The terms ' diffusion ' and ' dialysis ' should be distinguished frorn 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 mem- 

 brane the term ' dialysis ' is employed. The word osmosis is properly restricted 

 to the passage of water through membranes, and can be best studied when 

 semi-permeable membranes are employed. See more fully article OSMOSIS in 

 Appendix. 



Crystallisation. Haemoglobin, the red pigment of the blood, is a 

 protein substance, and is crystallisable (for further details, see THE 

 BLOOD). Like other proteins, it has an enormously large molecule ; 

 though crystalline, it is not crystalloid in Graham's sense of 

 that term, although it probably forms a true solution with water. 

 Blood pigment 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 similar protein 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 half its 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 much more rapid and perfect if a little acetic or 



