THE PROTEIDS 



23 



Indiffasibility. — 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 Ls very largely 

 used (see figs. 7 and 8). 



Proteids 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 proteids, albumin and globulin. 



Fig. 7. — Dial;. pcuing of tiie bell jar 



suspeuded lu wa.er i< ii^^iciy covere<l with parchment 

 paper. The fluid to be dialysed is placed within this 

 vessel ; the crystalloids pass out into the distilled water 

 outside through the parchmeui pajjcr. 



Fir;. 8.— la lui? i-iu; ... ■u..;;,^c. 



the substance to be dialysed 

 is placed within the piece of 

 tubing suspended in the 

 larger vessel of water. The 

 tubing is made of parchment 

 paper. 



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

 which previously kept it in solution have been removed. 



The terras ' diffasion' and 'dialysis ' should be distlBguished 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 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 fully article Osmosis 

 in Appendix. 



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

 proteid substance, and is crystalUsable (for further details, see The 

 Blood). Like other proteids it has an enormously large molecule ; 



