6 COLLOIDAL CHARACTER OF PROTEIDS. [BOOK I. 



When solutions of the proteids are dried at a gentle heat so as to 

 drive off the water in which they are dissolved, or with which they 

 are combined, they appear as translucent and perfectly amorphous 

 solids, which break with a vitreous fracture, and furnish, when 

 triturated, a yellowish-white or white powder. Unless it has been 

 subjected to a high temperature, the powder thus obtained by 

 evaporating watery solutions of proteids, is found to be again soluble 

 in water. By exposure to too high a temperature the body may be 

 rendered insoluble. 



Proteids Solutions of all proteids are found to be non-dif- 



are Colloids, fusible through parchment-paper, and this property 



i.e. non-dif- allows us in certain cases to separate proteids from 



other matters with which they are mixed, and in some 



cases even to separate one proteid from another. 



Thus the chief proteid constituent of the blood is a body termed 

 serum-albumin. If this body, which is soluble in water, be pre- 

 sent in a solution which contains saline ingredients and diffusible 

 organic bodies, such for instance as sugar or urea, we can effect 

 the separation of the albumin by taking advantage of its properties 

 as a colloid. If we place the solution in a dialyser (Fig. 1 and 

 Fig. 2), i.e. in a suitable vessel where it may be in contact with 

 one side of a surface of parchment-paper, the other side of which 

 is immersed in pure water, which is frequently renewed, the diffu- 

 sible or so-called crystalloid constituents, such as the soluble salts, 

 the sugar and the urea, will pass through the parchment-paper into 

 the water, and there will be ultimately left within the dialyser a 

 solution of pure serum-albumin; if there be present in the original 

 solution not only albumin which is soluble per se in water, but such 

 a proteid as paraglobulin, which is held in solution by the water in 

 virtue of the salts which may be present, as these diffuse out it is 

 precipitated, so that 'by the process of dialysis alone we may succeed 

 in separating not only the proteids from diffusible admixtures, but, in 

 certain cases, to separate partially one proteid from another. 



The process of dialysis is one which is frequently of great use in 

 physiological chemistry. Various methods of carrying on the process are 

 employed. In some cases the dialyser is made by stretching and tying a 

 sheet of moist parchment-paper over a Loop of gutta percha ; the liquid 

 to be dialysed is then placed in this dialyser, which is immersed in a 

 larger vessel containing water (Fig. 1). A convenient form is made of glass 

 of the shape shewn in Fig. 2, the parchment-paper being tied across the 

 wide open mouth of a bell of glass, which is suspended iu water by its 

 narrower neck. 



Of late, hollow tubes of parchment-paper have been sold for the manu- 

 facture of sausages, and these serve admirably as dialysers ; the fluid to 

 be dialysed being placed within the tube, which is suspended in water. 

 In this case, as also in using the instruments shewn in Fig. 1 and Fig. 2, 

 it is often advisable to arrange for a constant influx and efflux of water 

 from the vessel in which the dialyser is immersed. 



