26 THE GENERAL CHARACTERS OF THE PROTEINS 



due to the serum protein alone could be directly estimated, and the 

 errors due to the presence of salts thereby eliminated. 



Moore has modified this method for estimating the osmotic pres- 

 sure of colloids. He employs a special form of osmometer, consist- 

 ing of two similar platinised capsules each of about 20 c.c. capacity, 

 each with a flange. By a special arrangement of rubber bands these 

 capsules can be screwed together. When in this position they are 

 separated by a thick platinum grid which passes between the flanges 

 and supports a parchment membrane. One capsule containing the 

 colloid is connected with the manometer, whilst the other contains 

 a liquid of which the non-colloid contents are approximately iso- 

 tonic with those of the solution containing the colloid. In this way 

 the errors due to the presence of the non-colloidal, dialysable sub- 

 stances, etc., are readily eliminated. 



Various estimations of osmotic pressure have been carried out 

 with the use of apparatus of this character, chiefly by Weymouth 

 Reid, who used the Starling form of osmometer, and by Moore and 

 Parker, and Moore and Roaf. 



Reid showed that proteins that have been well washed have 

 practically no osmotic pressure. He attributes the osmotic pressure 

 of protein solutions recorded by other observers to the adsorbed 

 inorganic substances. He found, however, for purified dogs' haemo- 

 globin an osmotic pressure varying from 3'63-4'35 mm. Hg per 

 I per cent, concentration. This is in accord with an observation of 

 Gamgee (published in abstract, Proc. R. S., 1902, 70, 79) that haemo- 

 globin possesses conductivity and is therefore in true solution. 



Moore and his co-workers observed quite appreciable pressure 

 with different proteins, which varied with the salt contents of the 

 solution. Moore maintains, in opposition to Reid, that the proteins 

 exert a definite osmotic pressure, but that their state of aggrega- 

 tion varies in solutions containing different quantities of salt ; he 

 conceives that by repeated reprecipitation, washing, etc., the aggre- 

 gates become so large that they exert no osmotic pressure. 



From the above short summary it is obvious that cryoscopic 

 methods, at any rate in the present state of our knowledge of 

 colloids, can bear but little value in fixing the character of individual 

 proteins. 



SECTION IX. THE ELECTROLYTIC CONDUCTIVITY OF PROTEIN 



SOLUTIONS. 



The difficulty of obtaining protein solutions free from electrolytes 

 with relatively high conductivity must be taken into account again 

 when measuring the electrolytic conductivity of protein solutions. 



The method has been employed in a large number of cases, as an 

 adjunct to chemical methods, e.g.> in determining the capacity of 

 proteins to form salts with acids or bases, in determining the disso- 

 ciation constants of such salts, and also in investigating the rate of 

 hydrolysis of proteins by acids or enzymes. It will be necessary to 

 refer to the conductivity of protein solutions again later when discuss- 

 ing the action of alkalies and bases (p. 39). 



