v THE BLOOD: PLASMA 141 



ability of peptonised blood depends on the fact that under the 

 action of albumoses the leucocytes and endothelial vessels pro- 

 duce a substance which gives rise in the liver to a large secretion 

 of anti-thrombin, which is subsequently poured out into the 

 circulatory torrent. 



VII. The blood plasma, of which we have enumerated the 

 principal constituents, presents as a whole a solution of organic and 

 mineral substances, which are partly in chemical combination, 

 partly a simple mixture in which the corpuscles are suspended. 

 After the physico-chemical theory of solutions had been established 

 by the work of Pfeffer, H. de Vries, Kaoult, Van't Hoff and 

 Arrhenius, the method was, later on, applied to physiology. The 

 determination of molecular concentration, osmotic pressure, elec- 

 trical conductivity and viscosity in the blood serum and other 

 tissue fluids of the body, is now of some importance, since it has 

 brought out certain striking facts which are the starting-point 

 of a new chapter on the physical properties of blood plasma. 



Let us commence with certain theoretical considerations. 



By the molecular concentration of a solution we mean the 

 number of dissolved molecules (irrespective of their chemical 

 nature) in relation to a given weight of solvent, which in the case 

 of the organic fluids is always represented by water. 



Such a solution, introduced into the graduated tube of a 

 Dutrochet's eiidosmometer, in connection with a mercury mano- 

 meter, and separated from the solvent by a semi -permeable 

 membrane (i.e. one which permits the passage of the solvent, but 

 not of the substances dissolved), sets up a current through the 

 membrane by which the solution is more and more diluted, so that 

 the manometer column rises to a certain height, after which it 

 remains stationary. The pressure then recorded by the manometer 

 represents the osmotic pressure of the given solution. 



Pfeffer showed experimentally that the osmotic pressure is 

 in direct ratio with the molecular concentration. Given this 

 relation, it follows that when the osmotic pressure of a certain 

 solution is known, its molecular concentration is known also, and 

 vice versa. 



Perfect osmotic equilibrium between two solutions is obtained 

 each time that the solutions, separated by a semi-permeable 

 membrane, contain the same number of molecules in the same 

 volume of water, even if they are of different chemical constitu- 

 tion. Suppose, for instance, a solution of urea and one of sugar 

 to be separated by a membrane that is permeable to water but 

 not to the dissolved substances. So long as one of the two 

 solutions contains a larger number of molecules dissolved in 

 the same volume of water than the other, there will be a diffusion 

 of water from the more dilute to the more concentrated solution. 

 This diffusion ceases as soon as the number of molecules in the 



