142 PHYSIOLOGY CHAI>. 



two solutions, in respect of the same volume of water, becomes 

 equal for the two fluids, although their chemical constitution 

 remains unlike, since the one contains only urea and the other^ 

 sugar. 



Solutions which are of equal molecular concentration, and 

 are therefore called equi-molecular, have also the same osmotic 

 pressure and are termed isotonic (from lu-os, equal, and TOI/OS, 

 tension). In fact, when separated by a semi-permeable membrane 

 they are found to be in osmotic equilibrium. 



According to a law discovered by Eaoult (1S82), each molecule 

 of any substance dissolved in a given quantity of water lowers the 

 freezing-point of the water by a certain and always constant 

 degree, so that the lowering of the freezing-point depends on the 

 number of molecules dissolved, and not upon their weight or their 

 chemical constitution. The determination of the freezing-point 

 of different solutions is termed Cryoscopy, and the difference 

 between the freezing-point of the solution and that of the pure 

 solvent is indicated by the symbol A. 



The cryoscopic method serves indirectly, by an easy technique, 

 to determine the molecular concentration and the osmotic pressure 

 of any given solution. 



The salts in general are an exception to Eaoult's Law, since 

 their solutions indicate a higher osmotic pressure than that which, 

 according to the law, should be exerted by the number of their 

 molecules. The molecules of these salts behave as if a portion of 

 them were split up. This led to Arrhenius' hypothesis of the 

 electrolytic dissociation, or ionisation, of dissolved saline molecules 

 a phenomenon which is in strict relation with the electrical 

 conductivity of the solutions of salts, acids and bases, which are 

 called electrolytes, as distinguished from the solutions of non- 

 ionisable molecules which do not conduct electricity well, and are 

 known as anelectrolytes. This ionisation again is in relation with 

 the electrolysis which can be verified in these solutions on the 

 passage of a galvanic current. The dissociation of the molecules 

 of salts increases with dilution of their solutions. 



Such in a few words is the modern physico-chemical theory of 

 solutions. We must now go on to examine some of the most 

 important results obtained by this method in regard to the osmotic 

 pressure and molecular concentration of blood serum. 



These investigations were initiated in Holland by Hamburger, 

 and continued by others. 



Hamburger's method is founded upon the resistance offered by 

 erythrocytes to diffusion of their haemoglobin when they are im- 

 mersed in a hypotonic solution, i.e. one in which the concentration 

 is less than isotonic. He sought to alter the molecular concentra- 

 tion of the circulating blood plasma, in order then to study its 

 effects on the serum collected from a small quantity of shed blood. 



