142 PHYSIOLOGY 



A more convenient form of osmometer has been devised by B. Moore, 

 using parchment paper as the membrane. With this osmometer, the 

 existence of an osmotic pressure in colloidal solutions has been definitely 

 established both by Moore in the case of haemoglobin, proteins, and soaps, 

 and by Bayliss in the case of colloidal dyes, such as Congo red. The osmotic 

 pressure of haemoglobin was found y Hiifner to correspond to a molecular 

 weight of about 16,CCO, i.e. a molecular weight already deduced from its 

 composition and its combining powers with oxygen. Often however the 

 osmotic pressure is yery much smaller than would be expected from the 

 molecular weight of the substance, owing to the fact that colloids in solution 

 may be in many different conditions of aggregation. Thus the molecule 

 of colloidal silica must be many, probably thousands of times larger 

 than the molecule as represented by H 2 Si0 3 . The osmotic pressure 

 being proportional to the number of molecules in a given volume of solution, 

 the larger the aggregate the smaller would be tne total number of molecules, 

 and the smaller therefore the osmotic pressure of the solution. 



It is in consequence of the huge size of the molecular aggregates that 

 colloidal solutions, such as starch or glycogen, and probably globulin, display 

 no appreciable osmotic pressure. We cannot divide colloidal solutions into 

 two classes, viz. those which form true solutions and present a feeble osmotic 

 pressure, and those which form only suspensions and therefore exert no 

 osmotic pressure. In inorganic colloids, such as arsenious sulphide, Picton 

 and Linder have shown that all grades exist between true solutions and 

 suspensions. With increasing aggregation of the molecules, the suspension 

 becomes coarser and coarser until finally the sulphide separates in the form 

 of a precipitate. 



The measurement of the osmotic pressure of the colloids of serum points 

 to their having a molecular weight of about 30,000. Chemical evidence 

 shows that haemoglobin has a molecular weight of about 16,000, and we 

 have every reason to believe that the much more complex molecules forming 

 the cell proteins may have molecular weights of many times this amount. 

 When however we arrive at molecular weights of these dimensions, the 

 disproportion between the size of the molecules and those of the solvent, 

 water, becomes so great that a homogeneous distribution of the two sub- 

 stances, solute and solvent, is no longer possible. The size of a molecule 

 of water has been reckoned to be -7 x 10 8 mm. A molecule 10,000 

 times as large would have a diameter of -7 x 10 4 mm. -07^, a size 

 just within the limits of microscopic vision. Long before molecules attained 

 such a size they would no longer react according to the laws which have 

 been derived from the study of the behaviour of the almost perfect gases, 

 but would possess the properties of matter in mass. They have a surface 

 of measurable extent, and their relations to the molecules of water or solvent 

 will be determined by the laws of adsorption at surfaces rather than by 

 the laws of interaction of molecules. As a matter of fact we find that such 

 solutions present an amazing mixture of properties, some of which betray 

 them as mechanical suspensions, while others partake of the nature of the 



