144 PHYSIOLOGY 



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 the 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 only form 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/x, 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 

 chemical reactions such as those studied in the simpler compounds usually 

 dealt with by the chemist. 



OPTICAL BEHAVIOUR OF HYDROSOLS. Nearly all colloidal solu- 

 tions present what is known as the Faraday-Tyndall phenomenon. When 

 a beam of light is passed through an optically homogeneous fluid, the course 

 of the beam is invisible. A beam of sunlight falling into a dark room is 

 rendered visible by impinging on and illuminating the dust particles in its 

 course. Each of these particles, being illuminated, acts as a centre of dis- 

 persion of the light, so that the course of the beam is apparent to a person 



