160 PHYSIOLOGY 



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 substances, 

 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 /u, 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 mole- 

 cules. As a matter of fact we find that such solutions present an 

 amazing mixture of properties, some of which betray them as mechani- 

 cal 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 

 solutions 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 illu- 

 minated, acts as a centre of dispersion of the light, so that the course of 

 the beam is apparent to a person standing on one side of it. Tyndall 

 showed that, if the particles were sufficiently minute, the light dis- 

 persed by them at right angles to the beam was polarised. This 



