COLLOIDS 41 



There is no sharp line between the two groups, the passage is 

 gradual, ami it is impossible to say where one group ends and 

 the other begins. Many colloids such as haemoglobin are 

 crystallizable, and many crystallizable substances are coagul- 

 able. Many substances appear at one time in the crystalloid 

 state and at another time in the colloidal state, so that instead 

 of dividing substances into colloids and crystalloids, we should 

 rather consider these expressions as denoting different phases 

 assumed by the same substance. 



In order to define clearly our various classes and divisions, 

 we are apt to exaggerate slight differences of properties or 

 composition. We say that colloids have no osmotic pressure, 

 whereas in fact the osmotic pressure of the colloids though 

 feeble plays a very important part in the phenomena of life. 



So in other departments of science — a factor which is 

 almost infinitesimal may yet exercise a vast influence on the 

 results. It is by infinitesimal variations of pressure, a 

 thousandth of a millimetre or less, that we obtain the various 

 degrees of penetration in the Rontgen rays. 



The division into solutions and pseudo-solutions or sus- 

 pensions is also an arbitrary one. A true solution is also 

 a suspension of the molecules of the solute. There is no 

 essential difference between a solution and a suspension, but 

 only a difference in the size of the molecules, or agglomerations 

 of molecules, in one case so small as to be transparent, and in 

 the other case just big enough to diffuse light. There are 

 moreover many properties common to colloidal solutions and 

 suspensions of fine powders, such as kaolin, mastic, charcoal, or 

 Indian ink. These particles in suspension are precipitated by 

 solutions of electrolytes in a manner similar to the coagulation 

 of colloids. 



The surface of every liquid is covered by a very thin layer, 

 a sort of membrane slightly differentiated from the rest 

 of the liquid. This membrane may be a chemical one, a 

 pellicular precipitate like that which is formed by the contact 

 of two membranogenous liquids. On the other hand, the 

 membrane may not differ from the subjacent liquid in 

 chemical composition, but only in physical properties. If we 



