COLLOIDS, OR THE MATERIAL OF LIFE 121 



applicability. But, perhaps, it is in the biological field that the 

 subject is finding the most interesting of its applications. 

 The properties of living things are intimately connected with 

 their colloid condition, and colloid chemistry has already gone 

 a long way towards the explanation of the fundamental pro- 

 cesses which take place in the organism. 



Although its foundations were laid by Graham some sixty 

 years ago, most of our knowledge of colloids has been acquired 

 during the last two or three decades. The rapid progress 

 of the science, together with the far-reaching importance of its 

 applications, make it peculiarly fascinating for study. What, 

 then, is the fundamental distinction between " crystalloid " and 

 " colloid," to which the striking properties of the latter condi- 

 tion are due ? How does the state of a lump of washing-soda 

 or sugar differ from that of a piece of jelly ? Merely in the size 

 of their ultimate particles. A crystal of soda is built up of an 

 ordered arrangement of the molecules of that substance alone. 

 Excepting those in the surface layers of the crystal, each 

 molecule of soda is surrounded by others of like kind. The 

 substance is the same throughout. On the other hand, a piece 

 of jelly or charcoal is composed of an enormous number of 

 extremely tiny particles of gelatin or carbon, too small to be 

 seen with the most powerful microscope, but which are yet 

 larger than single molecules, and the spaces between the 

 particles are filled with liquid, or air, as the case may be. 

 Ultimately the substance is granular. It is to this extremely 

 fine subdivision that the properties of colloids are due. In 

 this condition the surface area of the particles has been 

 developed enormously in comparison with their bulk. Now 

 molecules in the surface layers of a particle are in a condition 

 very different from that of those in the interior. The latter 

 are subject to molecular attraction in every direction, so that 

 the bulk of a substance is under an enormous pressure, to 

 which its cohesion is due. In the case of a liquid like water 

 the internal pressure amounts to over 10,000 atmospheres. In 

 many solids it must be very much greater. Since molecules in 

 the surface skin are not attracted by similar particles on the 

 outside, the internal pressure falls off, through this layer, to that 

 of the air or medium with which the body is in contact, and 

 there is a free external force of attraction, emanating from the 

 molecules in the surface, which is exerted upon anything 

 coming within the small radius through which these universal 

 molecular forces are appreciable. Such unbalanced forces 

 are experienced at the surfaces of all bodies. With liquids 

 they give rise to the effects of surface tension, as when 

 a small quantity of water collects into a drop. Probably they 

 are greater in solids, though less easy to observe. But, 



