COLLOIDS, OR THE MATERIAL OF LIFE 125 



are regulated by the presence of specific bodies known as 

 enzymes. These are organic bodies in the colloid state, whose 

 chemical structure is as yet unknown. Their general charac- 

 teristics and individual properties are, however, sufficiently 

 established to leave no doubt of their existence. Their 

 general properties are exactly similar to the known catalysts. 

 For instance, a colloidal solution of platinum may be prepared 

 by passing an electric spark through water between two threads 

 of the metal. The platinum gradually disintegrates, the liquid 

 turns yellow and finally brown. Three hundred litres of this 

 solution contain about one gram of platinum in an extreme state 

 of subdivision, the particles having a size of only some hundred 

 thousandths of a millimetre. By this and other methods, 

 sols of gold, silver, cadmium, iridium, and numerous other 

 substances have been prepared. Examples are shown in the 

 Science Museum, South Kensington. Some of them are 

 beautifully coloured, or exhibit a characteristic opalescence. 

 Such a platinum sol decomposes hydrogen peroxide, reddens 

 aloin, turns guiac blue, and is characterised by an optimum 

 temperature. It is first activated, then inhibited, by a trace of 

 alkali, and its action is destroyed by poisons such as prussic 

 acid, sulphuretted hydrogen, and mercuric chloride. It has, in 

 fact, quantitatively, the properties of an oxidase, a definite 

 class of enzyme. The study of biochemical catalysts has 

 already gone a long way to explain the assimilation by plants 

 of carbon, hydrogen, and oxygen, their condensation into 

 carbohydrates, the origin of fats, the decomposition of these 

 substances in the organism, the functions of nutrition and 

 respiration, and divers other phenomena of life. 



Of many other characteristic properties of colloidal solutions, 

 their electric charge, their precipitation by salts, its analogy 

 with the coagulation of the blood and the agglutination of 

 bacteria, and the utilisation of the opposite effect in the 

 transfusion of blood, which saved so many valuable lives during 

 the war, space forbids us to speak. We pass to the consideration 

 of the second type of colloid condition — the gel state. 



The coagulation of a sol results, as a rule, in the formation 

 of either a gelatinous precipitate or a jelly occupying the whole 

 volume of the liquid. The transformation may be permanent 

 or reversible. The coagulation of egg white by heat is an 

 example of the former change, the setting of a hot gelatin solu- 

 tion illustrates the latter. The structure of gels has been a 

 matter of controversy until the present time. Consideration 

 of the causes of " gelation " has indicated the general solution 

 of the problem. We have to inquire why a hot concentrated 

 solution of soda should deposit crystals on cooling and a gelatin 

 sol set to a jelly. Only little by little has the necessary evidence 



