THE LIVING SUBSTANCE 21 



drought, hence it is almost certain that the living substance is a 

 thici solution of certain substances in water. It has been found 

 that when solutions of complicated chemical substances, of such 

 substances as gelatine for instance, are made, the solution may 

 be perfectly transparent and yet refuse to pass through filter 

 paper. This is attributed by biochemists to the circumstance 

 that the molecules of such substances are arranged in groups 

 which are of enormous size compared to the molecules of such 

 simple substances as common salt, &c. They suppose that 

 these complicated molecules are in fact bigger than the pores 

 in the paper. "When the molecules reach an extreme size, the 

 solution ceases to be absolutely transparent and appears slightly 

 cloudy, and such a mixture we can hardly call it a solution 

 is termed an emulsion. Living substance is then supposed to 

 be such a thick solution, or in some cases an emulsion. But 

 what kind of substances form the stuff which is dissolved or sus- 

 pended in the water ] To such a question no perfectly satis- 

 factory answer can be given, for when we begin to analyse 

 living substance chemically, we kill it and then our results 

 give us the composition of dead and not of living material. 



But we may perhaps assume that when living substance is 

 killed by gentle heating its composition is not very much 

 changed. It then forms what is called proteid, which, as 

 we have learned, forms a necessary constituent in the food 

 of all animals. By treatment with solutions of salt and 

 by boiling with dilute acids, proteid can be split up into 

 still simpler substances, and by a continuance of this treatment 

 on the products of its decomposition we at last reach com- 

 paratively simple compounds whose chemical structure can be 

 recognised. These compounds contain carbon, hydrogen, 

 nitrogen and oxygen, and are of the class called amino-acids. 

 The great characteristic of an acid is that it can combine with 

 another kind of chemical compound called a base to form a 

 neutral compound termed a salt. The acid has as it were a 

 hand, by which it can grasp the hand of another compound. 

 An amino-acid is an acid which is also at the same time a 

 base in a word, it has two hands, viz. an acid hand by 

 which it can grasp a base, and a basic hand by which it can 

 grasp an acid. The base to which it unites itself may be the 

 basic portion of another amino-acid, and the acid which it 



