424: CHEMICAL DISCOVERY AND INVENTION 



only the dextroforms among the hexoses which are capable of 

 entering into fermentation. The majority of the saccharoses 

 containing less and more carbon are unfermentable. These 

 facts are difficult to account for, except on the assumption that 

 there is a relation of spatial structure between the sugars and 

 the enzymes which appear to be the agents in the fermentive 

 process which is analogous to the interaction of lock and key. 

 There is considerable evidence that in the first stages of fermenta- 

 tion there is production of compounds into which the molecule 

 of the sugar enters, and, in fact, a hexose phosphate C 6 H 10 4 

 (H 2 P0 4 ) 2 has actually been isolated. 



The whole process, however, is still imperfectly understood 

 and will continue to furnish problems for research. Changes of 

 a kind similar to those which go on in fermentation probably occur 

 in the tissues of both animals and plants and the subject is one 

 which is of great importance in physiology. 



CHAPTER XXIX 



PROTEINS OR ALBUMINOUS SUBSTANCES 



THESE are slimy, glutinous, or gelatinous substances which form 

 the basis of the animal body, and which also occur in the juices 

 of vegetables. 



Chemists and physiologists between them have been so busy 

 that they are able to distinguish and characterise some fifty 

 substances which are gathered under this name. Some of these 

 are familiar enough in white of egg, in the serum of blood, in 

 milk, in animal cartilage, bone, and horn, in the gluten of wheat 

 flour, and in many seeds such as peas, beans, and almonds. They 

 are all, as found in nature, mixtures of highly complex compounds 

 containing the elements carbon, hydrogen, nitrogen, oxygen, 

 usually sulphur, sometimes also phosphorus and iron. 



The study of these compounds has been proceeding, it may be 

 said, long before systematic chemistry began, for some of the 

 old chemists two hundred years ago learned to recognise a 

 chemical difference between substances of this kind which are 

 particularly characteristic of animal matters, and the materials 

 of which wood and vegetable tissues chiefly consist. When the 



