ALBUMENOID SUBSTANCES. 85 



corresponding with albuminous matters in chemical composition. It 

 is known as " vegetable albumen." In peas and beans there is also 

 a substance, termed " legumine," similar to the caseine of milk. It is 

 not coagulated by heat, but is thrown down both by the organic acids 

 and by magnesium sulphate in excess. According to some observers it 

 is also coagulable by rennet. The cereal grains, and especially wheat, 

 contain a substance insoluble in water, which, in its tenacity, extensi- 

 bility, and elasticity, resembles coagulated fibrine. It is this substance 

 which gives consistency to the dough made from wheaten flour, enabling 

 it to retain the starchy materials in a consistent mass. 



FERMENTS. 



The substances belonging to this group are distinguished from the 

 preceding ; First, by their inferior quantity. Their amount is usually 

 too small to allow either of exact chemical analysis or quantitative 

 determination ; and they are known more from their effects than from 

 their physical characters. Secondly, their action is one of catalysis. 

 They do not directly form the materials of nutrition, but they cause in 

 these materials the changes requisite for assimilation. For this reason 

 they are not perceptibly consumed in the process; a very small quan- 

 tity of the ferment being sufficient to produce the needed result in a 

 large quantity of material. Thirdly, when heated, in watery solution, 

 to the boiling point, their properties are changed and they become 

 inactive as ferments. This fact, which is denied by some writers, has 

 been unmistakably evident in our own observations, and appears to be 

 amply confirmed by the experience of others.* Fourthly, the ferments 

 are precipitated from their solutions by alcohol in excess. But there is 

 a marked difference, in this respect, between them and the albuminous 

 matters proper. An albuminous matter, coagulated by alcohol, is per- 

 manently altered, and cannot be again rendered soluble except by 

 means which still further modify its character. A ferment, on the 

 other hand, when thrown down by alcohol, may be kept in this con- 

 dition for an indefinite time ; and, after the removal of the alcohol, if 

 redissolved in water, will again exhibit its characteristic activity. The 

 ferments may also be extracted and preserved by the action of glycerine, 

 which is often used as a convenient means for their preparation. 



1. Ptyaline. 



This is a ferment belonging to human saliva, which has the prop- 

 erty of converting boiled starch into sugar. Its action takes place 

 most readily in a slightly alkaline solution, at the temperature of the 

 living body. It is obtained in comparative purity by adding to 

 the saliva dilute phosphoric acid, and neutralizing the solution with 

 lime-water. The precipitate of lime phosphate thus produced brings 



* Hoppe-Seyler, Physiologische Chemie. Berlin, 1878, p. 113. Gorup-Besanez, 

 Lehrbuch der Physiologischen Chemie. Braunschweig, 1878, p. 504. Ewald, Die 

 Lehre von der Verdauung. Berlin, 1879, p. 1:2:2. 



