238 CHYMISTRY APPLIED TO AGRICULTURE. 



CHAPTER XV. 



ON FERMENTATION. 



The process of decomposition commences in all the pro- 

 ducts of vegetation, as soon as they are ripe, or separated 

 from the plant. Air, heat, and water, which before this 

 concurred in promoting their formation and growth, be- 

 come now the principal agents in the changes which they 

 undergo. 



The appearances and the new products resulting from the 

 decomposition of bodies, vary according to the nature of 

 their constituent principles. 



Generally speaking, all vegetable substances decay when 

 left to experience spontaneous decomposition ; but when, by 

 being exposed to mechanical pressure, those parts of the 

 fruits which have been separated are again mixed, there 

 arises a new product. The grape, for instance, rots upon 

 the vine, whilst the juice expressed from it undergoes 

 the vinous fermentation. It has for a long time been in 

 the power of art to excite, retard, and modify decomposition, 

 so as to form new articles of food and drink, for men and 

 animals. 



In vegetable products, all the principles are in a state of 

 combination and saturation, one with the other : whilst 

 the plant lives, its organic energies exert an influence 

 over external agents, and preserve in their natural propor- 

 tions the elements, which enter into the composition of the 

 products^ 



As soon sls a plant dies, or a fruit becomes ripe, a new 

 order of things tcd^es place ; the different parts of the vege- 

 table, being no longer influenced by the laws of vitality, 

 become subject to the power of external agents ; air, wa- 

 ter, and heat exercise over them an almost absolute sway ; 

 oxygen, by depriving them of their carbon, destroys the 

 proportions of their constituent principles; water produces 

 the same effect by dissolving certain portions of the sub- 

 stance ; and heat, by separating the particles, weakens 

 their union, and facilitates the action of the two other 

 powers. 



According to the experiments of M. Gay-Lussac, the 

 juice of grapes expressed in a vacuum does not ferment ; 

 but from the moment that the air is allowed admittance to 



