330 EREMACAUSIS OR DECAY 



It is a very general error to suppose that organic 

 substances have the power of undergoing change 

 spontaneously, without the aid of an external cause. 

 When they are not in a state of change, it is neces- 

 sary, before they can assume that state, that the ex- 

 isting equilibrium of their elements should be dis- 

 turbed ; and the most common cause of this distur- 

 bance is undoubtedly the atmosphere which surrounds 

 all bodies. 



The juices of the fruit or other part of a plant 

 which very readily undergo decomposition, retain 

 their properties unchanged as long as they are pro- 

 tected from immediate contact with the air, that is, 

 as long as the cells or organs in which they are con- 

 tained resist the influence of the air. It is not until 

 after the juices have been exposed to the air, and 

 have absorbed a certain quantity of oxygen, that the 

 substances dissolved in them begin to be decom- 

 posed. 



The beautiful experiments of Gay-Lussac upon 

 the fermentation of the juice of grapes, as well as 

 the important practical improvements to which they 

 have led, are the best proofs that the atmosphere 

 possesses an influence upon the changes of organic 

 substances. The juice of grapes which were ex- 

 pressed under a receiver filled with mercury, so that 

 air was completely excluded, did not ferment. But 

 when the smallest portion of air was introduced, a 

 certain quantity of oxygen became absorbed, and 

 fermentation immediately began. Although the juice 

 was expressed from the grapes in contact with air, 

 under the conditions therefore necessary to cause its 

 fermentation, still this change did not ensue when 

 the juice was heated in close vessels to the tempera- 

 ture of boiling water. When thus treated, it could 

 be preserved for years without losing its property 

 of fermenting. A fresh exposure to the air at any 

 period caused it to ferment. 



Animal food of every kind, and even the most 

 delicate vegetables, may be preserved unchanged if 



