OF BODIES DESTITUTE OF NITROGEN. 271 



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 

 existing equilibrium of their elements should be 

 disturbed; and the most common cause of this 

 disturbance 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 

 contained 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 

 decomposed. 



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 of the atmosphere 

 having an influence upon the changes of organic 

 substances. The juice of grapes which were 

 expressed 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. When the juice 



