ORGANIC CHEMISTRY. 7 



to react upon one another, and to produce, from 

 slight variations of circumstances, a totally new 

 order of combinations. Thus a degree of heat, 

 which would occasion no change in most mineral 

 substances, will at once effect the complete disunion 

 of the elements of an animal or vegetable body. 

 Organic substances are, in like manner, unable to 

 resist the slower, but equally destructive agency 

 of water and atmospheric air ; and they are also 

 liable to various spontaneous changes, such as those 

 constituting fermentation and putrefaction, which 

 occur when their vitality is extinct, and M'hen they 

 are consequently abandoned to the uncontrolled 

 operation of their natural chemical affinities. This 

 tendency to decomposition may, indeed, be regarded 

 as inherent in all organized substances, and as re- 

 quiring for its counteraction, in the living system, 

 that perpetual renovation of materials which is sup- 

 plied by the powers of nutrition. 



It would appear that during the continuance of 

 life, the progress of decay is arrested at its very 

 commencement ; and that the particles, which first 

 undergo changes unfitting them for the exercise of 

 their functions, and which if suffered to remain, 

 would accelerate the destruction of the adjoining 

 parts, are immediately removed, and their place 

 supplied by particles which have been modified for 

 that purpose, and which, when they afterwards 

 lose these salutary properties, are in their turn 

 discarded, and replaced by others. Hence the 

 continued interchange and renewal of particles, 

 which take place in the more active organs of 

 the system, especially in the higher classes of 

 animals. In the fabric of those animals which 



