ORGANIC CHANGES. 153 



352. It is certainly a very surprising fact that so 

 many different substances should be formed by the 

 combination of the same elements in different pro- 

 portions. Nothing can well be more dissimilar than 

 oil and sugar, flax and starch; yet it is easily proved 

 that they all consist of the same elements — carbon, 

 oxygen, and hydrogen. 



353. The knowledge of this might naturally lead 

 one to suppose, that if the whole difference between 

 such substances consists in the relative proportions of 

 carbon, oxygen, and hydrogen which they contain, it 

 might be possible, by some chemical operation, to 

 take away a small portion of carbon or hydrogen, 

 and thus, by altering the relative proportions of the 

 elements, to convert starch into gum, or flax into 

 starch. Now this can really be done ; and strange 

 as it may appear, it is nevertheless true, that, by 

 very simple means, it is easy to change gum, starch, 

 and lignin, &c., into each other. 



354. The various vegetable substances, though so 

 different in properties, are very similar in chemical 

 composition, and may for the most part be readily 

 converted or changed into each other, by simple 

 means. They may, when pure, be preserved un- 

 changed for an unlimited time, if quite dry ; but 

 when exposed to air and moisture they sooner or 

 later begin to decompose. Those which consist of 

 carbon, hydrogen, and oxygen alone, are far less 

 prone to decompose than those which contain in addi- 

 tion nitrogen ; and these latter, when decaying, posr 



