116 BIOLOGY OF PLANTS. 



2. Of hydrogen and oxygen, as water, which is formed of 

 OH. 



3. Of carbon and oxygen, as oxalic acid, composed of 

 C203. 



4. Of carbon and nitrogen, as cyanogen, composed of C^N. 



II. Oi'ganic principles, composed of three constituents, are 

 much more numerous than the preceding. 



1. The most common combinations are carbon, hydrogen 

 and oxygen. Sugar, gum, etc. and the greatest number of 

 acids, are thus constituted. 



2. Compounds of carbon, hydrogen and nitrogen, as azul- 

 mic acid which is thus constituted, C^HN^. 



3. Compounds of carbon, oxygen and nitrogen, as carba- 

 zotic acid, CisN^O^s. 



III. Organic principles of four constituents, as aspartic 

 acid, C^H^NO^. Almost all the alkalies contain these four 

 substances. 



Of the simple substances, carbon is most abundant, and 

 next are oxygen and hydrogen ; while nitrogen, although not 

 absent from any part of a plant, exists in very small quan- 

 tities in all. 



But the substances which constitute the principal mass of 

 every vegetable, are compounds of oxygen and hydrogen (in 

 the proportions to form water) and of carbon. In a second 

 class, oxygen is in excess, as in the numerous organic acids. 

 In a third class, as the volatile and fixed oils, carbon and hy- 

 drogen exist, but no oxygen, and in a fourth class nitrogen 

 is added, as in vegetable albumen, in indifferent substances, 

 and in some acids. 



by the number represcnlinir the equivalent for that substance, the ex- 

 act amount by weight may be obtained. The whole added together 

 gives the equivalent of the compound, which is also represented by 

 numbers. This mode of represention is now adopted by all chemical 

 writers, and seems useful to present tlie compounds to the eye in a 

 condensed form. 



