CHEMICAL COMPOSITION OF HUMAN BODY. 9 



most abundant substances of this class, in the more highly 

 organized tissues of animals, are composed of five elements, 

 carbon, hydrogen, oxygen, nitrogen, and sulphur. The 

 most abundant inorganic substance, water, has but two 

 elements, hydrogen and oxygen. 



2. Not only are a large number of elements usually 

 combined in an organic compound, but a large number of 

 equivalents or atoms of each of the elements are united to 

 form an equivalent or atom of the compound. In the case 

 of carbonate of ammonium, as an example among inorganic 

 substances, one equivalent of carbonic acid is united with 

 two of ammonium; the equivalent or atom of carbonic acid 

 consists of one of carbon with two of oxygen ; and that of 

 ammonium of one of nitrogen with three of hydrogen. But 

 in an equivalent or atom of fibrin, or of albumen, there 

 are of the same elements, respectively, 72, 22, 1 8, and 

 112 equivalents. And together with this union of large 

 numbers of equivalents in the organic compound, it is 

 further observable, that the several numbers stand in no 

 simple arithmetical relation one with another, as the 

 numbers of equivalents combining in an inorganic com- 

 pound do. 



With these peculiarities in the chemical composition of 

 organic bodies we may connect two other consequent facts; 

 first, the large number of different compounds that are 

 formed out of comparatively few elements ; secondly, their 

 great proneness to decomposition. For it is a general 

 rule, that the greater the number of equivalents or atoms 

 of an element that enter into the formation of an atom of 

 a compound, the less is the stability of that compound. 

 Thus, for example, among the various oxides of lead and 

 other metals, the least stable in composition are those in 

 which each equivalent has the largest number of equiva- 

 lents of oxygen. So, water, composed of one equivalent 

 of oxygen and two of hydrogen, is not changed by any 

 slight force ; but peroxide of hydrogen, which has two 



