CHEMICAL COMPOSITION OF HUMAN BODY. 19 



posed into the organic and the inorganic is convenient, and 

 will be here employed. 



No very accurate line of separation can be drawn between 

 organic and inorganic substances, but there are certain pecu- 

 liarities belonging to the former which may be here briefly 

 noted. 



1. Organic compounds are composed of a larger number of 

 Elements than are present in the more common kinds of inor- 

 ganic matter. Thus, albumen, fibrin, and gelatin, the most 

 abundant substances of this class, in the more highly organized 

 tissues of animals, are composed of five elements, carbon, hy- 

 drogen, 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 com- 

 bined in an organic compound, but a large number of equiva- 

 lents or atoms of each of the elements are united to form an 

 equivalent or atom of the compound. In the case of carbon- 

 ate of ammonium, as an example among inorganic substances, 

 one equivalent of carbonic acid is united with two of ammo- 

 nium ; 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, 18, 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 or- 

 ganic 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 proneuess 

 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 equivalents 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 

 equivalents of oxygen to two of hydrogen, is among the sub- 

 stances most easily decomposed. 



The instability, on this ground, belonging to organic com- 



