12 ANIMAL PHYSIOLOGY. 



oxygen at a high temperature; and, when oxygen and 

 hydrogen gases are mixed, and a light applied to them, they 

 combine with explosion, producing water, which consists of 

 two equivalents of hydrogen and one of oxygen. Ammonia 

 consists of one of nitrogen and three of hydrogen; and, in 

 the complete combustion of organic matter, this hydrogen 

 may be obtained partly from the organic matter itself, and 

 partly from water, the oxygen of which is used in the forma- 

 tion of carbonic acid. In less perfect combustion, cyanogen 

 in a state of combination may be evolved instead of ammonia, 

 by the nitrogen of the organic matter combining with part 

 of the carbon, in the proportion of one equivalent of carbon 

 to one of nitrogen. 



The combustibility of organic matter depends on its con- 

 tained oxygen being less than sufficient to combine with its 

 carbon and hydrogen to form carbonic acid and water, and 

 on the complexity of its molecules. While substances found 

 native in the inorganic world consist of elements grouped in 

 pairs, in which the number of equivalents of the one sub- 

 stance bears a simple proportion to the equivalents of the 

 other, organic substances present groups of three, four, or 

 more elements gathered together in common union, with 

 many equivalents of each combined in one molecule, often 

 in proportions by no means simple; and more especially are 

 the molecules of the nitrogenous constituents of the textures 

 complex. 



The oxidation of organic matters may take place Ly other 

 means besides a burning heat. Thus it occurs in the form 

 of putrefaction at much lower temperatures, especially when 

 aided by abundant moisture. So also, oxidation of organic 

 matter and the resolution thereof either into carbonic acid, 

 water, and ammonia, or into products of less complete 

 decomposition, take place in the interior of organisms during 

 life, and are sometimes alluded to under the name of com- 

 bustion. 



4. The organic world is divisible into two kingdoms, the 

 animal and the vegetable. The power of building up the 

 complex molecules of organic matter from the separate 

 elements, or such simple combinations as carbonic acid, 

 water, and ammonia, is peculiar to vegetables, while intelli- 



