ORGANIC MATTER. 9 



being expelled from most of its combinations by a moderate 

 heat. Carbonic acid is a relatively feeble acid: the carbon- 

 ates being decomposed by the majority of other acids and by 

 ignition. The various hydro-carbons are but narrow in the 

 range of their comparatively weak affinities. The com- 

 pounds formed by ammonia have not much stability: they 

 are readily destroyed by heat, and by the other alkalies. 

 The affinities of cyanogen are tolerably strong, though they 

 yield to those of the chief acids. Of the several oxides of 

 nitrogen, it is to be remarked that, while those containing 

 the smaller proportions of oxygen are chemically inert, the 

 one containing the greatest proportion of oxygen (nitric acid) 

 though chemically active, in consequence of the readiness 

 with which one part of it gives up its oxygen to oxidize a 

 base with which the rest combines, is nevertheless driven 

 from all its combinations by a red heat. 



These diatomic compounds, like their elements, are to a 

 considerable degree characterized by the prevalence among 

 them of allotropism ; or, as it is more usually called when dis- 

 played by compound bodies isomerism. Professor Graham 

 finds reason for thinking that a change in atomic arrange- 

 ments of this nature, takes place in water, at or near the 

 melting point of ice. In the various series of hydro-carbons, 

 differing from each other only in the ratios in which the 

 elements are united, we find not simply isomerism but poly- 

 merism occurring to an almost infinite extent. In some 

 series of hydro-carbons, as, for example, the terpenes, we find 

 isomerism and at the same time a great tendency to undergo 

 polymerisation. And the relation between cyanogen and 

 paracyanogen is, as we saw, a polymeric one. 



There is one further fact respecting these diatomic com- 

 pounds of the chief organic elements, which must not be 

 overlooked. Those of them which form parts of the living 

 tissues of plants and animals (excluding water which has a 

 mechanical function, and carbonic acid which is a product of 

 decomposition) belong for the most part to one group the 



