24 BIOLOGY. [BOOK L 



position, and, like it, also exerts a special action on one of the 

 gases of the atmosphere. 



How summary soever may be the short enumeration which 

 precedes, it suffices to establish from a thorough knowledge of the 

 matter a parallel between the composition of animals and that 

 of plants, and to give saliency to the analogues and the differences. 



3. The Organic Substances of the two Kingdoms. 

 A supreme fact is evolved from the preceding examination, 

 namely, that there is in the ternary and quaternary substances a 

 dominant element common to them all, carbon. Of all organic 

 substance, carbon is the base. In weight it forms the principal 

 element thereof. The albumine of the blood contains about fifty 

 per cent, of carbon. But in organic substances carbon plays a 

 much more important part still. It is the bond of all the various 

 atoms, which compose the complex molecules of organised bodies. 

 W e have already seen that carbon is a tetratomic body, that is to 

 say, capable of fixing, of keeping wedded to one of its atoms 

 four atoms of a monoatomic body, such as hydrogen, or two 

 atoms of a diatomic body, such as oxygen ; and so on. We have 

 besides remarked that the atoms of carbon could unite with each 

 other in neutralising reciprocally one only of their affinities, the 

 others remaining free and fit to satisfy themselves, in attracting 

 and fixing either atoms of other elements or even aggregates 

 more or less complex, radicals comporting themselves as a single 

 atom. But these atoms, these radicals, are often only aggre- 

 gated to the atom of carbon which attracts them by one of their 

 affinities, while the others remain active, exciting the aggrega- 

 tion of new atoms. Let us take, for instance, the iodide of 

 methyl, that is to say, of carbonised hydrogen, an atom of iodine 

 taking the place of an atom of hydrogen : 



H 



H C I 



i 



