4 MANUAL OF ZOOLOGY. 



three elements united, and are therefore called "binary" or 

 "ternary" compounds. On the other hand, organic bodies 

 are composed of few elements, and these are almost always 

 combined. Indeed there are only four principal organic ele- 

 ments namely, carbon, hydrogen, oxygen, and nitrogen ; and 

 of these the first is so much the most important, that Organic 

 Chemistry has been appropriately termed the " Chemistry of 

 Carbon." Furthermore, the combinations of the elements in 

 organic compounds are complex, the resulting substances being 

 mostly " ternary," "quaternary," or "quinary" compounds; 

 and there is generally a larger number of atoms or equivalents 

 of the combining elements than is usually the case among in- 

 organic bodies. Thus, carbonate of lime consists of no more 

 than one atom of calcium, one of carbon, and three of oxy- 

 gen. On the other hand, albumen, which may be taken as a 

 typical organic substance, consists of 144 atoms of carbon, 

 no atoms of hydrogen, 18 atoms of nitrogen, 42 atoms of 

 oxygen, and 2 atoms of sulphur. Haemoglobin (the red col- 

 ouring-matter of the blood), again, is stated by Thudichum to 

 consist of no less than 1875 atoms of no more than six ele- 

 ments. Iron, however, exists in the blood, not improbably in 

 its elemental condition ; and copper has been detected in the 

 liver of the mammalia, and largely in the red colouring-matter 

 of the feathers of certain birds, in the latter instance being in 

 a condition of loose chemical combination. 



(2.) As the result of the large number of atoms which enter 

 into the composition of organic bodies, we find that substances 

 of this class are singularly unstable the stability of all chemical 

 combinations, even amongst inorganic bodies, generally de- 

 creasing in direct proportion to the increased number of atoms 

 associated in the compound. Organic bodies, being composed 

 of much larger aggregations of atoms than inorganic, are pro- 

 portionately more unstable; and this instability is increased 

 by the fact that many organic substances contain nitrogen, an 

 element of feeble and undecided affinities, and also by the fact 

 that all those which are of natural and normal occurrence in 

 the living body, are in this state more or less completely per- 

 meated with water. 



Hence, the primary organic substances, such as enter directly 

 into the composition of living beings, are so unstable that we 

 usually speak of them as decomposing or breaking up " spon- 

 taneously," when removed from the influence of the living 

 organism. So long as they form part of the actually living 

 body, they are to some extent stable, but when removed from 



