8 ZOOLOGY. 



stantly kept many degrees higher than that of surrounding 

 objects ; therefore there must be a constant renewal of 

 energy to balance that lost by radiation. Whence comes 

 the energy which is necessary to a rabbit's existence ? 



12. Source of the Rabbit's Energy. The chemical 

 study of the bodies of animals like the rabbbit has shown 

 them to be more than half composed of water, while the 

 remainder consists for the most part of the class of chemical 

 compounds called organic, which are combinations of carbon, 

 nitrogen, oxygen, and hydrogen, with smaller proportions of 

 sulphur, phosphorus, and other elements. Now these 

 organic compounds are exceedingly complex in their 

 chemical constitution, and as the elements of which they 

 are composed have (for the most part) less affinity for one 

 'another than for oxygen, they are unstable. All such 

 complex and unstable compounds, in the presence of oxygen 

 with which their elements can combine, afford a great 

 source of energy, and it is when this energy is transferred 

 into other forms that work is done. Every bit of work (in 

 the widest physical sense) done by the rabbit implies the 

 decomposition and oxidation of a definite amount of these 

 organic compounds. Further evidence of this is readily at 

 hand. 



13. Removal of Waste Material. A rabbit is con- 

 stantly breathing taking air into its lungs and expelling 

 it again. The composition of the ingoing and outcoming 

 air differs very definitely the latter containing less oxygen 

 and more carbon dioxide and water vapour than the former. 

 At intervals, too, the rabbit expels from its body a fluid 

 called urine. This consists of water containing in solution 

 a compound called urea (a partially-oxidized compound 

 of carbon, nitrogen, and hydrogen), with phosphates, sul- 

 phates, and other inorganic salts. From the skin also a 

 certain amount of water containing inorganic salts is 

 given off (perspiration). All these waste-materials are 

 called excretions. 



Thus the rabbit is continually taking in oxygen, and 

 giving out compounds which are obviously the products of 



