CHEMICAL EFFECTS OF RESPIRATION. 239 



Priestley. The exact quantity of oxygen, which is lost in 

 natural respiration, varies in different animals, and even in 

 different conditions of the same animal. Birds, for in- 

 stance, consume larger quantities of oxygen by their res- 

 piration; and hence require, for the maintenance of life, a 

 purer air than other vertebrated animals. Vauquelin, how- 

 ever, found that many species of insects and worms pos- 

 sess the power of abstracting oxygen from the atmosphere 

 in a much greater degree than the larger animals. Even 

 some of the terrestrial mollusca, such as snails, are capa- 

 ble of living for a long time in the vitiated air in which 

 a bird had perished. Some insects, which conceal them- 

 selves in holes, or burrow under ground, have been known 

 to deprive the air of every appreciable portion of its oxygen. 

 It is observed by Spallanzani, that those animals, whose 

 modes of life oblige them to remain for a great length of 

 lime in these confined situations, possess this power in a 

 greater degree than others, which enjoy more liberty of 

 moving in the open air: so admirably have the faculties of 

 animals been, in every instance, accommodated to their re- 

 spective wants. 



Since carbonic acid consists of oxygen and carbon, it is 

 evident that the portion of that gas which is exhaled from 

 the lungs is the result of the combination of either the whole, 

 or a part, of the oxygen gas, which has disappeared during 

 the act of respiration, with the carbon contained in the dark 

 venous blood, which is brought to the lungs. The blood 

 having thus parted with its superabundant carbon, which 

 escapes in the form of carbonic acid gas, regains its natural 

 vermilion colour, and is now qualified to be again transmit- 

 ted to the different parts of the body for their nourishment 

 and growth. As the blood contains a greater proportion of 

 carbon than the animal solids and fluids which are formed 

 from it, this superabundant carbon gradually accumulates in 

 proportion as its other principles, (namely, oxygen, hydro- 

 gen, and nitrogen) are abstracted from it by the processes of 

 secretion and nutrition. By the time it has returned to the 



