THE LIBERATION OF ENERGY 



53 



absence of free oxygen, being able to obtain any 

 necessary supplies of that gas from compounds 

 containing it. Most plant embryos, also, are able for 

 a time to take the oxygen they need from like sources, 

 but, generally speaking, we may say that free oxygen 

 is essential to life. The chief compounds ultimately 

 resulting from the oxidation of organic compounds 

 are carbon di- 

 oxide and water. 



So long ago 

 as 1757 Black 

 showed that the 

 final product 

 both of combus- 

 tion and of re- I 

 spiration was 

 carbon dioxide, 

 but it was not 

 until Priestley, 

 twenty years 

 later, discovered 

 oxygen, that it 

 became possible 

 to compare the 

 two processes 

 in detail, as was 

 Saussure. 



As every one knows, in all the higher animals, 

 oxygen, along with nitrogen and carbon dioxide, 

 enters the lungs, gills or other respiratory organs, 

 either as a free gas or in solution in water. In mam- Respira- 

 mals, the respiratory organ, the lung (Fig. 21), consists animals, 

 of an immense number of minute cavities in whose 

 walls lies a network of extremely delicate blood- 

 vessels known as capillaries. The oxygen becomes 



FIG. 21. A.r spaces in .ung. The darker 

 lines are capillaries. ( x 350.) 



done by Lavoisier and De 



