26 HUMAN PHYSIOLOGY 



in other words, they are organic in character. Wood, 

 animal grease, vegetable oils, alcohol these have but 

 recently come from the sphere of living organisms. 

 Coal, petroleum, and natural gas have arisen in connec- 

 tion with pre-historic life. We say of such materials that 

 they are energetic or that they have distinct fuel-values. 



The burning of fuels is a chemical process in which oxy- 

 gen, usually supplied from the atmosphere, unites with 

 their principal elements. Two of these elements, carbon 

 and hydrogen, have particularly to be considered. When 

 oxygen has united with carbon to the full extent that it 

 will do so a gaseous product, carbon dioxid, is formed. 

 This is also known as carbonic acid gas; it is familiar in 

 the bubbles of soda water and it is the chief gas going 

 up the chimney from a coal fire. When oxygen combines 

 with hydrogen, water is formed and much more water 

 arises from combustion than is commonly realized. In 

 the flame of a candle oxygen is uniting with the carbon 

 and hydrogen of the wax to produce carbon dioxid and 

 water vapor. The ascending column of hot gases above 

 the flame is composed very largely of these two compounds. 



The generation of carbon dioxid and water by the lit- 

 eral burning of organic substances is not the only mode of 

 their formation. Similar materials are constantly under- 

 going the same resolution in a more gradual way and 

 without the accompaniment of flame and smoke. If we 

 were to add " without heat" we should be wrong for it 

 has been shown that just as much heat is evolved in the 

 slowest as in the quickest oxidation of a given compound. 

 Of course the heat production will be much less obvious 

 if it is extended over a long time. 



Physiological Oxidation. The most interesting cases 

 of oxidation of the more gradual type are those which go 

 on under the influence of living matter and, indeed, con- 

 stitute the most conspicuous part of its activity. Where 

 there is life there will usually be perceptible oxidation. 

 In the plant, as in the candle, organic matter is continu- 

 ally being degraded and again, carbon dioxid and water 



