8o PHYSIOGRAPHY. [chap. 



the combustible substances in all cases combining with the 

 oxygen to fonii oxides. Some of these oxides are solid 

 substances, whilst others are gaseous. Every act of com- 

 bustion in air depends on the presence of oxygen. When 

 a piece of magnesium wire burns with its dazzling splendour, 

 the metal combines with the oxygen of the air to form 

 oxide of magnesium or magnesia, which, after the combus- 

 thus is left behind as a light white solid substance. When 

 a piece of charcoal burns in air, the solid disappears, with 

 exception of a little ash ; the charcoal has, in fact, combined 

 with oxygen to form an oxide which is an invisible gas known 

 as carbon dioxide or more commonly as carbonic acid. All 

 our ordinary combustibles — such as coal, wood, oil, tallow, 

 and wax — contain a large proportion of carbon ; and, con- 

 sequently this gas is produced in considerable volume 

 during their combustion. In like manner, the respiration of 

 animals depends upon the presence of oxygen in the 

 medium by which they are surrounded, whether air or 

 water. Respiration is, in fact, a kind of slow combustion, 

 in which the oxygen, taken into the system through either 

 lungs or gills, is consumed in the formation of oxidised 

 products, such as carbonic acid gas. Oxygen is therefore 

 as needful to support animal life as to support flame, and 

 hence it was at one time known as " vital air." After 

 death, again, the matter which was once living is subject 

 to a process of oxidation or slow combustion, by which it is 

 converted, for the most part, into compounds which contain 

 a larger proportion of oxygen. Oxygen is therefore essential 

 to the maintenance of combustion, respiration, decay, and 

 many other natural and artificial processes in daily operation. 

 In pure oxygen, all these actions would be carried on with 

 undue energy, and the great use of the nitrogen in the air 

 appears to be that of tempering the activity of the oxygen 



