COMBUSTION. 151 



er hand, requires a much higher temperature to ignite it. 

 Friction will do it, but it must be brisk and long continued. 

 By increasing the quantity of oxygen present combustion 

 will take place with less heat than is ordinarily required. 

 This is the cause of spontaneous combustion in many cases, 

 as noticed in 192. In the brisk and continued burning of 

 iron or steel in oxygen gas, 59, we see the influence of an 

 abundance of oxygen about the iron, in contrast with the 

 mere spark that flies off in striking fire in the air, which is 

 only one-fifth part oxygen. 



195. Ordinary Oxidation a Slow Combustion. As carbon 

 and hydrogen in burning unite with oxygen, forming car- 

 bonic anhydride and water, so do the metals, forming ox- 

 ides. It is indeed this union which is the combustion. It 

 follows, then, that the gradual oxidation of the metals, the 

 rusting of iron, copper, zinc, etc., is a combustion a slow 

 fire. And it undoubtedly produces as much heat in the ag- 

 gregate as rapid oxidation does, though the process is so 

 very slow that the heat at any one moment is so little as 

 to be imperceptible. 



196. Sun-Bleaching is Combustion. The old mode of 

 bleaching by exposure to the sun, grass-bleaching as it is 

 termed, is an example of oxidation that is, combustion. 

 By the influence of the sun's light the oxygen of the air is 

 made to unite with the coloring matter of the cloth, and so 

 this is burned up, the product passing off in the air, just as 

 the products of ordinary combustion do. If the cloth be 

 exposed too long, some of the substance itself is burned up, 

 lessening the strength of the cloth, or rotting it, as it is 

 commonly expressed. The reason that the coloring matter 

 is affected before the substance is that it is more combusti- 

 ble, or, in other words, more readily oxidized. 



197. Animal Heat. The heat of the body is maintained 

 by a real combustion, though without light. To produce 



