252 OXYGEN. 



is always attended with the production of heat. The smoul- 

 dering combustion of iron-pyrites and some other metallic 

 ores in the atmosphere, is a phenomenon of the same nature. 

 Most bodies which burn with flame, also admit of being oxi- 

 dated at a temperature short of redness, and exhibit the phe- 

 nomenon of low combustion. Thus, tallow thrown upon an iron 

 plate not visibly red-hot, melts and undergoes oxidation, dif- 

 fusing a pale lambent flame only visible in the dark (Dr. C. J. 

 B.Williams). If the tallow be heated in a little cup with a 

 wire attached, till it boils and catches fire, and the flame then 

 be blown out, the hot tallow will still continue in a state of low 

 combustion, of which the flame may not be visible, but which 

 is sufficient to cause the renewal of the high combustion, if the 

 cup is immediately introduced into a jar of oxygen gas. A candle 

 newly blown out is sometimes rekindled in oxygen, although 

 no point of the wick remains red, owing to the continuance of 

 this low combustion. When a coil of thin platinum wire, or a 

 piece of platinum foil is first heated to redness and then held 

 over a vessel containing ether or hot alcohol, the vapours of 

 these substances, mixed with the air, oxidate upon the hot 

 metallic surface, and may sustain the metal at a red heat for 

 a long time, without the occurrence of combustion with flame. 

 The product, however, of the low combustion of these bodies 

 is peculiar, as is obvious from its pungent odour. 



Combustion in air. The affinity for oxygen o.f ah 1 ordinary 

 combustibles is greatly promoted by heating them, and is in- 

 deed rarely developed at all except at a high temperature. Hence, 

 to determine the commencement of combustion, it is commonly 

 necessary that the combustible be heated to a certain point. 

 But the degree of heat necessary to inflame the combustible, 

 is in general greatly inferior to what is evolved during the 

 progress of the combustion, so that a combustible, once in- 

 flamed, maintains itself sufficiently hot to continue burning 

 till it is entirely consumed. Here the difference may be ob- 

 served between combustion and simple ignition. A brick 

 heated till it is red-hot in a furnace, and taken out, exhibits 

 ignition, but has no means within itself of sustaining a high 

 temperature, and soon loses the heat which it had acquired in 

 the fire, and on cooling is found unchanged. The oxidable 

 constituents of wood, coal, oils, tallow, wax, and all the or- 

 dinary combustibles are the same, carbon and hydrogen, which 



