CHARCOAL FIRE. 



disappear altogether from the furnace, under which a small portion, 

 of ashes consisting of incombustible matter mil remain. If the 

 charcoal had been pure that is, if it had been carbon it would 

 have altogether disappeared, no ash whatever remaining. 



This phenomenon is an example of FIEE. The heat and light de- 

 veloped during the process here described are commonly called fire. 



7. To comprehend what takes place in this process, we must 

 consider that, as the air passes through the charcoal, the oxygen 

 gas, which forms one-fifth part of it,* enters into combination 

 with the pure carbon. A compound is thus formed consisting of 

 carbon and oxygen. The formation of this compound is attended 

 with so great a production of heat, that not only the compound 

 itself, but the charcoal, from which it is evolved, is raised to a 

 very elevated temperature. 



The compound thus produced is a gas called carbonic acid, which 

 has been already briefly noticed in our Tract on Air. 



The air which enters the furnace being a mixture of azote and 

 oxygen,* that which rises from it after the combustion has been 

 produced is a mixture of azote and carbonic acid ; the azote having 

 passed through the furnace without suffering other change than 

 an increase of temperature, while the oxygen has been converted 

 into highly heated carbonic acid. 



Several questions, however, arise out of this explanation. How 

 is it known that such combination really takes place between the 

 carbon and oxygen ? If it do, in what proportion do they com- 

 bine ? How does it appear that the azote, which forms four-fifths 

 of the air which passes through the furnace issues unaltered ? 



8. To supply satisfactory answers to these questions, it is only 

 necessary to bring the two constituents of common air separately 

 into the presence of carbon under the conditions necessary to 

 favour combination, and to ascertain their weights before and 

 after the development of the phenomena. 



Let a glass flask containing sixteen 

 grains of oxygen gas be inverted over mer- 

 cury, as represented in fig. 1, and let a 

 piece of carbon weighing more than six 

 grains, supported in a platinum spoon, be 

 introduced into it by means of a piece of 

 bent platinum wire ; let the sun's rays, 

 concentrated by means of a burning-glass, 

 be then directed upon the carbon through 

 the glass flask. The carbon will be ignited 

 by the solar heat, and will burn in the oxygen with great splendour. 



* See Tract on Air. 



o2 195 



