28 CHEMISTRY, AS APPLIED TO THE 







that the property of supporting combustion, which common air possesses, is 

 owing. 



Various have been the theories to account for the production of heat and 

 light during combustion. All have failed ; we can merely express the fact, 

 that combustion is the result of intense chemical action. In ordinary cases of 

 combustion this intense chemical action is produced by the union of oxygen 

 with the combustible substances. Common air consists of one volume of 

 oxygen to four of an element to be described, nitrogen. Now by the act of 

 combustion, the oxygen is absorbed, and this renders the air unfit to carry on 

 the process. If we invert what we improperly call an empty glass vessel, but 

 which is in reality filled with air, over water, having previously introduced a 

 lighted taper or jet of gas under the vessel, we shall observe the bulk of air 

 diminish, the flame gradually becoming more dim, until it is at last extin- 

 guished. The air no longer possesses its characteristic properties, having 

 been deprived by the combustion of the tallow, of its vital principle, oxygen. 

 We shall examine subsequently the products of combustion, for the oxygen is 

 not annihilated, since matter is indestructible, but by its union with the ele- 

 ments of the tallow has formed a new substance. It has been ascertained 

 that the heat developed by combustion is so nearly proportionate to the quan- 

 tity of oxygen absorbed by the combustible, that for all practical purposes it 

 may be assumed as true. Berthier has invented a beautiful process for ascer- 

 taining the value of a combustible founded on this fact : he mixes the sub- 

 stance to be tested with several times its weight of oxide of lead, and exposes 

 the mixed mass in a crucible to the action of a strong fire. Oxide of lead is 

 formed of oxygen and lead, in the proportion of 8 of the former to 104 of the 

 latter. At a red heat, the combustible matter seizes upon the oxygen, and 

 reduces the lead to a metallic state. This melts, and collects in the form of 

 a button at the bottom of the crucible. As we have shown that the propor- 

 tions in which bodies unite are always uniform, it is evident that for every 

 portion of oxygen consumed a certain portion of lead will be reduced, so that 

 the quantity of effective combustible matter in the compound examined will 

 be in due proportion to the weight of the button of lead. This process is not 

 entirely free from objections, but sufficiently so for every practical purpose. 



HYDROGEN was discovered in the year 1766 by Cavendish. It is gaseous, 

 and the lightest body known, its specific gravity being only O06896. This 



