58 On the Means most proper to he resorted to 



two distinct substances dissolved in caloric or heat, which, 

 forms a third ingredient. The two first are oxygen and 

 azote. 



The azote is in such strong chemical union with the ca- 

 loric, in which it is dissolved, that in no common process 

 of combustion is the union destroyed : or, in other words, 

 that portion of the heat of the atmosphere which is united 

 to the azote is never liberated to exercise its action in form- 

 ing new combinations *. 



it is otherwise, however, with the portion of heat united 

 to the oxygenous part of the atmosphere. These two have 

 so weak an affinity for each other, that a little increase of 

 temperature is all that is necessary to determine their sepa- 

 ration, if substances to which the oxygen can unite itself 

 be present. In proportion as the oxygen joins itself to these 

 substances, the heat thus liberated raises the temperature of 

 other portions of them to that point which determines their 

 union with oxygen ; thus more air becomes s[)eedlly decom- 

 posed, and all the phtenomcna of combustion arc rendered 

 more and more conspicuous, till complete deflagration pre- 

 cludes all possibility of checking the progress. 



What office does water perform when employed for the 

 purpose of checking the progress of a fire ? It extinguishes 

 the flame hy cutting off tke commiwicatiori bettveen the hiirn- 

 i!i<y body and the air which mainlains the combustion. But 

 this it can do only in certain cases. 



Water is known to consist of two substances, oxygen 

 and hijdrngen. The former, as has already been noticed, is 

 an ini^^redient also in atmospheric air, and is that substance 

 ,which unites itself to the burning body in every case of com- 

 bustion : the latter is the base of hydrogen gas or injlam- 

 viable air. 



Water, like atmospheric air, maybe decomposed by pre- 

 sentino- to it, under certain circumstances, substances for 

 which either of its consitituent principles has a stronger 

 affinity than the two have for each other. 



When a fire has got to such a height that water cannot 

 be thrown on it in sufficient quantity to interpose itself as 

 a trail of separation between the burning materials and the 

 atnwsphere, but is itself instantly converted into vapour and 

 decomposed, — in iha^i. caiCyitistead of extinguishing, it adds 



■"* It must be liore obsetved, ho\ve\cr, that this remark shoiihl b« 

 taken with some limiiation : modern chemistry is in some measure forced 

 to sLipposc that the azote goes to the formation of aliolis when they 

 result from the combustion ; in which case the caloric m^y be supposed 

 to exercise sonic aciioii , but that affects not our general artjumenr. 



to 



