132 CLASSICS OF MODERN SCIENCE 

 philosophers of old, that the matter of fire or of light is a very subtle, 

 very elastic fluid, which surrounds every part of the planet we live on, 

 which penetrates with more or less ease the substances which compose 

 that, and which tends, when it is free, to come to a state of equilib- 

 rium in all. 



I will add, borrowing the chemical phraseology, that this fluid is 

 the solvent of a large number of substances; that it combines with 

 them in the same way that water does with salt, and the acids with 

 metals, and that the bodies thus combined and dissolved by the 

 igneous fluid lose in part the properties which they had before the 

 combination and acquire new ones which bring them nearer (make 

 them more like) the fire matter. 



It is thus, as I have shown in a memoir deposited with the secre- 

 tary of this Academy, that every aeriform fluid, every kind of air, 

 is a resultant of the combination of some substance, solid or fluid, 

 with the matter of fire or of light ; and it is to this combination that 

 aeriform fluids owe their elasticity, their specific volatility, their 

 rarity, and all the other properties which ally (rapprocJient) them 

 to the igneous fluid. 



Pure air, according to this, what Mr. Priestley calls dephlogisti- 

 cated air, is an igneous compound into which the matter of fire or of 

 light enters as solvent, and into which some other substance enters 

 as a base ; but if, in any solution whatever, a substance is presented 

 to the base with which that has greater affinity, it unites with this 

 instantly and the solvent which it leaves is set free. 



The same thing happens with the air in combustion ; the substance 

 which burns steals away the base ; then the fire matter which served as 

 its solvent becomes free, regains its rights and escapes with the 

 characteristics by which we know it; that is to say, with flame, heat 

 and light. 



To elucidate whatever may seem obscure in this theory let us apply 

 it to some examples : when a metal is calcined in pure air, the base of 

 the air, which has less affinity for its own solvent than for the metal, 

 unites with the latter as it melts and converts it into metallic calx. 

 This combination of the base of the air with the metal is proved ist, 

 by the increase in weight which the latter undergoes in calcination ; 

 2nd, by the almost total using up of the air under the receiving bell. 



