VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 24() 



called a decomposition, which loss is not usually called a decomposition; for 

 water is never said to be decomposed when it becomes ice, nor metals when they 

 become solid on cooling. 



In answer to all this it will be said, that inflammable air undoubtedly contains 

 phlogiston, which produces all the beforementioned effects; but that the phlo- 

 giston it contains is united to some other substance, which some will have to be 

 an acid, some an earth, and others respirable air. To these hypotheses I shall 

 oppose one general observation, which is, that since inflammable air, when pure, 

 that is, when disengaged from all heterogeneous substances which no way con- 

 tribute fo its inflammability, has always the same properties; it must, if it con- 

 sists of phlogiston combined with any other substance, be always united to the 

 same specific substance; that is, if this be an acid, it must be always the same 

 species of acid, or if an earth, it must be always the same species of earth; for 

 we find, that substances, which are only generically the same, always produce, 

 with any other given substance, compounds whose properties are very different 

 from each other. Thus we sec that the different species of alkalis, or earths, or 

 metals, produce, with one and the same species of acid compounds essentially 

 different. This is a rule which, as far as I know, admits of no exception; and 

 if we apply it. to the abovementioned suppositions it will entirely destroy them ; for 

 it is impossible to think, that the phlogiston can in every substance, that pro- 

 duces inflammable air, meet either the same acid, or earth, or any res- 

 pirable air. 



But to be more particular, the following reasons demonstrate that an acid of 

 any sort cannot be the basis of inflammable air. 1st. Inflammable air has been, 

 by Dr. Priestley, separated from metals by mere heat. Now metals contain no 

 acid, except perhaps their dephlogisticated calx, which those eminent chemists, 

 Bergman and Scheele, suspect to be of an acid nature; but these calces cannot 

 enter into the composition of inflammable air, otherwise the inflammable air of 

 each different metal would have different properties, as already shown: nor indeed 

 are these the acids that have been supposed to enter into the composition of 

 inflammable air; but rather those acids by whose means it is extricated. But as 

 this air is extricated from metals, not only by acids, but also by alkalis,* this 

 supposition must vanish of course. 



The same reasons militate with equal strength against the supposition that an 

 earth of any kind enters into the composition of this air; nor is there an instance 

 of any earth rendered permanently fluid by any means, except in sparry air. 

 Besides, if it were a metallic earth, it must necessarily be supposed to be in 

 a metallic state; and how then could it escape the action of all kind of acids? for 



* Mem. Par. 1776, p. 687- 



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