246 ESSAY XXIII. 



true aerial acid (to which the author assigns the old 

 appellation, fixed air), more or less, or no humidity ; as also, 

 more or less, or no earth. Could Dr. Weber decompound 

 pure fixed air, and show, demonstrably, that phlogiston 

 actually is one of its constituent parts, his opinion would 

 acquire much weight. Were this effected (which it can 

 never be by conjectures), it will next be asked, What is 

 the other principle contained in fixed air ? The author's 

 supposition, that it is the electrical fluid, is a mere 

 hypothesis, just as much as if I were to suppose that it is 

 some acid of a stronger nature. The author can by no means 

 establish his opinion concerning the presence of phlogiston in 

 fixed air, by saying that the vapours of fixed air, extricated 

 from chalk by muriatic acid, have a smell ; fixed air separated 

 from magnesia alba by vitriolic acid is inodorous, and the 

 same fluid expelled from lapis suillus by muriatic acid has a 

 stinking smell ; and yet both these species of air coincide 

 entirely with respect to their principal qualities. Dr. 

 Black's experiments are so solid and convincing that it 

 seems to me impossible to form any objection against them. 

 He has indeed pushed his conclusion rather too far in saying 

 that the explosion of fulminating gold, and the increase of 

 weight in metallic calxes, prepared by heat, are owing to 

 fixed air ; but the chief discovery does not suffer from these 

 mistakes. 1 Dr. Weber cannot maintain, upon the authority 

 of his first and second experiments, that crude calcareous 

 earth contains phlogiston as a constituent part ; for, in the 

 first place, that which appears is to be deduced from a small 

 residuum of decaying mineral substances ; and, in the second, 

 saltpetre can be alkalised by being kept long in a red heat, 



1 It is but justice to this great philosopher, whose lectures not only 

 teach the principles of a science and the operations of an art, but contain 

 likewise a system of practical logic, that he never contended strenuously 

 forjthase conjectures, and that he has long abandoned them. T. 



