SI 8 Biographical Memoir q/'Dr Priestley. 



nature ; but it would only have been by seeing him thus in ac- 

 tion, that we should have truly known the most beautiful of all 

 the works of nature, the genius of a great man. 



It must not, however, be supposed that Priestley's discoveries 

 were all perceived by himself, or that he was able to develope them 

 in his book as clearly as we distinguish them in it, and as we would 

 at the present day develope them. When he made these disco- 

 veries, however, he was not acquainted with any other chemical 

 theory than that of Stahl, which being formed from experiments 

 in which the gases were of no account, could not embrace them, 

 and still less foresee all their phenomena. Hence there is a sort of 

 hesitation in his principles, a kind of embarrassment and uncertain- 

 ty in his results. Wishing to find phlogiston in all things, he is ob- 

 liged to suppose it at times quite differently constituted ; in fixed 

 air, very heavy and acid ; in inflammable air, very light ; in phlo- 

 gisticated air, as having a property possessing none of the qua- 

 lities of the other two. There are cases in which an accumula- 

 tion of phlogiston diminishes the weight of the combination, it 

 therefore communicates an absolute lightness to the mixtures in- 

 to which it enters : in other cases it produces a contrary effect. 

 Nothing seems uniform, and no general or precise conclusion 

 is the result. 



Modern chemistry alone could draw this conclusion, and for 

 this it only required one or two formulae : — There is no phlo- 

 giston ; pure air is a simple substance ; phlogisticated air and 

 iriflammable air are also simple substances ; combustion is only 

 a combination of pure air with the bodies burnt. Like the sub- 

 lime words related in the book of Genesis, these few expressions 

 have thrown light upon and disentangled all ; chaos is reduced 

 to order, each fact has assumed its place, and the whole has 

 formed the most magnificent of pictures. 



But, like the gods of the pagans, chemistry could create no- 

 thing out of nothing : it required matter, a subject for its pre- 

 scription ; and with this matter Priestley has had the principal 

 merit of furnishing it *. 



* See principally his memoirs 



On Phlogiston, and the apparent conversion of water into air ; Phil. Trans. 

 1783. 



On the Principle of Acidity, the Composition of Water, and Phlogiston. 

 Phil. Trans. 1788. Paris. On 



