2 Dr Henry's Estimate of 



are distinguished by great merits, and by great defects ; the lat- 

 ter of which are wholly undisguised by their author. He un- 

 veils, with perfect frankness, the whole process of reasoning, 

 which led to his discoveries ; he pretends to no more sagacity 

 than belonged to him, and sometimes disclaims even that to 

 which he was fairly entitled ; he freely acknowledges his mis- 

 takes, and candidly confesses when his success was the result of 

 accident, rather than of judicious anticipation ; and by writing 

 historically and analytically, he exhibits the progressive improve- 

 ment of his views, from their first dawnings to their final and dis- 

 tinct development. Now, with whatever delight we may con- 

 template a systematic arrangement, the materials of which have 

 been judiciously selected, and from which every thing has been 

 excluded that is not essential to the harmony of the general de- 

 sign, yet there can be no question that, as elucidating the opera- 

 tions of the human mind, and enabling us to trace and appreci- 

 ate its powers of invention and discovery, the analytic method 

 of writing has decided advantages. 



To estimate, justly, the extent of Dr Priestley's claim to philo- 

 sophical reputation, it is necessary to take into account the state 

 of our knowledge of gaseous chemistry at the time he began 

 his inquiries. Without underrating what had been already done 

 by Van Helmont, Ray, Hooke, Mayow, Boyle, Hales, Macbride, 

 Black, Cavendish, and some others, Priestley may be safely af- 

 firmed to have entered upon a field, which, though not altoge- 

 ther untilled, had yet been very imperfectly prepared to yield 

 the rich harvest, which he afterwards gathered from it. The 

 very implements with which he was to work were for the most 

 part to be invented ; and of the merits of those which he did in- 

 vent, it is a sufficient proof that they continue in use to this day, 

 with no very important modifications. All his contrivances for 

 collecting, transferring, and preserving different kinds of air, and 

 for submitting those airs to the action of solid and liquid sub- 

 stances, were exceedingly simple, beautiful, and effectual. They 

 were chiefly, too, the work of his own hands, or were construct- 

 ed under his directions by unskilled persons ; for the class of in- 

 genious artists, from whom the chemical philosopher now derives 

 such valuable aid, had not then been called into existence by 

 the demands of the science. With a very limited knowledge of 



