Dr. Henry on the Philosophical Character of Dr. Priestley. '21^ 



tern. The same eager direction of his mind to a single object 

 caused him also to overlook several new substances, which 

 he must necessarily have obtained, and which by a more 

 watchful care he might have secured and identified. At a 

 very early period of his inquiries (viz. before November, 

 1771,) he was in possession of oxygen gas from saltpetre, 

 and had remarked its striking effect on the flame of a candle ; 

 but he pursued the subject no further until August, 1774, 

 when he again procured the same kind of gas from the red 

 oxide of mercury, and, in a less pure state, from red lead. 

 Placed thus a second time within his grasp, he did not omit 

 to make prize of this, his greatest, discovery. He must 

 also have obtained chlorine by the solution of manganese in 

 spirit of salt; but it escaped his notice, because being re- 

 ceived over mercury the gas was instantly absorbed *. If he 

 had employed a bladder, as Scheele afterwards did, to collect 

 the product of the same materials, he could not have failed 

 to anticipate the Swedish philosopher, in a discovery not less 

 important tlian that of oxygen gas. Carbonic oxide early 

 and repeatedly presented itself to his observation, without 

 his being aware of its true distinctions from other kinds of in- 

 flammable air ; and it was reserved for Mr. Cruickshank of 

 Woolwich to unfold its real nature and characters. It is re- 

 markable also that in various parts of his works. Dr. Priestley 

 has stated facts that might have given him a hint of the law, 

 since unfolded by the sagacity of M. Gay Lussac, ' that 

 gaseous substances combine in definite volumes.' He shows 

 that 



1 measure of fixed air unites with If measure of alkaline air, 



1 measure of sulphurous acid with 2 measures of do. 



1 measure of fluor acid with 2 measures of do. 



1 measure of oxygen gas with 2 measures of nitrous, very 

 nearly ; 

 and that by the decomposition of ] vol. of ammonia, 3 vols, 

 of hydrogen are evolved. 



Let not, however, failures such as these, to reap all that 

 was within his compass, derogate more than their due share 

 from the merits of Dr. Priestley ; for they may be traced to 

 that very ardour of temperament which, though to a certain 

 degree a disqualification for close and correct observation, 

 was the vital and sustaining principle of his zealous devotion 

 to the pursuit of scientific truth. Let it be remembered that 

 philosophers of the loftiest pretensions are chargeable with 

 similar oversights ; — that even Kepler and Newton overlooked 



• Series II. p. 253. 



discoveries, 



