34 . Philosophical Character of Dr.. Priestley. 



lecting others, which were necessarily connected with it, and which, 

 if investigated, would have thrown great light on the main research. 

 As an instance, may be mentioned his omitting to examine the re- 

 lation of gases to water. This relation, of which he had indistinct 

 glimpses, was a source of perpetual embarrassment to him, and led 

 him to imagine changes in the intimate constitution of gases, which 

 were in fact due to nothing more than an interchange of place be- 

 tween the gas in the water and that above the water, or between the 

 former and the external atmosphere. Thus he erroneously supposed 

 that hydrogen gaS was transposed into azotic gas, by remaining long 

 confined by the water of a pneumatic cistern. The same eager di- 

 rection 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 No- 

 vember, 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 farther untill 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 with- 

 in his grasp, he did not omit to make prize of this, his greatest, dis- 

 covery. He must, also, have obtained chlorine by the solution of 

 manganese in spirit of salt ; but it escaped his notice, because, being 

 received over mercury, the gas was instantly absorbed.* If he had 

 employed a bladder, as Scheele afterwards did, to collect the pro- 

 duct of the same materials, he could not have failed to anticipate the 

 Swedish philosopher, in a discovery not less important than 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 inflammable air ; and h was reserved for Mr. Cruick- 

 shank 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 unfold- 

 ed by the sagacity of M. Gay Lussac, « that gaseous substances com- 

 bine in definite volumes.' , He shows that 



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

 I measure of sulphurous acid with 2 measures of do. 



Series II. p. 253. 



