66 
FIRST REPORT —1831. 
exclusively, neglecting others, which were necessarily connect¬ 
ed 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 relation 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 between 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 transmuted into azotic gas, by remaining long 
confined by the water of a pneumatic cistern. 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 im¬ 
portant 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 it was reserved for Mr. Cruickshank of Woolwich to 
unfold its real nature and characters. It is remarkable, 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 com¬ 
bine 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 nitrous, very 
nearly; 
* Series II. p. 253. 
