39 
CONCLUSION. 
The philosopher can have no remaining doubt as to the composition of 
atmospheric air; but the circumstances of these experiments might appear 
to him more correct, though probably at the time less clear to others, were 
it said, that mercury , at a certain temperature, overcoming the affinities * of 
It was a metallic calx that gave origin to Dr. Priestley’s famous discovery of vital air. “ On the 1st 
of August, 1773, I endeavoured,” says this illustrious philosopher, “ to extra# air from mercurius 
caicinatus per se; and I presently found that, by means of a very large burning-glass, an aerial 
fluid was expelled very readily. Having got three or four times as much as the bulk of the materials, 
I admitted water to it, and found it was not fixed air, because water did not absorb it. But what 
surprised me exceedingly was, that when a candle was put into this newly acquired air, the 
flame, besides being larger', burnt with considerable more splendour, and heat, than in common 
air ; and a piece of burnt wood that had any redness in it, was rekindled and burnt away very 
fast, resembling by its crackling noise paper that had been dipped in a solution of nitre." 
“ I extracted,” he adds, “ in the same way, a quantity of air, with the very same property, from 
the common red precipitate, which had been produced by a solution of mercury in spirit of nitre, 
and hence I concluded that this peculiar property w T as derived in both instances from nitrous particles. 
I even thought that what w r as usually sold as the mercurius caicinatus per se was contaminated with 
nitrous acid. However, upon mentioning this suspicion to Mr.Waltire, he furnished me with some, 
which he assured me was genuine. This being treated in the same manner as the former, only by 
a longer continuance of heat, I extracted much more air from it than from the other. This experi¬ 
ment might have satisfied any other; but being at Paris in the GHober following, and knowing that 
there were several very eminent chemists in that place, I did not omit the opportunity to get an ounce 
of mercurius caicinatus prepared by Mons. Cadet; of the genuineness of which there could not 
pofiibly be any suspicion; and at the same time I frequently mentioned my surprise at the kind of air I 
had got from this preparation to Mons. Lavoisier, and several other philosophers who honoured me 
with their notice in that city. 
“ At the same time that I had obtained the air above described from the mercurius caicinatus and 
the red precipitate, I also procured some of the same kind from minium, or red lead. As I never 
made the least secret of any thing that I observed, I mentioned this experiment also to all my philo¬ 
sophic acquaintance at Paris, and elsewhere; having no idea at that time to what these remarkable 
facts would lead.” 
Lavoisier, however, w r as the first who turned this important discovery to account. In a late 
work by Dr. Priestley he says, respecting the nature of the composition of the air, “ For my own 
part, I will frankly acknowledge, that at the commencement of the experiments recited, I was so 
far from having formed any hypothesis that led me to the discoveries these produced, that they ap¬ 
peared to me improbable when I heard of them; and when the decisive facts did at length obtrude 
themselves upon my notice, it was very slowly, and with great hesitation, that I yielded to the evi¬ 
dence of my senses. And yet, when I reconsider the matter, and compare my last experiments relating 
to the constitution of the atmosphere with my first discovery, I see the closest and easiest connexion 
between them, so as to wonder that I should not have been led immediately from the one to the other. 
That this wxis not the case, I attribute to the force of prejudice, which, unknown to ourselves, biasses 
not only our judgments properly so called, but even the perceptions of our senses; for we may take 
a maxim so strongly for granted, that the plainest evidence of sense will not entirely change, and often 
hardly modify, our persuasions; and the more ingenious a man is, the more effectually he is entangled 
in his errors; his ingenuity only helping him to deceive himself by evading the force of truth.” 
* If you take a bullet and divide it with a knife into two parts, provided these be smooth and 
rubbed together, they will firongly unite and form one whole. This is from a law impressed on 
matter called the attraction of cohesion. But should a particle of sand, or any roughness exist, the 
particles being divorced from each other, beyond the sphere of mutual attraction, they are no longer 
actuated by this law.-—The attraction of cohesion in mercury, at the common temperature, hinders 
the admission of oxygen, for which it has an elective attraction or affinity. But when exposed to a 
