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TRANSACTIONS OF SECTION B. 767 
also formulated then, as now, in an éloge read before that learned body. The 
assailant was M. Arago, who did battle, not for his countryman Lavoisier, whose 
claims are dismissed as ‘pretensions, but on behalf of James Watt, the great 
engineer, who was one of the foreign members of the Institute. 
It is not my wish to trouble you at any length with the details of what 
has come to be known in the history of scientific discovery as the Water Con- 
troversy—a controversy which has exercised the minds and pens of Harcourt, 
Whewell, Peacock, and Brougham in England; of Brewster, Jeflrey, Muirhead, 
and Wilson in Scotland ; of Kopp in Germany ; and of Arago and Dumas in France. 
This controversy, it has been said, takes its place in the history of science side by 
side with the discussion between Newton and Leibnitz concerning the invention of 
the Differential Calculus, and that between the friends of Adams and Leverrier 
in reference to the discovery of the planet Neptune. Up to now it has practically 
turned upon the relative merits of Cavendish and Watt. M. Berthelot is the first 
French savant of any note who has seriously put forward the claims of Lavoisier, 
his countryman and predecessor Dumas having deliberately rejected them. 
At the risk of wearying you with detail, I am under the necessity of restating 
the facts in order to make the position clear. Some time before April 18, 
1781, Priestley made what he called ‘a random experiment’ for the entertainment 
of a few philosophical friends, It consisted in exploding a mixture of inflammable 
air (presumably hydrogen) and common air, contained in a closed glass vessel, by 
the electric spark, in the manner first practised by Volta in 1776. The experiment 
was witnessed by Mr. John Warltire, a lecturer on natural philosophy and a friend 
of Priestley, who had rendered him the signal service of giving him the sample of 
the mercuric oxide from which he had first obtained oxygen. Warltire drew 
Priestley’s attention to the fact that after the explosion the sides of the glass 
vessel were bedewed with moisture. Neither of the experimenters attached any 
importance to the circumstance at the time, Priestley being of opinion that 
the moisture was pre-existent in the gases, as no special pains were taken to dry 
them. Warltire, however, conceived the notion that the experiment would afford 
the means of determining whether heat was ponderable or not, and hence he was 
led to repeat it, firing the mixture in a copper vessel for greater safety. The results 
of these observations are contained in Priestley’s ‘Experiments and Observations 
on Air,’ vol. v. 1781, App. p. 3965. 
' At this period Cavendish was engaged on a series of experiments ‘made, as 
he says, principally with a view to find out the cause of the diminution which 
common air is well known to suffer by all the various ways in which it is 
phlogisticated, and to discover what becomes of the air thus lost or condensed,’ 
(Cavendish, ‘ Phil. Trans.’ 1784, p. 119.) On the publication of Priestley’s work 
he repeated Warltire’s experiment, for, he says, as it ‘ seemed likely to throw great 
light on the subject I had in view, I thought it well worth examining more closely.’ 
The series of experiments which Cavendish was thus induced to make, and which 
he made with all his wonted skill in quantitative work, led him some time in 
the summer of 1781 to the discovery that a mixture of two volumes of the inflam- 
mable air from metals (the gas we now call hydrogen) with one volume 
of the dephlogisticated air of Priestley combine together under the influence 
of the electric spark, or by burning, to form the same weight of water. If 
Cavendish had published the results of these observations at or near the 
time he obtained them, there would have been no Water Controversy. But in 
the course of the trials he found that the condensed water was sometimes acid, and 
the search for the cause of the acidity (which incidentally led to the discovery of 
the composition of nitric acid) occasioned the delay. The main result that 
a mixture of two volumes of inflammable air and one volume of dephlogisticated 
air could be converted into the same weight of water was, however, communicated 
to Priestley, as he relates in a paper in the ‘ Phil. Trans.’ for 1783. Priestley was 
at this time interested im an investigation on the seeming convertibility of water 
into air, and he was led to repeat Cavendish’s experiments, some time in March 
1783, on what was apparently the converse problem. Priestley, however, made a 
fatal blunder in the repetition. With the praiseworthy idea of obviating the pos- 
