768 REPORT—1890. 
sibility of any moisture in the gases, he prepared the dephlogisticated air from 
nitre, and the inflammable air by heating what he calls ‘ perfectly made charcoal ’ 
in an earthenware retort. At this time, it must be remembered, there was no sharp 
distinction between the various kinds of inflammable air: hydrogen, sulphuretted 
hydrogen, marsh gas and olefiant gas, coal gas, the vapours of ether and turpen- 
tine, and the’gas from heated charcoal, consisting of a mixture of carbonic oxide, 
marsh gas, and carbonic acid, were indifferently termed ‘ inflammable air.’ Priestley 
attempted to verify Cavendish’s conclusion on the identity of the weight of the 
gases used with that of the water formed; but his method in this respect, as in his 
choice of the inflammable air, was wholly defective, and could not possibly have 
given him accurate results. It consisted in wiping out the water from the explo- 
sion vessel by means of a weighed piece of blotting-paper and determining the in- 
crease of weight of the paper. He says, however: ‘1 always found as near as I 
could judge the weight of the decomposed air in the moisture acquired by the 
paper. ... I wished, however, to have had a nicer balance for this purpose ; the 
result was such as to afford a strong presumption that the air was reconverted into 
water, and therefore that the origin of it had been water.’ These results, together 
with those on the conversion of water into air, were communicated towards the 
end of March 1783 by Priestley to Watt, who began to theorise upon them, and 
then to put his thoughts together in the form of a letter to Priestley, dated April 
26, 1783, and which he requested might be read to the Royal Society on the occa- 
sion of the presentation of Priestley’s memoir. In this letter Watt says: ‘Let us 
now consider what obviously happens in the case of the deflagration of the inflam- 
mable and dephlogisticated air. These two kinds of air unite with violence, they 
become red-hot, and upon cooling totally disappear. When the vessel is cooled, a 
quantity of water is found in it equal to the weight of the air employed. This 
water is then the only remaining product of the process, and water, light, and heat 
are all the products. Are we not then authorised to conclude that water is composed 
of dephlogisticated air and phlogiston deprived of part of their latent or elementary 
heat ; that dephlogisticated or pure air is composed of water deprived of its phlogiston 
and united to elementary heat and light, §c.?’ 
This letter, although shown to several Fellows of the Society, was not publicly 
read at the time intended. Priestley, before its receipt, had detected the fallacy of 
his experiments on the seeming conversion of water into air, and as much of the 
letter was concerned with this matter Watt requested that it should be withdrawn. 
Watt, however, as he tells Black! in a letter dated June 23, 1783, had not 
given up his theory as to the nature of water, and on Noy. 26,1783, he restated his 
views more fully in a letter to De Luc. In the meantime Cavendish, having com- 
pleted one section of his investigation, sent in a memoir to the Royal Society, 
which was read on January 15, 1784, in which he gives an account of his experi- 
ments and announces his conclusion ‘ that dephlogisticated air is in reality nothing 
but dephlogisticated water, or water deprived of its phlogiston; or, in other words, 
that water consists of dephlogisticated air united to phlogiston; and that inflam- 
mable air is either pure phlogiston, as Dr. Priestley and Mr. Kirwan suppose, or else 
water united to phlogiston.’ Watt thereupon requested that his letter to De Luc 
should be published, and it was accordingly read to the Royal Society on April 29, 
1784, Which of the two—Cavendish or Watt—is, under these circumstances, to 
be considered as ‘ the true and first discoverer’ of the compound nature of water is 
the question which has been hitherto the main subject of the water controversy. 
Let us now consider the matter as it affects Lavoisier. In 1783 Lavoisier had 
publicly declared against the doctrine of phlogiston, or rather, as M. Dumas puts 
it, ‘against the crowd of entities of that name which had no quality in common 
except that of being intangible by every known method.’ (‘ Lecons sur la Philosophie 
Chimique,’ p. 161.) How completely Lavoisier had dissociated himself from the 
theory may be gleaned from his memoir of that year. ‘Chemists,’ he says, ‘ have 
made a vague principle of phlogiston which is not strictly defined, and which in 
consequence accommodates itself to every explanation into which it is pressed, 
1 Watt, Correspondence, p. 31. 
iat 
