25 



passing through the retort, is presumed to phlogisticate and vitiate 

 the external air, is 7iitrogen ; and the dephlogisticated air of the water 

 is supposed to retain sufficient phlogiston to make, with the assistance 

 of heat, good air, of the same purity as the atmosphere. 



Such was Watt's " theory " and " way of solving the phaenomena," 

 in April 1783. But Priestley, though he did not prosecute Cavendish's 

 experiment on the conversion of inflammable and dephlogisticated air 

 into water, went on with his own on the conversion of water into com- 

 mon air, and discovered that instead of a decomposition of water it was 

 only a transmission of air through the pores oj the retort: and therefore 

 it was, because the proof had failed of the convertibility of water into 

 dephlogisticated air, and the phlogisticated air now called nitrogen, that 

 Watt laid aside his theory as not borne out by the facts, till assured by 

 the papers of Cavendish and Lavoisier, that the form in which water 

 did contain phlogiston, was solely that of injlammahle gas. 



But even after the publication of their luminous views, the ideas of 

 Watt remained equally vague ; for, in his " Thoughts on the consti- 

 tuent Parts of Water," printed in 1784, he says, "It appears that in 

 some circumstances dephlogisticated air can unite in certain degrees 

 with phlogiston without being changed into water. Thus Dr. Priest- 

 ley has found, that by taking clean filings of iron, which alone produce 

 only inflammable air of the purest kind, and mercurius calc.per se, which 

 gives only the purest dephlogisticated air, and exposing them to heat 

 in the same vessel, he obtained neither dephlogisticated nor inflamma- 

 ble air, but, in their place, fixed air*. Phlogisticated air seems to be 

 another composition of phlogiston and dephlogisticated air ; but in what 

 propox'tion they are united, or by what means, is still unknown. It 

 appears to me, that fixed air contains a greater quantity of phlogiston 

 than phlogisticated air does, because it has a greater specific gravity, 

 and because it has more affinity with water." He afterwards adds, 

 — "by some experiments of Dr. Priestley's, charcoal, when freed 

 from fixed air, and other air which it imbibes from the atmosphere, is 

 almost wholly convertible into phlogiston-\y Does M. Arago think that 

 one who had so confounded together hydrogen, nitrogen, and charcoal, 

 that with him either was as likely as the other to foi/U vater, was in a 

 condition to discover its composition, or to throw any light upon the 

 subject ? Cavendish, indeed, has shewn that it was possible to reason 



* Cavendish had already pro^'ed by an experiment related in the Philoso- 

 phical Transactions, vol. Ixxii. part 1, that Priestley was mistaken in this, and 

 that the fixed air was owing to carburet of iron (plumbago) in the iron filings. 



t Phil. Trans., vol. Ixxiv. part 2, pp. 334, 351. 



