28 



same result; and when Priestley in 1785* detected the difference be- 

 tween the gas distilled from charcoal, and pure hydrogen, he arrived, 

 as to the quantity of carbonic acid, at a similar conclusion ; in this 

 case we maj^ consider the mixture as containing 65 volumes of hydro- 

 gen, 6'66 of carburetted hydrogen, and 28*34 of carbonic oxide (in 

 addition to 25 of carbonic acid) ; and it will be found, on calculating the 

 results of burning such a composition, that the weight of the water 

 formed is to that of the gases burnt, as I :2. 



It is manifest therefore that Priestley could not possibly, even if he 

 had used a better method of collecting the dew, than wiping the sides 

 of the glass, have found the weight of the water equal, as MM. Arago 

 and Dumas suppose, to that of the gases which he burnt, nor to half 

 that weight ; it is manifest also, that he did not weigh the gases, either 

 before burning or afterwards, nor take their specific gravity, but esti- 

 mated them on the erroneous supposition that all inflammable gases 

 are alike. Watt was in the same error; and it was in consequence 

 of this error, that in his letter to De Luc, notwithstanding the notice 

 Cavendish had given that " the air from charcoal is a different kind of 

 inflammable airf," he states, on the authority of Priestley, that " charcoal 

 is almost wholly convertible into phlogiston J." Thus do we find that 

 Protean name bestowed pro hac vice on a mixture of gases probably 

 not containing more than -^^ or y^j of its weight of hydrogen. — '' Et 

 voila," as Berthollet said of a subsequent experiment made with no 

 greater accuracy, "a quoi ce reduit cette farneuse experience de M. 

 Priestley, dont les resultats avoient ete si bien prevus et analyses par 

 M. Cavendish §." 



Cavendish, as early as 1766, or very soon after, had discovered 

 that there was more than one species of inflammable gas : his Phlo- 

 giston therefore tvas hydrogen and nothing else ; whether obtained from 

 zinc or iron, he knew that it had a constant specific gravity, and he 

 found that it had likewise a constant combining quantity ; if he had 

 not attended to these points, he could neither have experimented, or 

 speculated, on the composition of water to any manner of purpose. 



* " Expending 94 grains of perfect charcoal (by which I mean charcoal made 

 with a very strong heat, so as to expel all fixed air from it), and 240 grains of 

 water, I procured 840 ounce measures of air, -\ of which was fixed air, and of 

 the inflammable part nearly ^ more appeared to be fixed air by decomposition." 

 (Phil. Trans., vol. Ixxv. part 1, p. 194, 1785.) 



t Phil. Trans., vol. Ixxiv. part 1, p. 135. 



X Ibid., part 2, p. 351. 



§ Consid. sur lesExper. de M, Priestley, relatives a la Compos, de I'Eau. 

 Annales de Chimie, torn. iii. p. 86. 



