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to have had no clear conception of the composition of water as consist- 

 ing of oxygen and hydrogen : it proves that with him hydrogen and 

 phlogiston were not convertible terms. In this I am sure MM. 

 Arago and Dumas will agree with me, whenever they shall take the 

 trouble to compare with attention Priestley's paper on " the Seeming 

 Conversion of Water into Air," and Watt's reference to that paper 

 in the preface to his letter to M. de Luc. The following is his own 

 account of his theory. " I first thought," he says, " of this way of 

 solving the phsenomena, in endeavouring to account for an experi- 

 ment of Dr. Priestley s wherein water appeared to be converted into 

 air ; and I communicated my sentiments in a letter addressed to 

 him, dated April 26, 1783, with a request that he would do me 

 the honour to lay them before the Royal Society ; but before he 

 had an opportunity of doing me that favour, he found, in the pro- 

 secution of his experiments, that the apparent conversion of water into 

 air by exposing it to heat in porous earthen vessels, was not a real 

 transmutation, but an exchange of the elastic fluid for the liquid, in 

 some manner not yet accounted for : therefore, as my theory was no 

 longer applicable to the explaining these experiments, I thought proper 

 to delay its publication, that I might examine the subject more delibe- 

 rately.'' Now, what were these experiments on the apparent conversion 

 of water into air? We learn from Priestley's paper, that he obtained 

 from the distillation of water in a porous earthen retort, a constant 

 supply of " air of the same purity as the atmosphere," so long as there was 

 free access of air to the outside of the retort ; and, " since," he says, 

 " pure external air was necessary to procure good air, it was concluded 

 by many of my friends, and especially Mr. Watt, that the operation of 

 the earthen retort was to transmit phlogiston from the water contained 

 in the [moist] clay [within the retort] to the external air*, and that the 

 water thus dephlogisticated was capable of being converted into respi- 

 rable air by the influence of heat." Here inflammable gas, or hydrogen, 

 is obviously out of the question ; the phlogiston of the water, which 



* In the unpublished part of his letter. Watt states his views thus : — " On 

 considering the last and most remarkable production of air from water imbi- 

 bed by porous earthen vessels, the only case wherein it appears almost incon- 

 trovertibly that nothing was concerned in the production except water and 

 heat, I think that the earth of the vessel attracts the phlogiston from the water 

 and gradually conveys it from particle to particle until it transmits it to the 

 external air, which it probably phlogisticates." " I omitted to mention in its 

 proper place, that clay when made hot has a very powerful attraction for 

 phlogiston, and under some circumstances, becomes quite black with it ; but 

 Tcadily parts with it to pure air and becomes white again." 



