This and the third paper in the series are classical, and this partly by reason of, and partly in spite of, their blunders, for they are the record of the work upon which James Watt largely based his conjectures concerning the real chemical nature of water, whereby his name has been associated with that of Cavendish and Lavoisier as the true discoverer of its composition. In the course of his inquiry Priestley studied the action of steam upon red-hot iron by an arrangement generally similar to that employed by Lavoisier, but his explanation of the phenomena is essentially different from that of the French chemist, as may be seen from the following quotation:— “Since iron gains the same addition of weight by melting it in _dephlogisticated air_, and also by the addition of _water_ when red-hot, and becomes, as I have already observed, in all respects the same substance, it is evident that this air or water, as existing in the iron, is the very same thing; and this can hardly be explained but upon the supposition that water consists of two kinds of air, _viz._, inflammable and dephlogisticated.” This, however, is how Priestley actually does explain it:— “When iron is melted in dephlogisticated air we may suppose that, though part of its phlogiston escapes to enter into the composition of the small quantity of fixed air which is then procured, yet enough remains to form _water_ with the addition of the dephlogisticated air which it has imbibed, so that this _calx_ of iron consists of the intimate union of the pure _earth of iron_ and of _water_; and therefore when the same calx, thus saturated with water, is exposed to heat in inflammable air, this air enters into it, destroys the attraction between the water and the earth, and revives the iron while the water is expelled in its proper form. “Consequently, in the process with _steam_, nothing is necessary to be supposed but the entrance of the water and the expulsion of the phlogiston belonging to the iron, no more phlogiston remaining in it than what the water brought along with it, and which is retained as a constituent part of the water or of the new compound.” No more striking illustration of how a man’s ingenuity may help him to deceive himself could be given than is afforded by this passage. Priestley to the end of his days never got a just conception of the real chemical constitution of water.