Note 810 — Chapter V · (continued 2)

On 26th November 1783, he writes: ‘For many years I have entertained an opinion that air was a modification of water; which was originally founded on the facts, that in most cases where air was actually made, which should be distinguished from those wherein it is only extricated from substances containing it in their pores, or otherwise united to them in the state of air, the substances were such as were known to contain water as one of their constituent parts, yet no water was obtained in the processes, except what was known to be only loosely connected with them, such as the water of the crystallization of salts. This opinion arose from a discovery that the latent heat contained in steam diminished, in proportion as the sensible heat of the water from which it was produced, increased; or, in other words, that the latent heat of steam was less when it was produced under a greater pressure, or in a more dense state, and greater when it was produced under a less pressure, or in a less dense state; which led me to conclude, that when a very great degree of heat was necessary for the production of the steam, the latent heat would be wholly changed into sensible heat; and that, in such cases, the steam itself might suffer some remarkable change. I now abandon this opinion, in so far as relates to the change of water into air, as I think that may be accounted for on better principles.’ See this remarkable passage, which is quite decisive as to the real history of Watt's discovery, in Correspondence of James Watt on the Composition of Water, London, 1846, pp. 84, 85. Compare p. cxxiv. and p. 248 note.