that part of the scale the whole pressure becomes of the order of errors of observation.

In conclusion, it appears to me that the following proposition, to which I have been led by the theoretical researches referred to at the commencement of this paper, is borne out by all the experiments I have quoted, especially by those of greatest accuracy, and may be safely and usefully applied to practice.

If the maximum elasticity of any vapour in contact with its liquid be ascertained for three points on the scale of the airthermometer, then the constants of an equation of the form

may be determined, which equation will give, for that vapour, with an accuracy limited only by the errors of observation, the relation between the temperature (1), measured from the absolute zero (274-6 centigrade degrees below the freezing point of water), and the maximum elasticity (P), at all temperatures between those three points, and for a considerable range beyond them.

Some Remarks on the Claims to the Discovery of the Composition of Water. By JOHN DAVY, M.D., F.R.S., Lond. and Edin., Inspector-General of Army Hospitals, &c. (Communicated by the Author.)

In editing the collected works of my brother Sir H. Davy, I had occasion to make some observations on the above subject, which, at that time (1839), had a good deal of attention given to it, in consequence of the claims then brought forward by the friends of Mr Watt, in favour of the merit of the discovery of the composition of water being due to him alone, to the disparagement of Mr Cavendish, whom the most zealous of those friends evidently wished to exhibit as a plagiarist, or as having derived the idea of the composition of water from Mr Watt, without acknowledgment.

In a work, published in 1846, by J. P. Muirhead, Esq., entitled, "Correspondence of the late James Watt, on his Discovery of the Theory of the Composition of Water," this

intention is most fully displayed, and in a manner, it appears to me, characteristic of the advocate-the special pleader rather than of the man of science-in the manner of one anxious to gain a verdict in favour of his client, rather than intent on dispassionate inquiry; and often, in accordance with forensic practice, using damaging expressions of an offensive kind, which never would have been employed in liberal and courteous discussion.

It is not my intention to enter into any lengthened commentary on this work (a quarto of 391 pages). After having carefully read it, I have found no reason to alter the opinion which I had previously formed and expressed, viz. that Mr Watt and Mr Cavendish independently arrived at the idea, or inference, that water is the compound it is now considered, -a conclusion, I have said, alike honourable to Mr Watt and to Mr Cavendish, and which is free from all the difficulties and painful consequences connected with the contrary.*

Such was my first impression, and thus has it been confirmed. The facts on which it was founded are principally the following, as then stated:-Dr Priestley, in his paper "On the seeming Conversion of Water into Air," bearing date Birmingham, April 21, 1783, distinctly mentions "an experiment of Mr Cavendish, concerning the reconversion of air into water, by decomposing it, in conjunction with inflammable air," a result which he confirmed by repetition. This result, derived or learnt from Dr Priestley, was the basis, it would appear, of Mr Watt's hypothesis respecting the nature of water, as stated by him to M. de Luc, and also the true basis of that hypothesis, as given in his earlier letter to Dr Priestley.‡

* Collected Works of Sir H. Davy, vol. vii., p. 133.

↑ Dr Priestley's words are,-" Still hearing of many objections to the conversion of water into air, I now gave particular attention to an experiment of Mr Cavendish's, concerning the reconversion of air into water, by decomposing it in conjunction with inflammable air."-Phil. Trans. for 1783, p. 426.

Mr Watt, referring to Dr Priestley's experiment on the firing of a mixture of dephlogisticated air (oxygen) and inflammable air, and the production of moisture, states, in a note,-"I believe that Mr Cavendish was the first who discovered that the combustion of dephlogisticated and inflammable air produced moisture on the sides of the glass in which they were fired." Mr Caven

This, his first letter on the subject, was written in the same month as Dr Priestley's paper, that referred to above, and, as will be mentioned further on, was quoted by Mr Cavendish. It was dated April 26, 1783. From a passage in Dr Priestley's paper, and from one in Mr Watt's first letter, that to Dr Priestley, it may be inferred that this his hypothetical conclusion was formed just before that letter was written. He mentions in it the abandonment of an opinion that he had entertained for many years "that air was a modification of water," "that, by a great heat, water might be converted into air."

Now, what is Mr Cavendish's statement relative to the discovery in question? After describing his experiments in proof of the production of water by burning hydrogen in close vessels with common air and dephlogisticated air, he remarks, "All the foregoing experiments on the explosion of inflammable air with common and dephlogisticated air except those which relate to the cause of the acid found in the water, were made in the summer of the year 1781, and were mentioned by me to Dr Priestley, who, in consequence, made some experiments of the same kind, as he relates in a paper printed in the preceding volume of the Transactions. During the last summer also, a friend of mine gave some account of them to M. Lavoisier, as well as of the conclusion drawn from them, that dephlogisticated air is only water deprived of phlogiston; but at that time so far was M. Lavoisier from thinking any such opinion warranted that, till he was prevailed upon to repeat the experiment himself, he found some difficulty in believing that nearly the whole of the two airs should be converted into water."t

It has been objected to this passage that it was an interpolation, after Mr Watt's letter to M. de Luc had been read

dish obtained 135 grains of water," pure water," as it seemed, in one of the experiments which he mentions. In Waltire's experiments, which led to Mr Cavendish's, a dew was observed on the inside of the vessel in which the explosion was made, mixed with soot, attended with a loss of weight. The experimenter referred the dew to moisture previously existing in and deposited from the airs used.

Phil. Trans. for 1784, p. 134.

at the Royal Society, and farther, that it was in the handwriting of Mr Cavendish's friend, Sir Charles Blagdon. That letter to M. de Luc it was, no doubt, which gave rise to the explanation contained in the interpolated statement; and the circumstance that it was not in Mr Cavendish's own handwriting in the MS., it seems most natural to infer would not denote that anything unfair was practised. Had Mr Cavendish not been confident that he was acting correctly, had he been performing the part of a plagiarist, he would, it may be presumed have acted with the caution of a plagiarist at the time. The correctness of the statement, it should be remembered, was not impugned, as, if incorrect, it surely ought to have been.

I have spoken of Mr Watt's conclusion of the compound. nature of water as an hypothesis, or as an inference from an experiment requiring to be confirmed by further experiments. In that light, he himself evidently first viewed it; thus, in his letter to Dr Priestley, of the 21st of April 1783, he says, On considering your very curious and important discoveries on the nature of phlogiston and dephlogisticated air, and on the conversion of water into air, and vice versa, some thoughts have occurred to me on the probable causes of these phenomena, which, though they are mere conjectures, seem to me more plausible than any I have heard on the subject, and, in that view, I have taken the liberty to communicate them to you." And he concludes the same letter with this remark,- "If you shall think that a hypothesis so hastily compiled, deserves to have the honour of being communicated to the Royal Society, or published in any other way, with the account of your experiments, I shall be obliged to you to present it to the Society, or to the public, as you shall think proper." Again, in his letter to Sir Charles Blagdon respecting the publication of his paper (his letter to M. de Luc), he says,-“I am really at a loss what title to give the paper; but propose the following, conjectures-Thoughts on the constituent parts of water, and of dephlogisticated air, with an account of some experiments on that subject." And, in his letter to M. de Luc, he prefaces it with the remark, "I feel much

reluctance to lay my thoughts on these subjects (the probable causes of the production of water from the deflagration of a mixture of dephlogisticated and inflammable air) before the public, in their present indigested state, and without being able to bring them to the test of such experiments, as would confirm or refute them, and should therefore have delayed the publication of them until these experiments had been made, if you, Sir, and some other of my philosophical friends, had not thought them as plausible as any other conjectures which have been formed on the subject; and that, though they should not be verified by further experiments, or approved by men of science in general, they may, perhaps, merit discussion, and give rise to experiments which may throw light on so important a subject," adding “I first thought of this way of solving the phenomena, in endeavouring to account for an experiment 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 as before he had an opportunity of doing me that favour, he found, in the prosecution 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 ways applicable to the explaining these experiments, I thought proper to delay its publication, that I might examine the subject more deliberately, which my other avocations have prevented me from doing to this time."

Mr Watt's paper in the Transactions of the Royal Society, bearing the date of November 26, 1783, and which was read the 29th April 1784, consisted, it must be remembered, of portions of his original letter to Dr Priestley, and of additions, some of which, it may be inferred, were made shortly before it was read, viz., in the "corrected copy,"* and when

*It is designated, in Mr Watt's handwriting, "Corrected copy of a letter from James Watt, Engineer, to M. de Luc, dated November 26, 1783, corrected April 1784." It is in the Archives of the Royal Society.