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to supply the imperfect admission of the French author, and to prevent those inferences as to priority of the theory, which otherwise might have been drawn in favour of Lavoisier. Considering the object thus manifestly in view, here, if anywhere, we ought to look for an explicit statement of the earliest date at which Mr. Cavendish's theory could be said to have been formed, which, at that time, there was no difficulty in ascertaining, and there could have been little in establishing; and we are fairly entitled to hold, that the earliest date consistent with the fact would be assigned, if not by the author of the paper, at least by his zealous and assiduous friend who is so much mixed up with the transaction. All this we say on the supposition, that the question as to priority had arisen merely between Lavoisier and Cavendish: for that is the whole length that our statement has as yet gone. We shall presently see whether other circumstances had not in the meantime arisen, which called still more loudly for that full, clear, and precise declaration which was to have been expected; and which was absolutely indispensable, in order to authenticate for the theory which Mr. Cavendish stated to the Royal Society on the 15th January, 1784, an earlier date than its publication on that day could ensure.




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MR. Watt, in whose neighbourhood Dr. Priestley says he had “the happiness to be situated," and with whom, as has been mentioned, he was on terms of friendship and frequent intercourse, had, previous to 1783, for many years entertained an opinion that air was a modification of water; and that, if steam could be made red-hot, so that all its latent heat should be converted into sensible heat, either the steam would be converted into permanent air, or some other change would take place in its constitution. “You may remember,"

" he writes to Mr. Boulton,* " that

* “ that I have often said, that if “ water could be heated red-hot or something more, it would

probably be converted into some kind of air, because steam “ would in that case have lost all its latent heat, and that it “ would have been turned solely into sensible heat, and pro

bably a total change of the nature of the fluid would ensue.'

." And, so early as 13th December, 1782, he talks of processes“ by which,” he says, “I now believe air is gene“ rated from water;" using the expression, “if this process “ contains no deception, here is an effectual account of many phenomena, and one element dismissed from the list.” +

Being thus, even at that time, prepared to expect that water was, in some way or other, convertible into air, he di

• 10th December, 1782.
† Mr. Watt to Mr. De Luc, 13th December, 1782.


rected his attention to Dr. Priestley's experiment, which he thus accurately relates: “He puts dry dephlogisticated air " and dry inflammable air into a close vessel, and kindles " them by electricity. No air remains, at least if the two “were pure, but he finds on the sides of the vessel a quantity “ of water equal in weight to the air employed.”. In less than a month after he thus mentions his knowledge of that experiment, we find him writing to Dr. Black that he “ be“ lieves he has found out the cause of the conversion of water “ into air;"f and giving the very words in which, both on that day, and a few days later, he stated his conclusions in the letter to Dr. Priestley, which he desired might be read to the Royal Society :—“In the deflagration of the inflammable " and dephlogisticated airs, the airs unite with violence,-be

come red-hot,—and, on cooling, totally disappear. The

only fixed matter which remains, is water; and water, light, “ and heat, are all the products. Are we not then authorized “ to conclude, that water is composed of dephlogisticated and “ inflammable air, or phlogiston, deprived of part of their “ latent heat; and that dephlogisticated, or pure air, is com“ posed of water deprived of its phlogiston, and united to “ heat and light; and if light be only a modification of heat, “ or a component part of phlogiston, then pure air consists of “ water deprived of its phlogiston and of latent heat?” The same conclusions are given in other letters written nearly at the same time; but nowhere are they more clearly, briefly, or forcibly stated, than in that to Mr. Gilbert Hamilton of the 22nd of April, where, after a short enumeration of Facts, beginning with the result of Dr. Priestley’s experiment, follow these DEDUCTIONS. Pure inflammable air is phlogiston itself.

Dephlogisticated air is water deprived of its phlogiston, and 6 united to latent heat.

Water is dephlogisticated air deprived of part of its latent heat, and united to a large dose of phlogiston.”

In writing to Mr. De Luc, four days afterwards, “ These,”

* To his brother-in-law, Mr. Gilbert Hamilton, 26th March, 1783.
† 21st April, 1783.


says Mr. Watt, “seem bold propositions, but I think they “ follow from the present state of the experiments; and if I “were at leisure to write a book on the subject, I think I “ could prove that no experiment hitherto made contradicts

them, and that the greater number of experiments affirm “ them.”* To others of his correspondents he announced his theory in similar terms. To Mr. Smeaton, writing that he has

attempted to demolish two of the most ancient elements, “ air and water;" † and to Mr. Fry, giving particular directions for the production of water and of [dephlogisticated] air :-“Dr. Priestley, as you observe, converts water into air, “ and air into water, and I have found out the reason of all “ these wonders, and also what air is made of, and what water “ is made of; for they are not simple elements.—I have “ written a paper on the subject, and sent it with Dr. Priest

ley's to the Royal Society. It is too long to give you even “ an abstract of it, but if you will forgive me the reasoning, I “ will add the receipt below for making both these elements.

To make Water. “R. Of pure air and of phlogiston Q.S., or if you wish to “ be very exact, of pure air one part, of phlogiston, in a fluid “ form, two parts, by measure. Put them into a strong glass “ vessel, which admits of being shut quite close ; mix them, “ fire them with the electric spark; they will explode, and “ throw out their elementary heat. Give that time to escape, “ and you will find the water, (equal in weight to the air), " adhering to the sides of the vessel. Keep it in a phial close « corked for use.

To make Air.“ Take pure water Q.V., deprive it of its phlogiston by any practicable method, add elementary heat Q.S. and distil

. “ You will obtain pure air, to be preserved as above." I

It will be remembered, that in the letter to Mr. Hamilton he had shown his belief to be, that pure inflammable air and

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* 26th April, 1783.

† 27th April, 1783.

| 28th April, 1783.



phlogiston were exactly synonymous; and it is very remarkable, that the proportions of the two gases which he directs to be fired, viz., of pure air one part, and of inflammable air two parts, by measure, are exactly those which chemists of the present day would employ.

It appears from the letter to Dr. Black of the 21st of April, that Mr. Watt had, on that day, written his letter to Dr. Priestley, to be read by him to the Royal Society ; but on the 26th he informs Mr. De Luc, that having observed some inaccuracies of style in that letter, he had removed them, and would send the Doctor a corrected copy in a day or two, which he accordingly did on the 28th ; the corrected letter, (the same that was afterwards embodied verbatim in the letter to Mr. De Luc, printed in the Philosophical Trans'actions'), being dated 26th April, and containing, almost at its very commencement, the following passages :

“ The same ingenious philosopher mixed together certain proportions of pure dry dephlogisticated air and of pure " dry inflammable air in a strong glass vessel, closely shut, and then set them on fire by means of the electric spark. “ The first effect was the appearance of red heat or inflam“mation in the airs, which was soon followed by the glass "vessel becoming hot. The heat gradually pervaded the

glass, and was dissipated in the circumambient air, and as " the glass grew cool, a mist or visible vapour appeared in it, “ which was condensed on the glass in the form of moisture

or dew. When the glass was cooled to the temperature of " the atmosphere, if the vessel was opened, with its mouth “immersed in water or mercury, so much of these liquids "entered as was sufficient to fill the glass within about oth

part of its whole contents; and this small residuum may safely be concluded to have been occasioned by some impurity in one or both kinds of air. The moisture adhering

to the glass, after these deflagrations, being wiped off, or “ sucked up by a small piece of sponge paper, first carefully

weighed, was found to be exactly, or very nearly, equal in weight to the airs employed. In some experiments, but “ not in all, a small quantity of a sooty-like matter was found

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