the nature of the subject, that he made further trials during the year 1782.

That Mr. Watt formed his theory during the few months or weeks immediately preceding April, 1783, seems probable.* It is certain that he considered the theory as his own, and makes no reference to any previous communication from any one upon the subject, nor of having ever heard of Mr. Cavendish drawing the same conclusion.

The improbability must also be admitted to be extreme, of Sir Charles Blagden ever having heard of Mr. Cavendish's theory prior to the date of Mr. Watt's letter, and not mentioning that circumstance in the insertion which he made in Mr. Cavendish's paper.

It deserves to be farther mentioned, that Mr. Watt left the correction of the press, and everything relating to the publishing of his paper, to Sir Charles Blagden. A letter remains from him, to that effect, written to Sir Charles Blagden, and Mr. Watt never saw the paper until it was printed.†

Since M. Arago's learned 'Eloge' was published, with this paper as an Appendix, the Rev. W. Vernon Harcourt has entered into controversy with us both, or, I should rather say, with M. Arago, for he has kindly spared me; and while I acknowledge my obligations for this courtesy of my reverend, learned, and valued friend, I must express my unqualified admiration of his boldness in singling out for his antagonist my illustrious colleague, rather than the far weaker combatant against whom he might so much more safely have done battle. Whatever might have been his fate had he taken the more prudent course, I must fairly say, (even without waiting until my fellow champion seal our adversary's doom), that I have seldom seen any two parties more unequally matched, or any disputation in which the victory was so complete. The attack on M. Arago might have passed well enough at a popular meeting at Birmingham, before which it was spoken; but as a scientific inquirer, it would be a flattery running the risk of seeming ironical to weigh the reverend author

That the idea existed in his mind previously, is proved by his declarations to Dr. Priestley, cited by the latter; by his own assertions, p. 335 of his paper; and by the existing copies of his letters in December, 1782.-[NOTE BY MR. JAMES WATT, JUN.]

The notes of Mr. James Watt formed part of the manuscript transmitted to me by Lord Brougham; and it is at the express desire of my illustrious fellowmember, that I have printed them, as a useful commentary upon his essay.[NOTE BY M. ARAGO.]

against the most eminent philosopher of the day; although upon a question of evidence, (which this really is, as well as a scientific discussion), I might be content to succumb before him. As a strange notion, however, seems to pervade this paper, that everything depends on the character of Mr. Cavendish, it may be as well to repeat the disclaimer already very distinctly made of all intention to cast the slightest doubt upon that great man's perfect good faith in the whole affair; I never having supposed that he borrowed from Mr. Watt, though M. Arago, Professor Robison,* and Sir H. Davy, as well as myself, have always been convinced that Mr. Watt had, unknown to him, anticipated his great discovery. It is also said by Mr. Harcourt that the late Dr. Henry having examined Mr. Watt's manuscripts, decided against his priority. I have Dr. H.'s letter before me of June, 1820, stating most clearly, most fully, and most directly, the reverse, and deciding in Mr. Watt's favour. I must add, having read the full publication with fac-similes, Mr. Harcourt has now clearly proved one thing, and it is really of some importance. He has made it appear that in all Mr. Cavendish's diaries and notes of his experiments, not an intimation occurs of the composition of water having been inferred by him from those experiments earlier than Mr. Watt's paper of Spring, 1783.

1

No. III.

SUMMARY OF THE HISTORY OF THE PROGRESS TOWARDS THE DISCOVERY OF THE COMPOSITION OF WATER, AND OF THE DISCOVERY ITSELF.

1776.

Volta fires inflammable air by the electric spark.

1776-77.

Macquer explodes mixtures of inflammable and common airs, and of inflammable and dephlogisticated airs, (but not by the electric spark), in glass vessels, not close. He makes his ob

Encyc. Brit., vol. xviii. p. 808. This able and learned article enters at length into the proofs of Mr. Watt's

claims, and it was published in 1797, thirteen years before Mr. Cavendish's death.

servation of the moisture formed when inflammable air is burned in common air, and of that moisture being pure water.

1778.

Macquer publishes his observations.

1781.

Before the 18th of April, Mr. Warltire, being encouraged by Dr. Priestley, fires, by the electric spark, a mixture of common and inflammable air in a close metal flask, weighing the vessel before and after the explosion, observing the dewy deposit, and finding only a very trifling loss of weight.

Dr. Priestley fires mixtures of common and inflammable airs, and of inflammable and dephlogisticated airs, in a close glass vessel, and observes a deposit of water on the sides of the vessel. Mr. Warltire repeats Dr. Priestley's experiment in the close glass vessel, and confirms his observation of the dewy deposit. In July, after the publication of Dr. Priestley's and Mr. Warltire's experiments, Mr. Cavendish repeats them.

No conclusion as to the real origin of the water, published by Mr. Cavendish; nor communicated to any individual, nor contained in the Journal and Notes of his experiments; nor alleged by himself, nor by any one else, to have been then drawn by him.

1782.

13th December.-Mr. Watt, in writing to Mr. De Luc and Dr. Black, mentions an opinion which he had held for many years, that air was a modification of water; and that if all the latent heat of steam could be turned into sensible heat, the constitution of the steam would be essentially changed, and it would become air.

1783.

"Dr. Priestley having put dry dephlogisticated air and dry "inflammable air into a close [glass] vessel, and kindled them by "the electric spark, finds on the sides of the vessel a quantity of "water equal in weight to the air employed."

26th March.-Mr. Watt mentions as new to him, that experiment of Dr. Priestley's.

21st April.-Mr. Watt states in his letters, both to Dr. Priestley and to Dr. Black, his conclusions, viz.: "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 composed of water deprived of its phlogiston, and united "to heat and light." He requests his letter to Dr. Priestley to be read to the Royal Society.

26th April.-Mr. Watt having re-written his letter of the 21st, sends it to Dr. Priestley, who receives it in London,-shows it to several members of the Royal Society,-among whom was Mr. Cavendish's intimate friend and private assistant, Dr. Blagden,-and then delivers it to Sir Joseph Banks the President, for the purpose of being publicly read to the Society.

Prior to the 23rd of June, Mr. Watt requests the public reading of his paper to be delayed till he should examine new experiments, said by Dr. Priestley to contradict his theory.

24th June. - MM. Lavoisier and La Place perform their experiment at Paris, at which Blagden is present. They are informed, as Lavoisier says, of Mr. Cavendish having burned the two airs and obtained water;-as Blagden subsequently says, of the conclusions of Watt and Cavendish ;-(this being the first time that any conclusion of Mr. Cavendish on the subject is referred to by any one.)

25th June.-MM. Lavoisier and La Place give an account of their experiment to the Academy of Sciences, and Lavoisier states the conclusion as to the compound nature of water, to have been drawn by La Place and himself.

June and July.-M. Monge performs his experiments at Mézières; and repeats them in October.

Martinmas.-M. Lavoisier reads to the Academy of Sciences his memoir on the composition of water.

26th November.-Mr. Watt being fully satisfied of the correctness of his theory, and hearing that Lavoisier was passing it off as his own, repeats it in his letter to Mr. De Luc, which he requests may be read to the Royal Society.

No conclusion published, nor known to have been committed to writing, nor alleged, (excepting by Dr. Blagden), to have been drawn, by Mr. Cavendish.

1784.

15th January.-In his paper read to the Royal Society this day, Mr. Cavendish, for the first time, states publicly in writing, and in his own person, his conclusions as to the compound nature of water; coinciding generally with those of Mr. Watt, but omitting the consideration of latent heat, as well as the mention of Mr. Watt's name.

March.-Mr. Watt, finding that in Mr. Cavendish's paper his

own theory had been fully explained and proved, and his name excluded, expresses his indignation, and takes immediate steps for having his own letters, of 26th April and 26th November, 1783, read at the Royal Society, with their true dates.

21st April.-MM. Meusnier and Lavoisier read to the Academy of Sciences their memoir on the decomposition of water, which is printed in the same year.

22nd April.-Mr. Watt's first letter, which had till now remained in the custody of the President, is, according to his request, read at the Royal Society.

29th April. Mr. Watt's second letter is also read. Both letters are ordered to be printed in the Philosophical Transactions.

5th May.-Dr. Blagden is appointed Secretary to the Royal Society, and is entrusted with the superintendence of the printing of both of Mr. Watt's letters, to be embodied in one paper, with marks distinguishing each from the other.

June?-M. Lavoisier's memoir is printed with additions.

July?-Mr. Cavendish's paper is printed ;-the separate copies, with the erroneous date of 1783 instead of 1784 on their titlepage; with the correct date, 1784, omitted from the heading of the paper; and the paper itself containing two interpolations, made by Dr. Blagden some months after it had been read to the Society. In one of these, Mr. Watt's name is for the first time mentioned as if by Mr. Cavendish, and his theory alluded to as his own.

August. Mr. Watt's paper is printed, under the sole superintendence of Dr. Blagden, and with the erroneous date of 1784 instead of 1783.

1786.

The paper of M. Monge is published; no date being mentioned at which it had been read.

J. P. M.

No. IV.

ADDITIONAL NOTE ON THE DISCOVERY OF THE

COMPOSITION OF WATER.

MR. ROBERT BROWN, (eminent as a botanist), who died in 1858, used in conversation to hint that he knew of the existence of some writing favourable to the claims of Cavendish; but, with an