Whether it was an

that fixed air was an oxide of carbon. acid oxide or not, could not be determined by analysis. That problem could be solved only by ascertaining whether or not it formed salts by combining with bases. That is the only method possible at the present day, and was the one Black followed."

So very easy is it for ill-informed and inaccurate writers to launch charges of ignorance and inaccuracy and carelessness against others! M. Arago will no doubt be fully sensible of this truth, though he will furnish no example of it in his own person or in his defence of himself.

As for the mysterious passage in p. 117, which states that the critic had prepared a commentary on my account of Mr. Cavendish's experiment regarding the density of the earth, but that, possibly through pity towards a fellow creature, he suppressed it, giving, however, as the result, that it would show "the most ingenious and entire distortion, not merely of nearly every step in the process itself, but of nearly every principle involved in it,"-I can only, with all humility, but with all comfort, mention, that the passage is none of my own, being taken very closely from the work of a most profound mathematician, professor of the science in one of our Universities; and that, in borrowing it, I find that I have avoided two errors in the original, one the misprint (apparently) of friction for torsion, the other the confining the comparison to the time of the oscillation, whereas I make it general, including therefore both the length and the duration. I wrote the account at a distance from Mr. Cavendish's paper, and therefore took it at second hand. If friction is intended, and not torsion, in the account which I copied, it is an omission certainly. How it can be called a distortion, I cannot comprehend, nor can the learned Professor himself, whom I have consulted. I say nothing of a similar charge respecting the Torricellian experiment, except to observe, that my reference to it is most studiously framed to exclude the very construction put upon it by the critic, as the sentence beginning "unless" must plainly show to any candid reader.

Now I write with great and unfeigned personal respect for the learned critic, who, had his work been given under the sanction of his name, would have been more careful in

all likelihood. But one discovery having been mentioned, I must add, that he also has made another, a discovery which, I think, would have surprised my friend Mr. Vernon Harcourt himself, as much as it did his other readers, "that there are very few amongst the most distinguished of our countrymen superior to" that reverend and excellent person, “either as a writer or as a man of science;" so great a length will zeal for his friend and fellow polemic carry a critic engaged in a controversy.

But this zeal is readily explained by the reflection that fellow-combatants in any controversy which heats their tempers, are blind to each other's deficiencies, and exaggerate each other's perfections; they are also prone to exaggerate the services rendered by each other to the common cause. "The unanswerable arguments of my noble, or my honourable friend," is a very familiar expression on every side in Parliamentary debates, which one thus finds are conducted on both sides by combatants equally invincible, and therefore ought always to prove drawn battles. So the critic holds Mr. Vernon Harcourt's publication from Mr. Cavendish's Journals, to be decisive in favour of his contention; whereas those extracts demonstrate, that Mr. Cavendish never had, even privately, given the explanation of his experiment until after Mr. Watt's theory was in the hands of the Royal Society. I am very far from arguing upon this important publication of Mr. Vernon Harcourt's, that Mr. Cavendish borrowed the hint from Mr. Watt; but at least it demonstrates that Mr. Watt had reduced his theory to writing before Mr. Cavendish, and could not by possibility have borrowed it from him.

It must once more be repeated, that I never charged or thought of charging Mr. Cavendish with having obtained from Mr. Watt's paper his knowledge of the composition of water, and having knowingly borrowed it, however suspicious a case Mr. Harcourt's publication may seem to make. Both those great men, in my opinion, made the discovery apart from each other, and ignorant each of the other's doctrine. Mr. Cavendish was a man of the strictest integrity, and the most perfect sense of justice. His feelings were very far inferior to his principles. He was singularly callous to the ordinary calls of humanity, as

there exist positive proofs sufficient to satisfy the polemical writer upon whose paper I have been commenting if he has any mind to see them. Nor do they rest on my assertion, for I never had any intercourse with him except in society. But the pursuits of a philosopher and the life of a recluse, which had so entirely hardened his heart, had not in the least degree impaired his sense of justice; and my own belief is, that he as entirely supposed himself to have alone made the discovery in question, as Sir Isaac Newton believed himself to be the sole discoverer of the nature of light, and the theory of the solar system.

Mr. J. Watt and M. Arago may now safely be left to carry on the controversy, whether with the reverend author, or with his able and ingenious, though somewhat over-zealous critic. The subject left in their hands is safe, and the truth is sure to prevail. In these circumstances I am far from feeling any anxiety as to the result, or any desire to anticipate the arguments and the statements which must so soon be brought forward. But as I have been freely and most rashly charged with inaccuracy, with inattention to facts, even with having omitted to read the original papers on which the question turns, and charged, in company with my friends M. Arago and Mr. J. Watt, (one of the most careful, laborious, and scrupulously exact of men,) I may simply assert, that as regards myself no imputation can well be more groundless; for there is not a single one of the whole papers which I have not repeatedly and sedulously examined, both alone and in company with others who took an interest in the controversy. I might add, that never was a charge made with a worse grace than this by the ingenious, and most careless, and very moderately-informed critic who has mixed in the discussion; for assuredly he has not taken the trouble to read the papers, or to make himself acquainted with the works which every chemist, even every student of chemistry familiarly knows. What shall we say of a writer who undertakes to discuss this question, with no better provision for handling it, than is betokened by his broadly affirming that Mr. Watt himself never preferred the disputed claim, when there exists his own paper of 1784 in the Philosophical Transactions,' referring to and indeed containing his letter of April, 1783? Nay, what

shall we again say of the same critic as broadly asserting, that no one ever in Mr. Cavendish's lifetime brought it forward, when Professor Robison in the Encyclopædia, Dr. John Thomson in his celebated Translation of Fourcroy, Dr. Thomas Thomson and Mr. Murray, each in their Elements of Chemistry,' and Mr. W. Nicholson in both his 'Dictionary' and his other works, all state Mr. Watt's claim in the very words in which M. Arago and myself now have urged it, nay, Sir C. Blagden states it in his letter to Crell, and all these long and long before Mr. Cavendish's death,* to say nothing of others, as Dr. Thomson, in his History of the Royal Society,' published since? As to Mr. Vernon Harcourt's appealing boldly to Dr. Henry's authority, and preserving a profound silence when I quoted his letter, expressly negativing that confident statement, I say nothing; because it is a matter not easily handled, consistently with the respect and esteem in which I have ever held my reverend friend.

* Professor Robison in 1797; the Translation of Fourcroy earlier.

NOTE TO THE LIFE OF SIMSON.

The remarkable circumstance of the case of the comet's motion, for which Sir I. Newton's solution was intended, proving to be the porismatic case of the construction, has been mentioned in the text. It has been sometimes considered as singular, that this did not occur to himself, the more especially as he evidently had observed two cases in which the problem became indeterminate—namely, when the lines were parallel, and when they all met in one point, for he excepts those cases in express terms (Prin. lib. 1. Lem. xxvii.) It may be observed, that such oversights could very rarely happen to the ancient geometers, because they most carefully examined each variation in the data, and so gave to their solutions such a fulness as exhausted the subject.

The commentators on the Principia (Le Seur and Jacquier) make no mention of the omission. The circumstance of the Porismatic case was not discovered till ten years after their publication, when F. Boscovich found it out, in 1749. But it is very extraordinary that Montucla appears to have been unaware of the matter, although the first edition of his work did not appear till 1758. Nor is the least reference made to it in the second edition, which was published the year he died (1799.) There are other omissions in both editions, and also in the continuation. He appears well to have understood the ancient method, and to have read and examined some of the most celebrated works upon it. He had given due praise to Simson in his first edition; and to Lord Stanhope, who sent him the 'Opera Reliqua ;' and we find in the second edition a full note upon the subject, II. 277. In the continuation— III. 11, and seq., we have further indications of the attention which he had bestowed upon the ancient geometry;