shows clearly that M. Cuvier never had read Mr. Cavendish's paper any more than he had read Dr. Black's Treatise, and his Lectures. Another proof is his asserting that Mr. Cavendish discovered the air evolved from burning charcoal to be fixed air. His paper contains not one word on that air as connected with burning charcoal. Nay, so far is Mr. Cavendish from assuming to himself the discovery of its identity with the air evolved in fermentation, that he expressly says Dr. Macbride had discovered the evolving of fixed air in that process, and that he himself only made his experiments to ascertain if any other air was also evolved, when he found inflammable air also to come. Apparently he had not been aware of Dr. Black's experiments in 1757. The Lectures would also have shown M. Cuvier that Dr. Black, as early as 1766, showed his friends the ascent of a bladder filled with inflammable air, long before the experiments of M. Charles, to whom the earliest observation of this fact is by M. Cuvier rashly ascribed.

M. Cuvier mentions Macquer as having first observed the deposit of moisture when inflammable air is burned. He says nothing of Mr. Warltire's experiment, though Mr. Cavendish himself states expressly ("Phil. Trans." 1784, p. 126), that it was the deposit of dew observed by Warltire, which set him on making his experiments. From this omission of M. Cuvier, it is plain that he never took the trouble to read the paper of Mr. Cavendish, which, as he refers to it by volume and page, he may, therefore, have seen-he never could have read it. This also accounts for his singular assertion, that Mr. Cavendish's discoveries were explained with an evidence and a clearness more astonishing than the discoveries themselves.

It is equally incorrect to affirm, as M. Cuvier appears to do, p. cxxxiii., that the decomposition of water suggested by M. de la Place, and performed by M. Lavoisier, became "la clef de la voûte," for the analytical experiment is equivocal, and the synthetical alone is precise. He says that M. Monge had made the same experiments as Mr. Cavendish, and had the same idea, "avoit en la même idée," probably meaning that of a quantity of water being formed equal to the quantity of airs burned, and had communicated the result to Lavoisier and La Place; and Monge seems really to give the first notion of water being composed of these airs, as La Place's; for he says, "Si la combustion de ces airs donne de l'eau, dit M. de la Place c'est qu'ils resultent de sa decompositions." Had M. Cuvier really read the work he so often cites, the "Philosophical Transactions," he would have found Mr. Watt's letter, and he could hardly have avoided mentioning the first idea of the composition as his.

But truly it is to be lamented that the history of science should be written with such remarkable carelessness, and such manifest inattention to the facts. To find mistakes so very gross in the works of ordinary writers might excite little surprise, but when they are embodied in the history of the National Institute, and when they come to us under the name, among the very first in all sciences, of Cuvier, we may at once wonder and mourn.

Since the Life of Watt was published, a very strange attack on both M. Arago and myself, but more on my illustrious colleague, has appeared in the "Quarterly Review." The ingenious and (as far as this controversy is concerned) not very learned critic appears to be led away by the excess of his zeal for Mr. Cavendish. I leave him in the hands of M. Arago, who will observe with some wonder that he has been attacked and judged and condemned by a chemist so well versed in that science, and so reflecting as to announce the astonishing novelty, that the exhibition of sulphur to sulphuric acid reduces that acid and restores it to its primitive state of sulphur! The writer had probably read somewhere that sulphuric acid is reduced to sulphurous by the process; for he is assuredly the first that had ever hit upon the acid's reduction by sulphur to its primitive state."* I have lying before me fifteen pages of statements of chemical errors in the thirty-four pages of the paper; and as these are the work of a most experienced and learned and practical chemist, whom I consulted on the above and other parts of the paper, I have entire reliance on his report and opinion. I must also add that he completely bears out, by the authority of his concurring opinion, the statements which I had ventured to make respecting Dr. Black's discoveries, with the single exception that he is not aware how far I am justified in stating the greater specific gravity of fixed air as known to him before Mr. Cavendish's experiments in 1766. My reason for so stating was my distinct recollection of Dr. B. having in his lectures shown us the experiment of pouring fixed air out of a receiver on a candle, and his having given this as a property originally known to himself when he discovered the gas, though it is very true that the published lectures do not decide either way the question of his early knowledge. His not mentioning Mr. Cavendish or any one else as having first taught it him is with me, who well knew his scrupulous exactness in such matters, quite decisive of his having himself observed it.

I shall only cite further my correspondent's note on the reviewer's statement, "that I was wrong in ascribing to Dr. Black the discovery that fixed air has acid properties." (p. 110.)—“The reviewer adds that the acidity of fixed air was indicated for the first time by Priestley and his fellow-labourers, and only completely established by Lavoisier, who showed fixed air to be carbonic acid, or a mixture of carbon and oxygen." His lordship is quite right, and the reviewer doubly and egregiously wrong. Priestley did not indicate for the first time the acidity of fixed air. Whether he understood Black's views concerning it does not appear, but he expressly disclaims the discovery as his own. His words are, 'It is not improbable but that fixed air itself may be of the nature of an acid, though of a weak and peculiar sort. Mr. Bergman of

The process of reducing phosphoric acid to its primitive phosphorus had just been stated, and the writer adds, "A similar succession of phenomena are presented by sulphur, &c. ;" and he enumerates sulphur as one of the bodies which reduce the acid to its primitive state.

Upsal, who honoured me with a letter upon the subject, calls it aerial acid; and among other experiments to prove it to be an acid, he says that it "changes the blue juice of tournesole into red." ("Phil. Trans.," 1772, vol. lxx., p. 153.) It does not appear whether Black was aware of the reddening action of fixed air on vegetable colours, but he was abundantly aware of the functions of fixed air as an acid; that is, of its power to neutralize bases, and to form salts by combination with them. Black's own words are, These considerations led me to conclude that the relation between fixed air and alkaline substances was somewhat similar to the relation between these and acids; that as the calcareous earths and alkalies attract acids strongly, and can be saturated with them, so they also attract fixed air, and are in their ordinary state saturated with it.' (Experiments upon Magnesia Alba,' &c, p. 50.) The whole page might be quoted. Nothing could be more satisfactory to a chemist than this statement. The modern definition of an acid is a substance which neutralizes bases, and by combination with them, forms salts.' Power to affect vegetable colours, or sour taste, the vulgar attributes of an acid, are wanting in many of the most powerful of them; for example, in silicic acid. The reviewer's reference to Lavoisier is quite meaningless. The French chemist showed that fixed air was an oxide of carbon. Whether it was an 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 for ascertaining 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 man. Now I write with great and unfeigned personal respect for the learned critic, who, had the 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 excelÏent 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 fellowcombatants, in any controversy which heats their tempers, are blind to each other's deficiences, and exaggerate their 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 these 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. 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 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 celebrated 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 in his letter to Crell, and all 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 for 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.

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* Professor Robison in 1795; the Translation of Fourcroy earlier.