note, (which, if they did not read, they at least ought to have read, and might have been supposed to have considered, because it is given both in the 'Philosophical Transactions' and in Lord Brougham's Historical Note), viz.: "Previous to “Dr. Priestley's making these experiments, Mr. Kirwan had "proved, by very ingenious deductions from other facts, that inflammable air was, in all probability, the real phlogiston in “an aërial form. These arguments were perfectly convincing "to me." 66 66 66 66 "it is So in Mr. Watt's paper we find these expressions :-" It "was reasonable to conclude, that inflammable air must be the pure phlogiston, or the matter which reduced the calces to "metals;"- -"the inflammable air being supposed to be wholly phlogiston;" "inflammable air or phlogiston;" worthy of inquiry whether the greater part of the heat let "loose was not contained in the phlogiston or inflammable air," + &c. &c. So, also, in writing to Dr. Black on the 21st of April, 1783,-the very day on which his letter to Dr. Priestley was first written, although the second edition, read a year afterwards at the Royal Society, was written on the 26th of the same month, he says, "therefore inflammable air is the thing called phlogiston." So to Mr. Hamilton, on the 22rd of April, the first of the three deductions he states is, "pure inflammable air is phlogiston itself." Above all, in the same letter to Dr. Black, as if to exclude all possibility of any cavil being raised, on the ground of the language in which his theory is expressed, he further states his conclusion to be, "that water is composed of dephlogisticated and inflam"mable air." 66 66 Neither is the objection, thus groundlessly stated, an original one, nor has it now been for the first time effectually answered. For, nearly half a century ago, a very able pen thus wrote: "We have said that the theory of Mr. Watt is 66 now demonstrated to be true. To this assertion, an objec"tion may be raised from the language in which he states "his theory; for he explains it by using the word 'phlo * Phil. Trans.,' 1784, p. 331. † Ibid., pp. 349, 350, 352. 'giston,' a word which is now exploded from philosophy as "the name of an imaginary substance. But it is sufficient to "reply, that Mr. Watt uses the word phlogiston as synonymous "with inflammable air." And that hydrogen, and not any other sort of inflammable air, (such as the gas from charcoal, on which also Priestley experimented), was intended by Mr. Watt, is evident from the circumstance of the equality of the weight of the water, on which, as reported by Priestley, he so much dwells; but which could not have been attained except by that particular gas being employed: as well as by Mr. Watt's expression of the inflammable air being "the "matter which reduced the calces to metals," a very exact description of hydrogen, and of no other gas. It is evident that the term hydrogen, derived from the Greek word for water, and designating one of its constituents, could not have been invented till after the composition of that fluid had been ascertained. Lavoisier himself, the inventor of the term, did not use it till a later period; and he expressly says, in the beginning of his paper, "The inflammable air "which I understand when I mention it in this Memoir, is "that which is obtained, either from the decomposition of "water by iron alone, or from iron and zinc dissolved in "vitriolic and marine acids; and, as it appears proved that "in all cases that air comes originally from water, I shall "call it, when it presents itself in the aëriform state, aqueous inflammable air; and when it is engaged in any combina"tion, aqueous inflammable principle." That passage is one of those additions to the paper, which are said not to have been made till after November, 1783; for it contains an allusion to the experiments made with M. Meusnier, which had not been performed at that date, but were described in the Memoir read at Easter, 1784. 66 But in what respect was Cavendish superior to Mr. Watt on this point? Even in 1784 he used neither the term hydrogen at all, nor uniformly the term inflammable air; for his conclusion is in that year thus stated:-"There seems "the utmost reason to think that dephlogisticated air is only "water deprived of its phlogiston, and that inflammable air is "either phlogisticated water or else pure phlogiston; but in "all probability the former,"-a conclusion infinitely more dim and distant from the truth than those which we have just cited from Mr. Watt's paper and letters. Such also is the language in which the rest of Mr. Cavendish's paper, on this subject, is couched; and even with all the additional lights supplied by Watt and Lavoisier to guide him, it is undeniable that his conclusions are at least as much embarrassed and disguised as those of either of the others: while M. Arago, that equal justice might be done to all parties, used exactly the same substitution in speaking of Cavendish's labours; thus making them, as well as those of Mr. Watt, more intelligible to those accustomed only to the modern nomenclature. Lastly, it has been asserted, that Cavendish's mere experiments, apart from the formation of any theory, "involved "the notion, and established the fact," of the composition of water. So in some sense did Priestley's ;-so did Warltire's; nay, on the same principles, it might be hard to withhold the merit of priority from Macquer and Sigaud de Lafond, who, by the combustion of gases, produced water which appeared to them to be pure. It may be true that Macquer's data, so far as he has recorded them, were scarcely sufficient to have led him readily to form a just opinion on the subject. But Priestley and Warltire, in their experiments of 1781, came very much nearer the last experimental step afterwards arrived at by Cavendish: the loss of weight which Warltire detected after the combustion was almost imperceptible, and was at once to be accounted for by the least imperfection in his apparatus. Yet they both confidently attributed the formation of the dew to the mere deposition of suspended moisture. So late as 1784, Meusnier and Lavoisier, in the commencement of their Memoir on the Decomposition of Water, remark, * 'Mémoires de l'Académie' for 1781, published in 1784. 66 66 66 66 that "there have nevertheless been doubts raised on that "entire reduction of two aëriform fluids into water; and, notwithstanding the precautions taken by M. Lavoisier, to ensure, as much as possible, precision in so delicate an experiment; notwithstanding the conformity of the result "obtained nearly at the same time by M. Monge, in the "laboratory of the school of Mézières, with a very exact apparatus and the most scrupulous attention, some persons "have believed, that the water which proceeds from that "operation may be attributed to humidity held in solution. by the airs, and deprived of support at the moment of their "combustion." Such was, then, the experience of MM. Meusnier and Lavoisier, who, it will not be denied, had the best means of ascertaining the impression really made on the scientific world by those experiments, which, to their own minds, had brought conviction of the truth of the theory of the composition of water. And in that Memoir, read as late as the 21st of April, 1784, when the conclusions of Watt, and the able reasoning of Lavoisier in his first paper, and the views of Cavendish, and the confirmatory observations of La Place, and Meusnier, and Monge, had all become well known, those two distinguished philosophers thus found it needful to begin anew their argument, by that positive and particular statement of the opposition which was made to the theory, or at least of the difficulties which, with some, stood in the way of its reception. In the same year, Mr. Kirwan appears to have thought that he ventured far in admitting himself to be "nearly con"vinced," that, when the two gases are fired, "water is "really produced."* The example of caution, which had been set by so many sage experimentalists, was further illustrated in the case of Dr. Black, who, in his correspondence with Mr. Watt, only remarks of the steps immediately preceding his discovery, that they appeared to him "very sur"prising ;" and, in 1790, thus wrote to Lavoisier:-"I long "experienced a great aversion to the new system, which * 'Phil. Trans.' for 1784, p. 167. 66 represented as erroneous that which I had regarded as a "sound doctrine; nevertheless, that aversion, which was "caused by the power of habit alone, has gradually dimi"nished, yielding to the clearness of your demonstrations, " and the solidity of your plan." Nay, the most conspicuous instance of the same truth, (at least in France, for it would be hard to point out a more signal one than Priestley), is to be found in the case of M. Monge himself. He, as has been shown,† was perfectly aware of the result of the combustion of the two gases; having performed the experiment on a greater scale, and obtained its product in a larger quantity, than was done by any other at so early a date; and yet he appears, at a period as late as 1786, when his paper was printed, to have entertained very uncertain notions as to the nature of the change which was operated, and very great doubts as to the theory, which would now be so idly represented to have been obvious to any one, who performed the experiments on which it might have been founded. After enumerating the various deductions which he thought possible, "either consequence," says M. Monge, " is equally extraordinary; and we could not decide between "them without experiments of another sort." And he concludes, "we have, then, need of much further light on this subject; but we are entitled to expect it, both from time, "and from the concourse of the labours of physical enquirers." The hesitation in yielding his assent to the new doctrine, which Monge thus philosophically, but perhaps even too cautiously expresses, is as great, as the incredulity of Priestley was persevering. It is very interesting to find Monge in 66 * Annales de Chimie,' viii. p. 227. It is said by Mr. Yeats, in his 'Obser'vations on the Claims of the Moderns to some Discoveries in Chemistry and 'Physiology' (1798, p. 247), that when Lavoisier was informed, by this letter from Dr. Black, of his conversion to the anti-phlogistic doctrine, so great was his joy in having acquired such able support, that he published it in all the newspapers in Paris. It is an affecting incident, which we re cord on the same authority, that |