than a forward movement in the march of chemical science. It is most probable, however, judging from internal evidence, that neither M. Arago nor Lord Brougham have ever read this or any other original scientific document connected with this controversy. Mr. Watt's paper in the Philosophical Transactions,' the only one he ever published (for there are others of his unpublished papers in the archives of the Royal Society), is singularly obscure, and perfectly unintelligible to any reader who is not familiar with the experiments and speculations of Dr. Priestley, the most remarkable of whose errors he has incorporated in the applications of his theory; presenting in this respect a singular contrast to the corresponding memoir of Mr. Cavendish, who clears away, at every step of his progress, the difficulties and anomalies, whose correct explanation was dependent on this theory, which had been accumulated in the experiments and the reasonings of his predecessors and contemporaries. But if, instead of adopting the unprofitable and worthless form of the theory of the composition of water proposed by Mr. Watt, we should assume it to be expressed in the clear and definite language assigned to it by M. Arago, upon what grounds could he claim the title of its discoverer? The chemist who first made the fundamental experiment, and showed, what was no easy task, that the explosion of definite proportions of oxygen and hydrogen, under whatever names they appeared, produced pure water and pure water only, equal in weight to the gases consumed, had already announced all that the most elaborate theory could furnish. Mr. Cavendish fully appreciated, from the beginning, the character and the value of the result which he had obtained, and required no theory to guide him to the interpretation of its meaning; and when Dr. Blagden communicated the details of these experiments to Lavoisier, neither the one nor the other conceived that the addition of any theoretical views were necessary to make its importance understood. We can discover, in fact, hardly an experiment or observation of primary importance in the history of the science which is more immediately and indissolubly connected with the theoretical inference deducible from it. We know that in expressing this opinion we are opposed to the authority of M. Arago 'Dois-je craindre,' says he, 'd'avoir attaché trop d'importance à la théorie que Watt imagina pour expliquer les expériences de Priestley? Je ne le pense pas. Ceux qui refuseraient un juste suffrage à cette théorie, parce qu'elle semble maintenant une conséquence inévitable des faits, oublieraient que les plus belles découvertes de l'esprit humain ont été surtout remarquables par leur simplicité. Que fit Newton, lui K 2 même, même, lorsque répétant une expérience déjà connue quinze siècles auparavant il découvrit la composition de la lumière blanche? Il donna de cette expérience une interprétation tellement naturelle, qu'il paraît impossible aujourd'hui d'en trouver une autre. Tout ce qu'on tire, dit-il, à l'aide de quelque procédé que ce soit, d'un faisceau de lumière blanche, y était contenu à l'état de mélange. Le prisme de verre n'a aucune faculté créatrice. Si le faisceau parallèle et infiniment délié de lumière solaire qui tombe sur sa première face, sort par la seconde en divergeant et avec une longeur sensible, c'est que le verre sépare ce qui dans le faisceau blanc était, par sa nature, inégalement refrangible. Ces paroles ne sont pas autre chose que la traduction littérale de l'expérience connue du spectre solaire prismatique. Cette traduction avait, cependant, échappé à un Aristote, à un Des Cartes, à un Robert Hooke.' It may be quite true that the highest generalizations of one age become the elementary truths of another, and that it requires an intimate knowledge of the precise conditions of opinion which prevail at every successive period of scientific history, to be able to form a just conception of the difficulties which embarrass or prevent those advances in the march of invention and discovery, which appear to us most open and unopposed. We are not disposed, however, to consider the interpretation of the memorable experiment referred to by M. Arago as presenting a parallel case to the one under consideration. It may be admitted-even without invoking the very apocryphal authority of Aristotle-that the formation and succession of the prismatic colours were perfectly well known before the time of Newton: but who before his time (though the acute historical vision of M. Arago will probably not fail to discover them) had taken the precautions requisite for forming the prismatic spectrum itself-for accurately defining the spaces which the successive colours occupied-for subjecting the successive colours themselves to a similar analysis, and thus determining their undecomposable character-and, finally, as an experimentum crucis, reversing the problem and reforming white light from the reunion of the colours into which it had been previously decomposed? If the successive steps of these beautiful experiments had been submitted to Des Cartes or to Hooke, it is most probable that they would not have hesitated to give them their correct interpretation: and though M. Arago, with all the lights of modern knowledge, and with the vigorous grasp with which he is accustomed to seize the great truths of philosophy and to resolve them into the most simple principles upon which they are dependent, may be enabled to translate the single experiment of the formation of the prismatic spectrum, as in the passage we have quoted, into its appropriate language, yet we greatly doubt whether either Newton or the most distinguished of his predecessors would have been equally success ful ful if the subsequent experimental steps in the process of investigation had never been made. But the question may be asked, if Priestley had been put, as has been asserted, into possession of all the early experiments of Cavendish, why did he hesitate to draw the correct and necessary conclusion from them? Whilst we are ready to admit the full force of the objection, we should observe, as we have indeed remarked before, that Priestley's mind was not disciplined to habits of correct inductive reasoning: he seldom appreciated the philosophical value of views which were different from his own, and when called upon to notice them, he rarely stated them correctly: he was embarrassed by his blind attachment to the phlogistic theory, and still more so by the real or apparently contradictory results of his own experiments: and though he himself states, in the first instance, that Mr. Cavendish had pointed out the influence of water in many of these experiments, and had then, it seems, satisfied his mind upon the correct interpretation to be given to them, yet we find his philosophical scepticism speedily recurring; and in his later papers he is equally disposed to impugn the correctness of these results, whether obtained by Mr. Cavendish or himself (the capital experiment to which M. Arago so often refers being included in their number), as well as of the explanation which had been given of them. Again, the theory of Watt, which was framed, as we have shown, not to correct the errors, but to reconcile the results of the experiments of Priestley by the sacrifice of the permanent character of the gases, was not, on that account, received by him with greater favour. He appears to have rejected it from the first, equally with that of Mr. Cavendish. M. Arago himself has quoted a passage from a letter to Mr. Watt, dated the 29th of April, 1783, three days after their joint papers had been sent to the Royal Society, in which he says- Look, with surprise and indignation, upon the drawing of an apparatus by which I have for ever upset your beautiful hypothesis;' and though we find indications, in Priestley's subsequent writings, of an occasional and passing belief in the prevalent theories of the composition of water, yet it appears that he speedily relapsed into his normal condition of fixed and determined incredulity. But to return to M. Arago and his argument : - "Une théorie,' says he, 'dont la conception n'eût présenté aucune difficulté, aurait été certainement dédaignée par Cavendish. Rappelez-vous avec quelle vivacité, sous l'inspiration de cet homme de génie, Blagden en réclama la priorité contre Lavoisier.' The name of Mr. Cavendish seems to exert a fatal influence both upon M. Arago and Lord Brougham: it rarely presents itself without giving rise to some extraordinary error of fact with one of them, and of philosophy with the other. It was not the theory of the composition of water, but the extent to which the results of the experiments upon which it rested had been communicated to Lavoisier, which was the subject of the reclamation made by Dr. Blagden on behalf of Mr. Cavendish. Though we are fully satisfied, for the reasons above stated, that no arrangement of dates could give countenance and support to the claim to this discovery which has been so imprudently put forward by the friends of Mr. Watt, we think it proper, in order that our readers may be enabled to estimate the force and relevancy of the remaining observations of M. Arago, to subjoin the following brief statement of the real chronology of the principal documents which are concerned in the dispute. On the 26th of April, 1783, Mr. Watt wrote a letter to Dr. Priestley, containing an outline of his theory, which was appended to his paper entitled Experiments relating to Phlogiston and the seeming conversion of water into air,' which was read to the Royal Society on the 19th June following: this letter, however, was not read, having been previously withdrawn by its author; and if we may judge from modern practice, it remained, in the mean time, locked up in the private box of the Secretary (Dr. Maty, not Dr. Blagden, as asserted by M. Arago), for no paper is deposited in the archives of the Royal Society until it has been publicly read, and its fitness for publication decided upon by the Council; there is no reason, therefore, to believe that the contents of this letter were made known to Mr. Cavendish, to Dr. Blagden, or to any other person. The substance of the first letter was incorporated into a second addressed to M. de Luc, which is dated 26th of November, 1783, but which was not read before April, 1784: it was published in the Transactions for 1784. Mr. Cavendish's paper was read on the 15th of January of that year, and published in the same volume. It further appears that Dr. Blagden communicated the result of Mr. Cavendish's experiments on the composition of water to M. Lavoisier, at Paris, on the 24th of June, 1783, two months after the date of Mr. Watt's first letter to Dr. Priestley; and it is the principal object of the documentary evidence and of the inferences drawn from it, which Lord Brougham communicated to M. Arago, to show that during this short interval Mr. Cavendish made his first experiments, which either had been or might have been suggested to him, by his obtaining, through private information or public report, a knowledge of the theory propounded by Mr. Watt. It is hardly necessary to add that either supposition would equally lead lead to the conclusion that Mr. Cavendish, in the statements which he authorized or made, had been guilty of a deliberate suppression or a misrepresentation of the truth; and, we may further add, to the necessary inference that the MS. papers which Mr. Harcourt has lithographed, had been fraudulently prepared with erroneous dates, with a view to give countenance to his claims, in case they should ever be called in question. But to return to the further statements of M. Arago: 'Parmi les prétendants à cette féconde découverte, nous allons maintenant voir paraître les deux plus grands chimistes dont la France et l'Angleterre se glorifient. Tout le monde a déjà nommé Lavoisier et Cavendish. 'La date de la lecture publique du mémoire dans lequel il développa ses vues sur la production de l'eau par la combustion de l'oxygène et de l'hydrogène, est postérieure de deux mois à celle de dépôt aux archives de la Société Royale de Londres de la lettre déjà analysée de Watt.' M. Lavoisier's memoir was not read in June, 1783, but partly in the November and partly in the December of that year, and additions were subsequently made to it. We have already stated that Mr. Watt's letter was not deposited in the archives of the Royal Society, so as to be accessible to its members. Le mémoire célèbre de Cavendish, intitulé Experiments on Air, est plus récent encore; il fut lu le 15 Janvier, 1784. On s'étonnerait avec raison que des faits aussi authentiques eussent pu devenir le sujet d'une polémique animée, si je ne n'empressais de signaler à votre attention une circonstance dont je n'ai pas encore parlé. Lavoisier déclara, en termes positifs, que Blagden, Secrétaire de la Société Royale de Londres, assista à ses premières expériences du 24 Juin, 1783, et " qu'il lui apprit que Cavendish ayant déjà essayé, à Londres, de brûler du gaz hydrogène dans les vaisseaux fermés, avait obtenu une quantité d'eau très sensible." 'Cavendish rappela aussi dans son mémoire la communication faite à Lavoisier par Blagden. Suivant lui, elle fut plus étendue que le chimiste Français ne l'avouait. Il dit que la confidence embrassa les conclusions auxquelles les expériences conduisaient, c'est-à-dire, la théorie de la composition de l'eau. 'Blagden, mis en cause lui-même, écrivit dans le Journal de Crell, en 1786, pour confirmer l'assertion de Cavendish. A l'en croire, les expériences de l'académicien de Paris n'auraient même été qu'une simple vérification de celles du chimiste Anglais. Il assure avoir annoncé à Lavoisier que l'eau engendrée à Londres avait un poids précisement égal à la somme des poids de deux gaz brûlés. "Lavoisier," ajouta enfin Blagden, "a dit la vérité; mais pas toute la vérité." Un pareil reproche est sévère; mais fut-il fondé, n'en atténuerai-je pas beaucoup la gravité, si je montre que, Watt excepté, tous ceux dont les noms figurent dans cette histoire s'y étaient plus ou moins exposés ? 'Priestley rapporte en détail et comme les siennes des expériences dont |