In the year 1781, I made fome ftay at Paris, where the new afpect of meteorology was the principal object of the converfations I had with many members of the Academy of Sciences, efpecially M. M. de la Place, Lavoifier, Monge and Vandermonde. Dr Crawford's theory on the phenomena of heat obferved when subftances undergo certain changes in their nature, was then in great agitation among these philofophers, and the two laft had, from the Academy, the fpecial commiffion to examine and follow that new view. Dr Crawford had already published, in 1779, his work on animal heat, in which he had expofed Dr Black's theory and experiments on latent heat, which therefore were by this time fufficiently known: he attributed that phenomenon to an increafe of capacity, in water comparatively to ice, and in fream comparatively to water; an explanation which could be adapted to the expreflion latent heat; for, when in a fyftem of insulated subftances, heat really diminishes by a change in their capacity, that portion of heat which disappears may be in fome manner confidered as latent. I mention that circumftance, in order to prove that Dr Black's experiments were well known; and I did not hear any doubt on his originality, though Mr Wilcke had published fimilar experiments at Upfal, which I knew, from my own cafe, might have happened without imitation. However, from well known dates, Dr Black had the priority with refpect to him, and it may be with me alfo, though it does not appear. But from that time, as I continued to do in my work Idées fur la Meteorologie, I refuted Dr Crawford's theory, by proving the neceffity, as well as the reality, of thofe combinations of fire with other fubftances, which, by depriving it of the faculty of producing heat, makes it really latent, till, by fome new chemical procefs, it is again fet free. And the year after, M. M. Lavoifier and de la Place, followed, by a very ingenious procefs, one of the inquiries by which I had foretold that Dr Crawford's theory would be proved erroneous. This was to determine, according to his theory, but in various cafes, the quantity of abfolute heat, in order to know if that determination would be always the fame; but the refults of their experiments were far from that neceffary confequence of the theory; and their Memoires fur la Chaleur, in which thofe experiments were defcribed, fhook very much that theory, which at first had appeared of great importance. Meteorology, as I have faid, was my principal object in those difcuffions at Paris. I maintained the theory of evaporation by the union of fire with water, against the diffolution of water by air, as a first step toward the explanation of atmospherical phenomena, with the perspective of the transformations of aeriform fluids ་ fluids an object which was alfo followed fuccefsfully among the chemifts at Paris, but with new theories, which I thought contrary to atmospherical phenomena, and were to be judged by them. This engaged me to begin at Paris, and afterward to write in England, under the form of Letters addreffed to M. de la Place, the fame fundamental theory which, with more informations that I fhall mention, I began to publifh in 1786, under the title of Idées fur la Meteorologie, in the Introduction, and the courfe of which, I mentioned thefe particulars. All thofe circumftances were paffed, when, in the later part of 1782, I went to Birmingham, and had the fatisfaction of making my first acquaintance with Mr Watt. Dr Priestley refided alfo there at that time; and after his experiments on aëriform fluids, in which he made a continual progrefs, none could be more interesting to me, than thofe of Mr Watt on the fleam of boiling water. He was fo kind as to explain to me, not only the construction of his fleam engine, but his motives for all the effential parts, which were derived from the nature of fteam; and the modifications of that fluid, which he had thoroughly investigated, were very effential to my purfuits. I traced in them the general character of aqueous vapour, but with fome important circumftances, that I could not have obferved, either in the course of my experiments, or from what I knew then of thofe of Dr Black, which were not made with the fame view. I ftated that analogy to Mr Watt, to which at firft he objected (a circumftance which I have happened to mention in § 553. of my work); but after having more confidered the subject, he gave me himself a parti cular proof of my theory, from a circumftance which he had obferved, namely, that the common evaporation of water deprived that liquid of as much heat as it loft by ebullition, proportionally to the quantity of water which is evaporated. In the intercourfe which then began between us, this was our conftant object, as equally interefting to us both, though from different motives; and all that I can recollect of my queftions with refpect to Dr Black's opinions, concerned only two objects, namely, whether, in his expreflion latent heat, he confidered heat (with fome natural philofophers) as only a particular modification of the fubftances themselves, communicable to one another, or as the effect of a particular fluid? And what he thought of common evaporation, that is, whether he admitted or not the diffolu tion of water by air? I do not recollect his answers. Some letters paffed at that time between Mr Watt and Dr Black, which Mr Watt has been fo good as to communicate to me lately; and they agree exactly with my recollection on the principal object, under this point of view, that I could not form ei ther ther an earnest wish, or any defign, of becoming the editor of Dr Black's difcoveries, which were already much fpread, as well as his first determinations on the quantities of latent heat. But furely I wished to be well informed of the particulars of his experiments; not to make them my own, which would have been as useless as impoffible, but to understand better the progrefs made by Mr Watt; and this is what I wished to publifh. But as his new steps were connected with Dr Black's discoveries, he thought himself obliged to communicate to him the ufe which I intended to make of them; and he approved it. In fome of the vifits I made afterward to Mr Watt at Birmingham, in which he was fo kind as to receive me in his houfe, I acquired thofe informations I desired on the modifications of feam in different circumstances. We had previously projected fome experiments on the production of steam of boiling water in vacus, which he had already made in 1765, and on the cooling of water by common evaporation, both with the view of comparing the quantity of latent fire in the vapour thus produced, with that of the vapour or steam of boiling water under the preffure of the atmofphere. Thefe comparative quantities had been effential for Mr Watt, to analyze an important phenomenon which he made me obferve in the fteam engine; and for me they were very useful to afcertain my fyftem of evaporation, and to determine fome precife laws of hygrology, in view of meteorology. Such was the object of thefe experiments, in which Dr Black did not interfere. The doctrine of latent heat was known, and taken for granted; and the object was only certain modifications, the determination of which belonged to Mr Watt. Unconscious, therefore, when I wrote my work Idées fur la Meteorologie, of any engagement with refpect to Dr Black, I nei- t. ther thought of any claim for him or myfelf, or of refuting the claims of any others. I expofed only a fyitem on the combinations of fire and water, which had fucceflively been formed in my mind, with a view to meteorology. In the courfe of that expofition, I was to begin by ftating fome fundamental facts concerning liquefaction and evaporation, and I naturally began by thofe which were already published in my former work; adding to them, with grateful acknowledgment, the facts which I had learned from Mr Watt.. But thefe were connected with determinations of abfolute quantities, as from them refulted the comparative quantities, which were our object; and as the firft attempt at thefe determinations belonged to Dr Black, I carefully recorded that circumftance. As all thefe tranfactions had paffed between Mr Watt and me, in the account of which I defired to be correct; as foon as my first volume was printed, but before it was published, I fent it to VOL. VI. NO. 12. Kk Mr Watt, defiring him to communicate to me his remarks. Among these he made me aware, that from my expreffion, that Dr Black had been the first who had attempted the determinations of the quantities of latent heat, it might be supposed that he had not dif covered the fact itself. I acquiefced in that remark; and, in confequence, I fully redreffed that equivocal expreffion in an Appendix to that first volume. The fecond was also published, when, in the summer of 1788, I had the pleasure of feeing Dr Black in my houfe at Windfor; and neither then, nor in a letter which fome time after I received from him, had I reafon to fuppofe that he had any complaint against me. Such, Gentlemen, is the true ftatement of facts concerning that fecond period. Pleafe to read the Appendix above mentioned, and no doubt, I think, will remain in your mind, that if Mr Robifon had not, by a defective recollection, tranfported the contents of an Appendix of the first volume to the fecond volume, with the idea that it was placed in this by a sort of compulsion of Mr Watt, he would not have accufed me, nor thus expofed you, by your repeating his accufation, to be troubled with my apology. But it is not sufficient for me to have proved my innocence, in a cafe where the contrary would be fhameful; I muft fhew you befides, by an inftance which belongs to the fubject, though epifodical, what may be concluded from all my works, how far I am from being eager for claims of discoveries. It is well known, that, founding principally on atmospherical phenomena, I have not embraced the new chemical theory, commonly called Lavoifier's; having found, first, that it is contradicted by meteorology, and then explained its facts by another theory, which agrees with the fixed points of that fcience. Being at Berlin in the year 1800, I wrote a paper on that fubject, which was read to a learned Society; and in the course of the expofition, I faid this: As long as the nature of aqueous vapour was not understood, that is to fay, while water in that state was confidered as obeying to a certain repulfion, increafed by heat, not knowing thus the latent fire which it contains in all temperatures, the firft difcovery of which is due to Dr Black, no good team engine could be made.' Then I went on explaining Mr Watt's additions to that discovery, and the knowledge which I had derived from them. There furely I forgot myself; but it would have been too long, and ufelefs to the fubject, to make the proper diftinction; and I praised with pleasure two men whom I esteemed. Now, the fame paper has ferved afterwards as an introduction to a work which I fent from Berlin to Paris, where it has been publifhed two years ago, under the title of Introduction à la Phyfique terreftre par les fluides expanfibles, in the first volume of which, p. 102, p. 102, is the paffage above tranflated; and I have faid the fame With confidence that you will do me juftice by publishing this vant, Windsor, April 18th 1805. DE LUC. [M. de Luc has been pleased to annex to this letter a very long |
