exposed to the rays of the sun they undergo remarkable changes. Thus the oxides of gold and silver, if in contact with combustible bodies, are reduced; chlorine and hydrogen gas explode, and form muriatic acid; water holding chlorine in solution emits oxygen gas, &c. Count Rumford suspected that these effects were occasioned solely by the heat evolved by the absorption of the light. The experiments related in this paper were instituted in order to determine the point. Though they cannot be considered as quite satisfactory, yet it seems established by the subsequent experiments of chemists, particularly of Gay-Lussac and Thenard, that the opinion entertained by Count Rumford on this subject is correct.

11. An Inquiry concerning the Weight ascribed to Heat. Phil. Trans. 1799. P. 179.-From an experiment of Dr. Fordyce, it was concluded that bodies become heavier the more they are cooled, and of consequence that heat diminishes their weight. But Count Rumford found, on repeating the experiment, that the supposed increase of weight was a deception, arising from vapour condensing on the surface of the glass vessel in which the experiment was made. Lavoisier had previously ascertained the same thing.

It does not seem necessary to give a particular account of the remainder of Count Rumford's writings. His two volumes of essays are of a very miscellaneous nature, and the most important of the essays are republications of those papers which have been already noticed. The seventh essay, in which the Count endeavoured to prove that fluids are nonconductors of heat, has been sufficiently refuted by the more decisive experiments of subsequent chemists. Indeed the Count himself, though abundantly obstinate, appears at last to have given up his opinion. The essays on the treatment of the poor, on cooking, on chimnies, and on the management of fuel, are not very susceptible of abridgment. His paper published in the Philosophical Transactions for 1804, entitled, An Inquiry concerning the Nature of Heat and the Modes of its Communication, gives us a number of curious facts respecting the effect of surface on the heating and cooling of bodies. But the publication of Mr. Leslie's book on heat, in which this subject is treated of at much greater length, and much more completely, have deprived this essay of most of its interest. It is not necessary to notice the papers published by the Count in Nicholson's Journal for 1805. An outline of his last paper, On the Quantity of Heat evolved during the Combustion of different Bodies, was given in the first paper in the third volume of the Annals of Philosophy, to which I beg

leave to refer the reader.

My Essay on Dew has lately been honoured in the Quarterly Review with a Criticism by Dr. T. Young, the depth and variety of whose knowledge entitle him, perhaps, to be considered as the most learned man in this country. I mention his name thus openly, because I am confident, that he has too much of the spirit of an English gentleman ever to desire to conceal his being the author of any publication, in which he discusses the merits of a literary work of another private person. With respect to the criticism of my work, indeed, he clearly shows, that he is the writer of it, by the manner, in which he speaks of his own works at its close.

I am necessarily much pleased with the general commendation, which has been bestowed upon my Essay by one of his high rank in literature. As several of his observations, however, respecting it do not appear to me entirely just, I beg leave to make a reply to these through the medium of your Journal.

I. Dr. Young has called my theory a simple and obvious consequence of principles deduced from the discoveries, concerning heat, by Mr. Leslie, and other observers. On this point I shall offer a few remarks.

1. The Inquiry of Count Rumford, and the Essay of Mr. Leslie, were both published in 1804, and in these works are to be found all the new facts relating to heat, which I have taken from others in forming my theory of dew. Whether Count R. ever afterwards treated of atmospherical appearances is unknown to me; but Mr. L. published, nine years after his Essay, a work on heat and moisture, in which, agreeably to the opinion of Aristotle, the production of dew is attributed to the condensation, by the cold of the night, of watery vapour diffused through a considerable portion of the atmosphere. Now, when the great ingenuity of Mr. L. is considered, if the theory of dew, which I have proposed, be an obvious consequence of his own discoveries, it would assuredly have occurred to him, in that long space of time, since he has shown, that the subject of dew had in the meanwhile occupied his attention.

2. Your own various publications demonstrate, both that you are well acquainted with the modern discoveries respecting heat, and that you have attended closely to atmospherical appearances; yet I remember, that you asked me, at an accidental meeting, shortly before the publication of my Essay, what my opinion was on the formation of dew, giving as a reason, that you were yourself ignorant of its cause.

3. Even Dr. Young, though his Lectures on Natural Philosophy prove him to have been well skilled in the new doctrine of heat, has advanced in that work an opinion upon the cause of dew, in the most ordinary form of its occurrence, which has not the least connection with any modern discovery, as I shall more particularly mention hereafter.

I might proceed in this way considerably further; but what has been said is, I think, sufficient to establish, that, when regard is paid to the imperfection of the human understanding, my theory of dew is not an obvious consequence of the recent doctrine of heat. My explanation, indeed, of the immediate cause of dew is altogether independent of that doctrine, being grounded on the simple fact, that bodies always become colder than the neighbouring air before they are dewed, and was consequently open to the discovery of every person since the invention of thermometers. It is true, that the next step in my theory could not have been taken, without the assistance of the late discoveries of others, and this has been amply acknowledged in my Essay.

II." Dr. Wells," says Dr. Y., appears, in his historical account of the doctrines relating to the nature and causes of dew, to have undertaken to afford us complete information respecting the sentiments, not only of Aristotle and Theophrastus, but also of the "most distinguished" philosophers of modern times: some of the works, however, of the persons whom he mentions, and some of the latest, have most unaccountably escaped his attention." In this sentence there are two small mistakes, which, from my respect for the author, I must suppose unintentional, though they give point to his statement. The first relates to the engagement into which he regards me to have entered concerning the opinions of preceding writers on dew; for I never gave the slightest hint, that could lead him to the conclusion which he has made. I knew that my account of opinions on the cause of dew was incomplete; and well it might be, since the whole of it, with the accompanying refutations, does not occupy more than three pages of my Essay. But my health, at the time of its being drawn up, was in such a state, that I scarcely hoped that I should ever finish my work, and my notes were so written, that no person besides myself could make use of them. I composed therefore in haste, and had neither leisure nor strength to search public libraries for all the works, which I wished to consult. I certainly thought, however, that what I had collected contained every thing of much importance, which had been said upon my subject. The second mistake consists in his applying the words "most distinguished," which were used by me concerning the authors, who had given opinions on the formation of ice in India, to those who had treated of dew.

I pass now to more important matters. Dr. Y. in support of what he has said respecting some very late and important works, connected with my subject, having most unaccountably escaped my attention, gives a long extract from the "Recherches sur la Cha

leur" of Mr. Prevost, published in 1792. This work was, indeed, unknown to me when I composed my Essay; but Dr. Y. I presume was ignorant, when he wrote his Criticism, that the very passages, which he has cited, are contained in another and much later work on heat by Mr. P. unaccompanied with any intimation, that they were copied from a preceding publication. Mr. P.'s later work, which was printed in 1809, was first seen by me in 1812, two years before my Essay came out. What appeared to me the most worthy of attention in the passages cited by Dr. Y. was spoken of in one of my notes, p. 79. I mentioned there, very distinctly, that Mr. P. had already accounted for the effect of clouds, in diminishing the cold of the air at night, by making this to depend upon their preventing the escape of its heat by radiation to the heavens, but only impliedly, that he had accounted, in this way also, for a similar effect produced by them upon the temperature of bodies on the surface of the earth, as I said only, that he did not seem to have known, that they have a much greater effect upon the temperature of such bodies, than upon that of the air. My full meaning was, that Mr. P. did not seem to know, that the degree of cold, which is prevented by clouds from appearing on the surface of the earth, is much greater, than that which they prevent from appearing in the air; or in other words, that he was ignorant, that, bodies on the surface of the earth become much colder than the air in a clear night, this being one of the principal facts, on which my theory of dew is built. That I had no desire to conceal any thing which Mr. P. had said upon this subject is shown, by my referring to the latest work, in which he has mentioned the effect of clouds upon the heat of the earth and atmosphere at night, and by my referring, likewise, in three different parts of my Essay, (p. 68, 74, and 118,) to Count Rumford as supposing, that the earth is cooled by radiation at night; since it cannot be thought, that, although unacquainted with both of those authors, I should withhold the knowledge of the possessions of one, and yet repeatedly speak of similar possessions of the other.

I have said in the preceding paragraph, that Mr. P. did not seem to know, that the earth ever becomes colder, at its surface, than the air by radiation. My reasons are, 1st, That he has not mentioned this fact: 2dly, That he has said what is equivalent to a denial of it in his late work, Du Calorique Rayonnant, p. 249: and 3dly, That it is apparently in opposition to an observation of his friend Mr. Pictet, from whom he seems to have derived all the facts which he has related in this disquisition on the effect of clouds ; for that philosopher found, that, although, in clear and calm nights, the heat of the air decreased from the height of 75 feet above the ground to within four lines of it, yet a thermometer, lying upon the ground, and having its bulb slightly covered with earth, precisement enterrée," was higher than all those which were suspended in the air above it.-Pictet sur le Feu, p. 180.

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Considering the purpose to which Dr. Y. has applied his quotation from Mr. P.; I shall venture to examine this a little closely.

1. What has hitherto been called by me Mr. P.'s explanation of the effect of clouds, at night, upon the temperature of the surface of the earth and of the lower atmosphere, is in reality only a conjecture; for, 1st, It is denominated by himself an "Essai d'Explication:" 2dly, It is supported by no experiment: 3dly, It takes as established, that air can radiate heat, whereas he says in p. 24 of his last work," On peut supposer, que les molecules de l'air rayonnent."

2. Mr. P. mentions, that clouds send back to the earth a little more heat than transparent air can do; which is equivalent to affirming, that clouds alter the temperature of bodies upon the surface of the earth at night only a little. Nothing can show more strongly Mr. P.'s want of practical knowledge upon this subject; since I have given in my Essay, p. 32, an instance of the temperature of grass having risen 15° in less than 45 minutes, on the sky becoming cloudy.

3. It is remarked by Mr. P. that the operation of clouds, in keeping the surface of the earth warm at night, is exactly similar, "exactement comparable," to that of cloathing on the human body; and Dr. Y. assents to this observation. Now it appears to me, that very dissimilar things are here confounded together. Cloaths keep us warm by being bad conductors of heat; but clouds warm the earth by radiating heat to it. Hence, the effect of clouds is immediate, but cloaths require to be applied some time, before they completely answer their purpose. Cloathing must touch the skin to produce its full effect, whereas clouds produce theirs at the greatest distance. The thicker our cloathing is, the warmer it renders our bodies; but, if a cloud be sufficiently dense to prevent the heat, that is radiated into it, from passing through its interstices, the earth is kept as warm by it, as far as the principle of radiation is concerned, as it would be by one many times deeper. This is shown by a piece of cambric preventing the occurrence of cold on the earth's surface, from radiation, as effectually as a thick blanket.

4. Annexed to the disquisition of Mr. P., are two meteorological facts communicated to him by Mr. Pictet. One is, that upon a night in January, 1777, a thermometer, suspended in the open air, rose nearly 31° of Fahr. in the space of an hour, the weather having in the mean time become cloudy. This was observed by himself; but it does not appear, that he afterwards made any similar observations. The second is given on the authority of husbandmen, who are said constantly to find that, though other circumstances are favourable for the production of dew, none, or almost none, appears, if the sky be cloudy; and that hoar frosts, which are so frequently injurious in spring and autumn, do not occur in those seasons, during cloudy weather. These facts, if