Since the preceding pages were prepared for the prefs, we have been called upon to fympathize with all the friends of literature, medicine, and philanthropy, for the untimely death of the amiable and ingenious author upon whofe work they are employed. Society, we think, has not recently fuftained a more deplorable loss; and it is painful to reflect, that he was fnatched away at a period when his matured talents and unabated activity held forth a reasonable hope of deriving from his future labours, as important benefits as had already refulted from the paft. ART. IV. An Experimental Inquiry into the Nature and Propa- THIS work comes before us with no ordinary claims to our attention and respect. It contains an expofition of the views to which the author was led by a long courfe of induction upon the fundamental doctrine of chemical fcience; and the fuccefs of his researches has obtained a moft honourable atteftation, in the award of the Rumford medal by the Royal Society, for the experiments, of which the history is here recorded. If the deci fion of that illuftrious Body required any confirmation, a fufficient interval has elapfed, to afford the fcientific world an opportunity of reviewing the fentence: and the general voice feems to have pronounced, that the facts brought to light by the labours of this ingenious perfon, are equally unexpected and important; opening the fairest prospects of more valuable acquifitions than the theory of heat has made fince the capital difcovery of Dr Black. The materials, however, with which Mr Leflie's experiments have furnished him, are far more precious than the workmanship he has bestowed in preparing them for the eye of the public. They come before us in a ftyle much too gaudy for the dignity of the occafion; they are difpofed with very little skill; and are mingled with a good deal of loofe, droffy matter, obtained from a very different fource. His language is defective in fimplis city and clearnefs: it unites the two great faults of misplac ed ornament and unneceffary abftraction. His arrangement is fingularly confused: the fpeculations that compose his work are put together without any plan, or fucceed each other according to fome arbitrary feries, of which we have been wholly unable to discover the law. A fubject ftrikes him; he pursues it, together with feveral other topics that crofs him on his way. He then takes takes up another thread; and, in following it out, is reminded of fomething which he had before broke off abruptly, or omitted in its right place. Frequently he pauses, and feems to make the likeness of a divifion; but he does not even indicate, by a title, the contents of what he calls a chapter. He is prone not only to run from one part of his fubject to another, but to leave his fubject altogether, and take a new one; in the course of which, fome third topic frequently draws him off: fo that many of his notes are, in truth, digreffions from his own epifodes. If his fections have no titles, it is scarcely to be expected that we should find the ordinary aids of index and table of contents. Hence, to examine any one branch of the Inquiry, it is neceffary that we fhould have all the reft prefent to our mind; and a reference from one part to another, for the purpofe of comparifon, must be preceded, either by the toil of abridging and claffifying the various difcuffions, or by fuch a minute and fevere ftudy of the whole work, as may give the reader an equal command of its contents with the author himself. Having fuffered exceedingly from the defect just now ftated, we truft Mr Leflie will not impute it to captioufnefs or difrefpect, if we add, that the rigorous examina tion of his work has proved the most painful talk to which the appearance of any fcientific performance has fubjected us fince the commencement of our labours. Our author's chief merit, we think, is the contrivance of fimple, conclufive, and elegant experiments, for afcertaining relations of no great generality; and the application of close argument, frequently of an ingenious calculus, to the comparison of his refults. He is often happy in the explanation of phenomena, by a reference to obvious, but unnoticed principles; and his practical views are fometimes ingenious and important. Even the parts of his book which we admire least,-his general differtations upon matters not immediately connected with the doctrine of heat,-and the too refined and often hafty theoretical inferences from his experimental difcoveries, are marked by the fubtlety and vigour that diftinguifh the more legitimate efforts of his understanding; and, with all the faults of his ftyle, and the abundance of crude diffufe writing scattered over his fpeculations, it would be difficult to find a fingle page which does not betray fome marks of a fertile genius and varied information. In conducting our examination of this work, we fhall be more anxious to lay before our readers an abstract of the folid improvements which chemical science has received from it, than to analyze and difcufs all the general fpeculations in which the author fancifully and fondly indulges. Of thefe it may for the most part be enough to give a fpecimen. The abundance and novelty of the the experiments claim our full attention; and while we endeavour to exhibit a sketch of the anomalous facts which they prefent as the groundwork of future, perhaps not very remote changes in phyfics, we fhall freely point out the imperfections of the general law under which the author has reduced them,-its want of fupport on one fide, and its repugnance to the phenomena on the other. The differential thermometer was invented by Mr Leflie for the measurement of minute variations of temperature. It is an in-. ftrument of the greatest delicacy, and has evidently been the main cause of the fuccefs with which his inquiries have been attended. Two tubes, each terminating in a small bulb of the fame. dimensions, are joined by the blow-pipe, and bent in the form of an U, a small portion of dark-coloured liquor having previoufly been introduced into one of the balls. After many trials, the fluid beft adapted to the purpose is found to be a folution of carmine in concentrated fulphuric acid. By managing the included air with the heat of the hand, this red liquor is made to ftand at the point required of the oppofite tube. This is the zero of a fcale faftened to that tube, and divided into equal parts above. and below nothing. The inftrument is then fixed upright on a ftand. It is manifeft, that when the liquor is at reft, or points at zero, the column is preffed oppofite ways by two portions of air, equal in elasticity, and containing equal quantities of caloric. Whatever heat, then, may be applied to the whole inftrument, provided both bulbs receive it in the fame degree, the liquor muft remain at reft. But if the ope ball receives the flighteft excess of temperature, the air which it contains will be proportionally expanded, and will push the liquid against the air in the other bulb with a force, as the difference between the temperatures of thofe two portions of air. The equilibrium, in fhort, will be destroyed, and the fluid will rife in the oppofite tube. The degrees of the fcale through which it paffes will mark the fucceffive augmentations in the temperature of the ball, which is expofed to the -greatest heat. This inftrument, therefore, is truly a balance of extreme delicacy, for comparing the temperatures with which its two scales may be loaded. If a heated body, or a body hotter than the furrounding atmofphere, is placed in front of a concave fpeculum, a thermometer in the focus is raised proportionally to the excefs of the temperature of the body above that of the atmosphere; and a cold body in the fame circumstances finks the thermometer. This communication of caloric has been long known under the name of the radiation of heat; and it appears to us that Mr Leslie's difcoveries have entirely refulted from the happy application of his new VOL. VII. NO. 13. inftrument E inftrument to the obfervation and measurement of that procefs.. He used a variety of reflectors, carefully conftructed of block-tin, and chiefly of the elliptical form. Sometimes he found the parabolic curvature convenient, especially when the reflection was made at confiderable distances. The heat was given out from cubical boxes of plain and polifhed tin, with an orifice at the top,. through which water of various temperatures was introduced, and a common mercurial thermometer occafionally placed in that fluid, to note the progrefs of its cooling. When the radiation of cold was to be tried, the canisters were filled with ice or fnow. The differential thermometer being placed fo that the ball containing the red liquor was in the focus of the fpeculum, and, the canister being filled with boiling water, the red liquor rofe to a certain height, and then began to fall in proportion as the water cooled. A fimilar effect, though in the contrary direction, was produced by a canifter filled with ice; and, in every cafe, the motion of the red liquor, above or below the point of equilibrium, was exactly proportional to the difference between the temperatures of the canister and the furrounding air. Such is the very fimple apparatus with which the greater part of our author's operations were performed; and fuch the general fundamental experiment the application of his differential thermometer to the measurement of radiant heat, whereof the others were only particular varieties and modifications.. If different fubftances are applied to the canister while giving out heat, the degree of its emiffion fuffers very fingular changes. Coat one fide of the canifter with lamp-black, another with writing paper, a third with crown glafs, and leave the fourth bare, or cover it with tin-foil: the differential thermometer will rife to 100, 98, 90, and 12 respectively, when these four fides are exposed to the fpeculum in fucceffion. The metal furface, then, gives out heat about eight times lefs copiously than the other three fubftances. By coating the focal ball of the differential thermometer with tin-foil, it is found to receive about five times lefs heat from any fide of the canister, than when it is expofed bare in the focus; and by coating the furface of a concave glafs-mirror, first with black pigment, then with tin-foil, and laftly expofing it bare to the heating body, it is found that the glass reflects very little heat, the pigment none at all, and the tin-foil ten times more than. the glafs. The metallic furface, then, has about five times less power of abforbing heat, eight times lefs power of emitting it, and ten times greater power of reflecting it, than the glass. Between the canister and the reflector place a frame, over which are stretched, fucceffively, tin-foil, glafs, and paper. The commu nication of heat or cold is altogether stopped by the first; at least fo little paffes, that the differential thermometer is not fenfibly affected, while the glass only ftops four-fifths, and the paper not fo much. The metallic fcreen, too, produces this effect, however near the canister it may be placed, provided the feparation is only fenfible. The other two fubftances interrupt the communication more and more, the nearer they are placed to the canister, but always permit a large portion of the heat or cold to pass. Instead of one screen, or a fcreen compofed only of one substance, let a combination of these be interpofed. A very remarkable illuftration of the general fact is obtained. The combinations used by Mr Leflie were, plates of glass, coated with tin-foil on one fide, and plates of tin, coated in like manner with lamp-black. Thefe he either placed in contact, so that their fimilar fides touched; or at a distance from each other, with their fimilar fides facing. He denominates the combination of glass and tin-foil the experimentum crucis. We conceive that of the tin-foil and lamp-black better entitled to this appellation. 'Cause two sheets of tin about ten inches fquare to be hammered quite flat and fmooth, and paint one fide of each with a thin coat of lamp-black. Arrange the apparatus as ufual, and, having joined together the tin-plates with their clear furfaces touching, fix them to the vertical frame the liquor of the differential thermometer will rise 23 degrees. Invert the pofition of the plates, fo that the blackened fides. come into contact, it will now fink down to zero. Remove either of the plates, and the liquor will again mount near 4 degrees. It is truly pleafing to witnefs this varied fpectacle, where the changes fucceed each other as if performed by the fancied operation of magic. But thofe tranfitions, and even the measures of the diverfified effects, are the neceffary refults of the principles already eftablished.Compare the cafe where both the external furfaces of the fcreen are metallic with that in which they are covered with pigment. On the one fide it receives five times lefs heat, and this heat is propagated with eight times less energy from the other. By the joint influence of those circumstances, therefore, its effect is 40 times lefs; which correfponds to about half a degree, a quantity fcarcely diftinguishable. When the fcreen confifts only of a fingle plate blackened on the one fide, the diminifhed effect is a mean between the receptive and the projecting powers, or 6 times fmaller than where both furfaces are painted. This enfeebled impreffion is confequently equal to about 4 degrees. P. 36. 37. Now, although the event of this experiment certainly has a general correfpondence with the difference between pigment and tin, in their receptive and propelling powers, it prefents at the fame time certain anomalies, which we had no reason to expect from the law previously established, and of which no notice whatever has been taken by Mr Leflie, either in this ftage of his in |