ducting power is a mere nonentity. This opinion might have appeared extremely ingenious to a chemist ignorant of the principles of mechanical fcience. But surely, if Dr Thomson, who fo often makes ufe of algebraic expreffions to render unintelligible to many of his readers what, in common language, could have been mistaken by no one, had allowed himself time for reflection, he never would have committed an error which has betrayed him into fo many inconfiftencies. He has himself proved that affinity is capable of accounting for the motion of caloric through conductors; it cannot therefore retard the motion which caloric is fuppofed to derive from its repulfive force, unless it acts in an oppofite direction: but in the cafe of conducting bodies, the affinity always acts in the fame direction with the repulfive force; and, instead of retarding the progrefs of caloric, it ought therefore to accelerate it. But fome bodies conduct caloric better than others; and Dr Thomson thinks it probable that their affinity is in all cafes in the inverfe ratio of their conducting power. The originality of this opinion is truly fingular; for no common mind would have conceived that an effect could be inverfely as its caufe. Bodies alfo differ in the distance to which they are capable of conducting caloric; and this difference, Dr Thomfon tells us, is always proportional to the temperature to which that body can be raifed before it changes its ftate. The reafoning upon which this opinion is hazarded, is perfectly inconclufive; for it proceeds on the fuppofition, not onfy that the decreafing feries of affinities for additional dofes of caloric, is the fame in all bodies, but also that the conducting power depends entirely on affinity. Now, the firft fuppofition is altogether arbitrary, and the fecond abfolutely erroneous. Dr Thomson's general law is equally contradicted by experiment; for lead, tin, and the other fufible metals, conduct caloric much farther than glafs; and that moft refractory substance, pure clay, cannot be made, by any intenfity of heat, to conduct caloric farther than the fufible metals. Count All folids conduct caloric; but fluids alfo carry it. Rumford was the first who paid particular attention to this fubject; and endeavoured to prove, by the moft ingenious experiments, that fluids only carry caloric, and never conduct it. This opinion of the Count, however, is now completely difproved, efpecially by the experiments of Mr Murray. On this fubject, Dr Thomfon has entered at more than ufual length, and has divided, between himself and Mr Dalton, the merit of having been the firft who, by various experiments, rendered the Count's opinion improbable; although it is inconceivable that he Thould be ignorant of being anticipated by Dr Hope, whose ingenious genious experiments on the fame fubject were publicly exhibited in his lectures. The tables of the conducting power of different bodies are extremely imperfect, and are conftructed upon no uniform principle. For example, M. Meyer's table is given, without any explanation, immediately after Dr Ingenhouz's; although the experiments of the latter fhowed the comparative length of waxcoating, which cylinders of different metals melted when their extremities were plunged in boiling water, and, thofe of the former, the times which equal fpheres of wood took to cool the fame number of degrees, from which the conducting powers were calculated, on the hypothetical fuppofition that they were inverfely as the times of cooling. Dr Thomfon has alfo determined the conducting power of fome fluids from his own experiments; but his ftatements can be of no ufe, until we know the data on which they are founded. He has, however, erroneously calculated the affinity of these bodies for caloric, from thefe obfervations on their powers of conducting it. The next fection is on the Equal Diftribution of Temperature. Some bodies cool much more quickly than others; and Dr Thomfon tells us, that, in general, other things being the fame, the rate of cooling may be confidered as nearly inverfely as the conducting power of fluids. But he before attempted to prove, that the affinity was inversely as the conducting power; so that the rate of cooling fhould be directly as the affinity, or, in other words, thofe bodies which have the strongest affinity for caloric, fhould part with it most readily! The equilibrium of temperature is principally produced by the repulfive force of the particles of caloric, which always tends to feparate them, until it be counterbalanced by an equal preffure in the oppofite direction. That affinity has alfo fome fhare in this process, seems to be proved, by the rate of cooling being flower in vacuo than in air.. But if the effects of this caufe were very confiderable, bodies fhould heat quickly and cool flowly, in proportion to their affinity, which, as far as we know, is not the cafe. The next fection treats of the Effects of Heat, the firft of which, Expanfion, is well explained according to the original and interesting ideas of Mr Dalton. In his obfervations on the caloric of fluidity, we meet with another instance of our author's rafhness in drawing general conclufions. In all Dr Irvine's experiments on this fubject, he fees this rule to hold, that the caloric of fluidity increases with the temperature at which liqui dity takes place. Dr Thomfon has evidently been induced to draw this conclufion, by imagining that Dr Irvine's numbers expreffed the caloric of fluidity of the fubftances which he exam Auidity 1 2 mined; mined; whereas they only expres the number of degrees by which the temperature of the bodies refpectively would have been raised by the quantity of heat absorbed during their que faction. For example, during the liquefaction of ice, as much caloric is abforbed as would have increased its temperature 140°; and during that of tin, as much as would have created its temperature poof; but the specific caloric of ice is to that of tin 28 9000 to 651; therefore, the caloric which is absorbed during the melting of tin, would only have increated the temperature of ice 36.72°, while that abíorbed by ice would have increated the temperature of tin 1956; whence it follows, that the caloric of fluidity of ice is 3.81 times as much as that of tin; or, taking that of ice as a ftandard, as I to 0.262, and not at ali propor tionate to their melting point. · The next fubject treated of is the Capacity for Heat, or ípecific caloric, of bodies; on which our author, as ufual, attempts to reafon profoundly, and to point out fome great general law which has escaped the obfervation of all former philofophers. Unfortunately, however, his whole reafoning is built upon erroneous data; and his law is inconfiftent with fact. The experiment, by means of which he explains what is meant by specific caloric, he has moft unaccountably miftated; for he tells us, that the caloric which raises the temperature of water 1°, will raife that of the fame weight of mercury 3.16°. Now, Crawford tells us, that the caloric which heats water 1°, heats mercury no lefs than 28°. At first, we thought Dr Thomfon had fallen into this important error, by inadvertently fubftituting equal weights for equal bulks: But even this will not anfwer; for the caloric which heats water 1°, heats an equal bulk of mercury only 1.5°. From whatever caufe this miftatement may have arifen, it certainly cannot be afcribed to an error of the prefs; for it is the bafis of a great deal of the fucceeding reafoning. . Becaufe fcarcely any two metals, when converted into oxides, combine with precifely the fame quantity of oxygen, Dr Thomfor fomehow concludes (vol. I. p. 394-), that the difference of specific caloric in bodies must therefore depend upon the affinity which exifts between bodies and caloric; and thinks it probable, that the fpecific caloric of bodies is always proportional to their affinity for caloric, and inverfely as their conducting power. This conclufion our author esteems of confiderable importance, not only becaufe it fimplifies the theory of the combinations of caloric with bodies, but because it enables us to determine the conducting power of bodies from their fpecific caloric, or the contrary. He is, however, fufficiently modeft to acknowledge that a fet of experiments would be neceffary to establish it completely. But, in thofe fubftances which he has examined, he finds the differ ence between the conducting power, as afcertained by experiment and by theory, lefs than could be imagined. In proof of which, he prefents us with the following table. A more erroneous table was perhaps never prefented to the public. Of the three fubftances which it contains, the first is the only one whofe correfponding numbers are right. Of the other eight numbers, feven are wrong:-one, we are perfuaded, in confequence of a typographical errors four from being cal culated on erroneous data and the laft two, the most important in the whole table, from miftatement. The fpecific caloric of mercury is fet down as ten times greater than it should be; while, in the cafe of linfeed oil, we actually find its specific gravity fubftituted instead of its caloric *. When these inexcufeable errors are corrected, Dr Themfon will have little reason to boaft of the coincidence between his theoretical and experimental eftimation of conducting powers. In the general table of specific calories, there are many errors not merely typographical, fuch, as the remarkable one of mer cury, but arising from Dr Thomfon inferting the mean of the obfervations of different experimenters, made in very different ways, inftead of felecting that which appeared to be derived from the jufteft principles, and most accurate experiments. In treating of the abfolute, quantity of caloric in bodies, Dr Thomfon examines, and endeavours to refute, the hypothefis of Dr Irvine and of Mr Dalton. The futility of his abjections to the. former, was: fo completely expofed by Mr Irvine, in Nicholfon's Journal, vol. V. p. 25, that we are aftonifhed to fee them retain 2 their This has probably happened in confequence of Dr Thomfon fol, lowing his own directions of infpecting his general table for the fpe. cific caloric, and ftumbling upon the wrong column, their place in this new edition; and his obfervations on the latter are equally inconclufive. On the fubject of Cold, Dr Thomfon quotes Pictet's celebrat ed experiment of its apparent radiation, as the only fad which gives any countenance to the opinion, that cold is a body. But although Prevoft's explanation of this phenomenon is, as Dr Thomfon justly obferves, unfatisfactory, we fee nothing in it but an example of the radiation of caloric. If we had room in this place for fuch a difcuffion, we think it would not be difficult to fhow, from what is now established as to the ra diation and reflection of heat, that the finking of the thermo meter in M. Pictet's experiment, is to be imputed entirely to the fubtraction of caloric occafioned by the introduction of a cold body; and that, in confequence of the intercepting and reflecting powers of the mirrors, this caloric is drawn in larger quantities from the focus where the thermometer is placed, than from any other point in the circumference. The heat which flows into the cold body is radiated in part from the furface of the nearest mirror, and the heat thus drawn from its furface is fupplied again by parallel rays reflected from the furface of the oppofite one, the whole of which will be found, from the angle of their reflection, to proceed from that focal point in which the thermometer is fituated. There is a greater drain upon the caloric of that focus, therefore, than upon any other point in the circumference; and its temperature is lowered proportionally. This explanation, perhaps, is too concife to be fatisfactory; but we are per fuaded, that all the facts may be accounted for by the radiation of caloric, and that the apparatus merely determines the point from which the radiation is to begin. The curious fact difcov ered by Mr Dalton, that the expanfion of water is the fame for any number of degrees above or below 42.5, is a much stronger argument for believing that cold is a body, and, if the fame law obtained in all other bodies, would be almost unanfwerable, The Sources of Caloric form the next fubject of confideration. Of these, Combustion is treated in a masterly manner. In the history of its theory, the opinions of others are fairly and candidly stated; while, in the account of that hypothefis which is adopted by our author, he allows their full fhare of merit to the German philofophers who advanced it, and ftates his own opi nions, which are very ingenious, with a degree of modesty and philofophical doubt which are extremely honourable to him. The heat produced by percuffion, is afcribed entirely to condenfation. This unquestionably is a fource of heat, but appears totally infufficient to account for the great increase of tempera ture |