This explanation, depending upon the assumed prin ciple that bodies, at all temperatures, radiate caloric in a degree proportional to the quantity of that power which they contain, is totally inconsistent with the most obvious facts attending the cooling of bodies. For, if it were true that a mutual participation of temperature takes place between the hotter and the colder body, un til they arrive at the same temperature, and that this re ciprocal action even goes on after an equilibrium is esta blished between them, it is evident that a hot body ought to cool more slowly when it is placed near a large body of inferior temperature, than near a small one ; because, in the former case, (as we must suppose, that if a body radiates at all, the radiation will at least be proportional to its surface,) it must receive more calorific emanations than in the latter ; and this, of course, ought to retard its cooling. But the fact is precisely the reverse. It is also very properly objected to this explanation by Mr Murray, that " of different surfaces, which at a given temperature radiate different quantities of caloric, that which radiates least must be least powerful in returning caloric to the thermometer, and must therefore have least effect in counteracting the reduction of its temperature ; in other words, must produce the greatest cold. From Mr Leslie's experiments, it is fully established, as has been already stated, that a blackened surface is that which, at a given temperature, radiates the largest quan tity of caloric, and a metallic surface that which radiates least. Were Prevost's explanation just, therefore, the blackened surface is the one w hich, in the experiment on radiant cold, ought to produce the least cooling effect on the thermometer, and the metallic surface the great est." Rejecting, therefore, this explanation of Pre vost, we shall proceed to consider that of Pictet.
According to this philosopher, the particles of caloric have a repulsion to each other, which increases as the distance between them diminishes. They therefore tend to recede from each other with a force proportioned to the degree of their accumulation in the same body, and separate, until they are counteracted by caloric in other bodies, of the same density or elasticity with themselves. So that, if this hypothesis be well-founded, the term equilibrium of temperature may be applied with strict propriety to the equal distribution of it, as being, in realir%, the balancing of equal and opposite forces. But if a both , a lower temperature, be introduced among a system of bodies in this state of equilibrium, the par ticles of caloric of tile colder body are forced to approach .
nearer to each other by the mole powerful elasticity of the caloric of the hotter bodies; a fresh supply enters :o occupy the space which they had left ; and the balance of tension being again restored, an equilibrium of tem perature is quickly established. When a cold body is placed in the focus of one of the reflectors, the equili brium of temperature of the surrounding bodies is sub verted, and caloric immediately rushes towards it from every side. The adjacent surface of the reflector is thus deprived of a portion of its caloric, and the deficiency is partially supplied by the opposite reflector. This reflector, in its turn, begins to act upon the thermome ter, which, by this series of radiations and absorptions successively repeated, is drained of its caloric, and reduced in temperature. This explanation, though free from the objections which we have stated against fact, hypothesis of Prevost, is irreconcileable with the fact, that the greatest reduction of temperature is produced in the thermometer, when the reflectors arc in the most unfavourable state for radiation and absorption, the effect in similar circumstances being greatest when the reflec• tors are well polished and resplendent. But if the theory
were just, it is obvious that the very reverse would be the case ; and that the greatest apparent radiation of cold would be exhibited when the surface of the reflectors were covered with a coating of lamp-black ; because, in that state, they possess the greatest radiating and absorb ing power, whereas the effect is then barely perceptible. This theory is therefore equally inadmissible with the .former.
With respect to the theory of Prevost, we may remark, that the same objection may be urged in applying it to the most simple case of cooling,—that of a thermometer being reduced in temperature when it is placed near a cold body, as in the experiments with the reflectors. The truth is, that as the only effect of the reflectors is to change mechanically the direction of the emanation, and thus establish a communication between the remote sides of the cold body and the thermometer, there is no greater difficulty in accounting for the reduction of the temperature of the thermometer in these circumstances, than when a thermometer is cooled directly in conse quence of its proximity to a cold body. So that the objection of Mr Murray against this theory, that the reduction of the temperature of the thermometer was greatest, when the surface of the cold body was most fivourable to radiation, may be applied with equal force •o the simplest case of radiation we can suppose. The .inperfection of Prevos'.'s theory does not consist, there fore, merely in its insufficiency to explain the experi ments with the reflectors, but in its being equally inap plicable to explain the phenomena attending the changes of temperature by direct and immediate radiation.
With regard to the explanation advanced by Pictet, the objections to which it is liable in its present form, i might, some measure, have been avoided, by taking a different %iew of the action of the mirrors. The ge neral theory which lie has proposed for explaining the processes of the heating and cooling of bodies, does not necessarily imply that the mirrors shall act solely by ab sorption and radiation ; and, indeed, the author of the theory seems to have adopted that view of the subject, from the difficulty which he had found of accounting for the emanations proceeding from the thermometer, when its temperature was the same as that of the surrounding medium. According to Pictet's hypothesis, if two equal and similar bodies, at the same reduced temperature,were placed, one in the focus of each reflector, they would have no effect upon each other, as the elasticity of the calorific fluid in both being exactly balanced, the equili brium of temperature would be maintained between them. But if a hot body, as a thermometer, were substituted for one of the cold bodies, the equilibrium would imme diately be subverted ; and the caloric of the thermome ter diffusing itself by its superior elasticity, would rush towards the adjacent mirror, undergo reflection at its surface, and proceed in parallel lines to the opposite mir ror, where it would suffer a second reflection, and at last be absorbed by the cold body. Of course, the tempera ture of the thermometer would be reduced, and thus in dicate the appearance of the radiation of cold from the cold body.