COLD. 



Explana- 

 tion of the 

 appatent 

 radiation 

 of cold by 

 Prevoo. 



Objection* 

 to it. 



ptiotcd with lamp black, another coated with writing 

 paper, a third covered with a pane of glass, and the 

 fourth bright and polished : he filled the vessel thus pre- 

 pared with ice, and having opposed it to the thermome- 

 ter in the focus of one of the reflector*, he found the 

 reduction of temperature sustained by the thermometer 

 to be least from the metallic surface, greater from the 

 pane of glais, and greatest of all fr >m the side coated 

 with lamp-black. From a variety of other experiments, 

 he ascertained, that the powers of different substances 

 in radiating, reflecting, and absorbing cold, are precise- 

 ly the same as their powers of radiating, reflecting, and 

 brorbing heat. Hence, it has been concluded, that we 

 have the same proofs for the existence of cold as a dis- 

 tinct and independent principle, that we have for the 

 existence of heat. 



The explanation of these facts, in conformity with the 

 doctrine that cold is merely a negation of heat, has given 

 greater difficulty to chemists than we conceive belongs 

 naturally to the subject. According to the hypothesis 

 of Prevost, caloric is radiated from all bodies at all tem- 

 peratures, the quantity radiated being proportional to the 

 difference of temperatures. When the body placed in the 

 focus of one of the reflectors has a more elevated tem- 

 perature than the thermometer in the focus of the op- 

 posite reflector, the temperature of the thermometer is 

 increased, because it receives more caloric by absorption, 

 than it communicates by radiation. When both the body 

 and the thermometer arc at the same temperature, no 

 change takes place, because each communicates the same 

 quantity which it receives ; but if the temperature of 

 the body be lower than that of the thermometer, though 

 it still radiates caloric, it radiates a smaller quantity than 

 the thermometer, and therefore the temperature of the 

 latter is reduced. 



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 O f 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 hoi 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. 1 1 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 lean 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 

 kast. Were Prevost's explanation just, therefore, the 

 blackened turface is the one which, 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- 

 vo$t, we ihalJ proceed to consider that of Pictet. 



According to this philosopher, the particles of caloric 

 have a repul-ion 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 

 reality, the balancing of equal and opposite forces. But 

 if a body, at a lower temperature, be introduced among 

 a system of bodies in this state of equilibrium, the par- 

 ticles of caloric of the colder body are forced to approach 

 nearer to each other by the more powerful elasticity of 

 the caloric of the hotter bodies ; a fresh supply enters 

 to occupy the space which they had left ; and the ba- 

 lance of tension being again restored, an equilibrium o 

 temperature; is quickly established. When a cold body 

 is placed in the focus of one of the reflectors, the i-qui. 

 libm:m 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 re- 

 flector, in its turn, begins to act upon the thermometer, 

 which, by this series of radiations and absorptions suc- 

 cessively repeated, is drained of its caloric, and reduced 

 in temperature. This explanation, though free from the 

 objections which we have stated against the hypothesis 

 or Prcvost, is irreconcileable with the fact, that the great, 

 est reduction of temperature is produced in the thermo- 

 meter, when the reflectors are in the most unfavourable 

 state for radiation and absorption, the effect in similar 

 circumstances being greatest when the reflectors are well 

 polished and resplendent. But if the theory were just, 

 it is obvious that the very reverse would be the cane ; 

 and that the greatest apparent radiation of cold would 

 be exhibited when the surface of the reflectors wero co- 

 vered with a coating of lamp-black j because, in that 

 state, they posseks the greatest radiating and absorbing 

 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 re- 

 mark, that the same objection may be urged in applying 

 it to the most simple case of cooling, that of a thermo- 

 meter beine reduced in temperature when it is placed 

 near a cold Dody, as in the experiments witlx the reflec- 

 tors. The truth is, that as the only effect of the reflec- 

 tors is to change mechanically the direction of the ema- 

 nation, snd 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 consequence 

 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 favourable to radiation, 

 may be applied with equal force to the simplest case of 

 radiation we can suppose. The imperfection of Pre- 

 vo.st'8 theory does not consist, therefore, merely in its 

 insufficiency to explain the experiments with the reflec- 

 tors, but in its being equally inapplicable to explain the 

 phenomena attending the changes of temperature by di- 

 rect and immediate radiation. 



With regard to the explanation advanced by Pictet, 

 the objections to which it is liable in its present torn), 



Cold. 

 - 

 Explana- 

 tion of 

 Tictct. 



Objection* 

 to ii. 



The theory 

 ol Prcvost 

 not mure 

 inapplica- 

 ble to the 

 expen 

 menu with 

 the reflec- 

 tor*, than 

 to ordinary 

 radiation. 



Modifica- 

 tion of Pic- 

 tet 'i tlie*- 



rr- 



