COLD. 



737 



Cold. 



The total 

 privation 

 of tempera- 

 ture tec m 

 to imply an 

 bnrxxMDi- 

 lit) ur ai 

 Icjn the 

 anmhiU 

 lion of 

 matter. 



dependent principle or condition of matter, receives no 

 additional support from the fact of its apparent radia- 

 tion. 



The effect of surface in modifying the heating and 

 cooling of bodies by radiation, has been long known ; 

 though we are indebted to Mr Leslie for the first scien- 

 tific investigation of the subject. Some experiments on 

 radiation had, indeed, been made by Lambert and Scheele, 

 but they were extremely obvious, and have little preten- 

 sions to precision. 



If a plate of polished metal be exposed to the heat of 

 a fire, almost all the rays of caloric are reflected from its 

 surface, and it requires but little increase of temperature. 

 But if a pane of glass be placed in similar circumstances, 

 the greater part of the rays are absorbed, and the pane 

 soon becomes very warm. The same effect is produced, 

 if the surface of the metallic plate be covered with a coat- 

 ing of lamp-black. From those experiments, and a va- 

 riety of others which will be detailed under HEAT, it 

 appears that the absorbing power of bodies for caloric is 

 considerably affected by the nature of their surfaces. It 

 is somewhat curious, that when their temperature is su- 

 perior to that of the surrounding medium, or adjacent 

 bodies, their tendency to radiate, or throw off caloric, 

 corresponds with their disposition in the former case to 

 absorb it ; so that the absorbing and radiating power, or 

 in common language, the tendency to heating and cool- 

 ing in opposite circumstances, are equally affected by 

 surface, and in a degree that has some relation to the dis- 

 tance and difference of temperature of the radiating and 

 absorbing bodies ; the radiation and absorption being car- 

 ried on between them, until an equilibrium of tempera- 

 ture is established. It is difficult to determine, whether 

 a mutual interchange of temperature takes place between 

 the warmer and the colder bodies, each radiating in a 

 degree corresponding to its quantity of caloric, or whe- 

 ther the radiation proceeds from the warmer body alone, 

 and the colder body is merely a recipient of its emana- 

 tions ; only, if we say that one body is hot, and the 

 other cold, we must affirm, in conformity with the ordi- 

 nary acceptation of the terms, that, in the latter suppo- 

 sition, there is a radiation of cold as well as of heat. 

 Thus, let the temperature of the hotter body be 60 of 

 Fahrenheit, and that of the colder 30 : if it be supposed, 

 that a mutual communication of temperature by radia- 

 tion takes place between them, we would admit without 

 hesitation that the colder body radiates cold ; or, if it be 

 supposed that the temperature of the one is 30, and that 

 of the other 20, and that the radiation proceeds solely 

 from the body at the more elevated temperature, we 

 might still say that the emanations were cold. But if 

 the temper iture of the hotter body were 120, and that 

 of the colder 60, we would scarcely be disposed to admit 

 that the latter radiated cold, even on the supposition of a 

 reciprocal exchange of temperature. In those instances, 

 however, it is obvious that we as-umr our own feelings 

 as the standard <.,{ reference ; and though, in the discus- 

 sion of the radiation of cold, this circumstance has been 

 sometimes brought into view, we suspect it has been too 

 little regarded ; and that it has contributed, either di- 

 rectly or indirectly, to produce all tlie obscun y in which 

 the radiation uf cold is at present involved. If we reject 

 entirely the decision of our feelings, ax vague and falla- 

 cious and have recourse to the li-ss objectionable mtans 

 of describing the degrees of temperature by the expan- 

 sion and contraction of bodies, vre shall be compelled to 

 acknowledge, that as we are still ignorant of the true 

 /.rn, iir the point of total privation of temperature, we 

 cann .t, in any rase, pronounce with certainty whether 



VOL. VI. PART II. 



Cold. 



the radiation be of a calorific or frigofific nature. Nay 

 as we judge ultimately of the reduction of temperature 

 in every case by the contraction of bodies, it is evident 

 that so long as this contraction takes place, we must in- 

 fer that caloric is abstracting from them ; and as this 

 may continue to go on until the bodies are reduced to 

 mere physical points, the true zero and the annihilation 

 of matter must be coincident ; in other words, matter 

 cannot exist without having a certain temperature, or a 

 certain degree of caloric is essentially connected with it 

 from its very nature. If this view of the subject be just, 

 the opinion of the radiation of cold must be rejected, a 

 founded entirely on the vague use of the terms which 

 we apply to the radiation of temperature in different cir- 

 cumstances, and which, by a slight change of these, may 

 become a radiation of caloric. 



Several experiments, however, have been adduced, 

 which seem to afford the same evidence for the existence 

 of a frigorific principle, that we have of caloric. , These 

 experiments are all connected with radiation or absorp- 

 tion : after describing them, we shall state the hypothe- 

 ses which have been proposed to explain them, in con- 

 formity with the opinion that cold is merely the abstrac- 

 tion of caloric. 



We have already mentioned, that when a plate of po- 

 lished metal is exposed to a heated body, almost all the 

 rays of caloric are reflected. If the rays strike the sur- 

 face of the plate perpendicularly, they are reflected di- 

 rectly backwards ; but if they strike it obliquely, they 

 are also reflected obliquely ; and the angle of reflection, 

 as in the case of light, is equal to the angle of incidence. 

 Such a figure, therefore, might be given to the metallic 

 surface, that when the heated body to which it is ex- 

 posed is placed in a certain position, all the rays incident 

 upon it might be reflected in a direction parallel to each 

 other. This will be the case if the metallic reflector 

 have the surface of a parabolic conoid, and the hot body 

 be placed in the focus of the generating parabola. Ac- 

 cordingly, if another mirror of the same form be placed 

 directly opposite, the foci being turned towards each 

 other, and the axes coincident, the emanations from the 

 hot body in the one focus will meet, after two reflec- 

 tions, in the other, and produce there an elevation of 

 temperature corresponding to that of the hot body. If 

 the surface of the reflectors be coated with lamp black, 

 the effect is scarcely perceptible, because almost 'all the 

 rays of caloric are then absorbed. In short, the great- 

 est effect is produced, when the surface of the hot body 

 is most favourable to radiation, the temperature being 

 supposed the same ; and when the reflectors are most 

 highly polished, and formed of some metal which has the 

 least disposition to absorb caloric. 



It was ascertained by Pictet, with an apparatus simi- Experi- 

 lar to the one we have described, that when a matrass ment of 

 filled with snow was placed in the one focus, and a deli- P' ctet< - 

 cate air thermometer in the other, the reflectors being 

 at the distance of 10^ feet, the thermometer immediate- 

 ly indicated a reduction of temperature of several de- 

 grees, and rose again when the matrass was withdrawn. 

 On replacing the matrass, and pouring nitrous acid upon 

 the snow, the temperature of the thermometer sunk 5 

 or 6 lower than before. In this experiment, cold seems 

 to have been radiated by the snow, and after being re- 

 flected from the surface of each mirror, to have been 

 absorbed by the thermometer ; and hence the reduction 

 of temperature which it sustained. 



In order to ascertain the nature of this radiant cold, Experi- 

 Mr Leslie, who has particularly investigated the subject nients of 

 by experiment, took a tin cubical vessel, having one side Mr 



