COLD. 



739 



CoW. 



Attempt to 

 shew that 

 the experi- 

 ment! with 

 the reflec- 

 tor-,, fur 

 nish no 

 new Jrgu. 

 mem tor 

 the radii- 

 tii. i! of 

 cold. 



PUTT 

 CXCIX. 

 Fig. 5 



might, in some measure, have been avoided, by taking 

 a different view of the action of the mirrors. The ge- 

 neral theory which he has proposed for explaining the 

 processes of the heating and cooling of bodies, does not ne- 

 cessarily imply that the mirrors shall act solely by absorp- 

 tion and radiation ; and, indeed, the author of the theo- 

 ry 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 hut 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 indi- 

 cate the appearance of the radiation of cold from the 

 cold body. 



Without adopting the explanation either of Prerost or 

 Pictet, or advancing any hypothesis of our own on the 

 subject, we shall deem it sufficient to shew that the- re- 

 sults of the experiments with the reflectors afford no ar- 

 gument for the support of any theory, which might not 

 be derived from direct radiation and absorption. 



That we may begin with the simplest case, let it be sup- 

 posed that the body A, Plate CXCIX, Fig.5, is surround- 

 ed on every side with bodies at an inferior temperature : it 

 will immediately begin to throw off rays of caloric in all 

 directions, and if the surrounding bodies be regularly and 

 uniformly disposed, and have the same absorbing powers 

 for caloric, it will cool equally on every side. IFanother 

 body B, in every respect similar to A, at a temperature 

 coniulei ably lower than that of the other bodies, be now 

 placed at no great distance from A. the body A will 

 cool more rapidly than before, particularly on the side 

 adjacent to B ; and still more rapidly if the surfaces a d, 

 and f It, possess great radiating or absorbing powers. 

 Since the cooling of A is promoted by the nature of the 

 surface / h of B, and since those surfaces which radiate 

 caloric most powerfully, also absorb it in the same de- 

 gree in opposite circumstances, it is obvious, that any 

 hypothesis which assumes, that emanations proceed mu- 

 tually from one body to the other, during the process 

 of cooling, must have the same difficulty in explaining 

 why, in the present case, A is more rapidly reduced in 

 temperature, when the radiation from B ought to be 

 most abundant, as it would have in similar circumstances 

 with the reflectors. Consequently, the objections which 

 we urged against Prevost's theory, deduced from the 

 experiment with the reflectors, that the greatest depres- 

 sion of the thermometer is exhibited when the surface of 

 the cold body, placed in the focus of one of the reflec- 

 tors, is most favourable to radiation, apply with equal 

 force in cases of cooling by direct radiation and absorp- 

 tion. 



Let us now suppose that the radiations from the side 

 b c, instead of being allowed to proceed from it in their 

 ordinary course, are intercepted by some reflecting sur- 

 face, and thrown back in the same direction with the 

 perpendicular emanations from the opposite side ad; as 



Natural 

 colj< 



this reflection can have no effect on the physical qualities Cold, 

 of the emanations, but merely to change their direction, '"Y""" 

 the only consequence of the arrangement we have sup- 

 posed will be to retard slightly the cooling of A, by the 

 partial re-absorption of those rays which are thrown back 

 upon b c. If we further suppose, that the reflected ema- 

 nations now moving parallel to one another, are again 

 intercepted by a reflecting surface placed behind B, and 

 thrown by means of its figure upon the surface eg, the 

 same effect must now be produced on this surface as if 

 there had been a direct radiation towards it from b c. In- 

 deed, it might be inferred, (i priori, that as the only 

 function of the reflecting surfaces is to change the di- 

 rection of the incident rays, we have no reason to expect, 

 when they are employed, any effect different from that 

 which is obtained by immediate and direct radiation. 



We accordingly find, that the greatest effect is pro- 

 duced in diminishing the temperature of A, when the 

 surfaces b c and eg are best fitted to radiate and absorb 

 the calorific rays, just as we found the same thing to 

 hold by direct radiation, when the surfaces ad wa&fh 

 were in like circumstances. It is evident, therefore, that 

 the rapid cooling of a hot body, when it is placed in the 

 focus of a reflector by means of a cold body placed in 

 the focus of an opposite reflector, admits ol being ex- 

 plained upon any hypothesis which affords a satisfactory 

 explanation of the simplest case of cooling, that by di- 

 rect radiation ; and that the experiment* with the reflec- 

 tors furnish no argument in support of the doctrine of a 

 frigorific principle of which we were not previously in 

 possession. 



The degrees of natural cold which occur in the more 

 inhospitable regions of the globe, are far surpassed by 

 the reductions of temperature obtained by artificial means. 

 The greatest cold that has hitherto been observed in the 

 open air, does not exceed 50 ; and probably the extreme 1 

 range of the inferior temperatures which occur in nature, 

 is but little below that point. 



The temperature of a particular place on the surface' 

 of the earth is determined by a variety of circumstances, 

 some of which are regular in their operation, others ac- 

 cidental. Among the former of these, may be stated, 

 the direct influence of the solar rays, and the latitude ; 

 among the latter, the winds, evaporation, and perhaps 

 the evolution or absorption of heat by operations going 

 on in the central regions. Even the causes of tempera- 

 ture which we have denominated regular, are not abso- 

 lutely so : the heat produced by the rays of the sun 

 may be affected by spots upon his surface ; and the tem- 

 perature resulting from geographical, position may be 

 modified by local peculiarities. Thus the sea limits the 

 range -of temperature, by moderating alike the extremes 

 of heat and cold ; while large tracts of land are equally 

 favourable to both. The winds have a very powerful 

 influence on the temperature of a place. When the sur- 

 face of the earth is much heated by the influence of the 

 solar rays, the air immediately above it is rarified, and 

 becoming specifically lighter, ascends into the higher re- 

 gions of the atmosphere. Its place is quickly occupied 

 by a fresh portion of air rushing in from every sidey 

 which, in its turn, being heated and rarified, also ascends.- 

 The warm air which has thus ascended, is gradually waft- 

 ed to colder regions, where it gives out its heat, and moj 

 derates the rigour of the climate to which it has been 

 transported. 



Evaporation is one of the principal sources of natural 

 cold, the conversion of water into vapour being neces- ableclun j 

 sarily accompanied with the absorption of much caloric ; mate'of ' 

 hence, the agricultural improvement of a country, or Europe. 



Remark- 



