1S15.] of tlie Effect of Ice on a Thermometer, 339 



substituted for the hot radiating body in the focus of the second 

 reflector, a thermometer in the focus of the first indicates a tempe- 

 rature below that of adjacent bodies not in that focus. This expe- 

 riment was, I believe, first brought forward by Mr. Pictet, and the 

 phenomenon at first appeared to favour the hyj)othesis of the actual 

 radiation of cold ; but a well-known explanation of it has been 

 given by Mr. Prevost. 1 have not been able to see this explanation 

 in Mr. Prevost's own words, but I collect it as referred to in other 

 books. 



He considers that caloric is radiated from bodies at all tempera- 

 tures. The thermometer, therefore, itself gives out heat by radia- 

 tion. In consequence of this universal radiation, all bodies must 

 also receive radiations. The thermometer, therefore, is receiving 

 heat also ; and while it receives an equivalent for what it gives out, 

 no change of temperature is to be expected. But he adds, that the 

 quantity radiated from all bodies is in proportion to the difference 

 of their temperatures. Since, then, comparatively cold bodies emit 

 comparatively weak radiations, when ice is placed in the focus of 

 the second mirror the thermometer receives less than it emits, and 

 thus loses in temperature. 



" But," says the writer of the article in the Encyclopsedia, " this 

 explanation is totally inconsistent with the most obvious facts 

 attending the cooling of bodies ; " for, he continues, " it is evident 

 tiiat on this hypothesis a hot body ought to cool more slowly when it 

 is placed near a large body of inferior temperature than when near 

 a small one; because in the former case it must receive more calo- 

 rific emanations than in the latter." 



/ This is precisely what I deny, notwithstanding the deference I 

 feel for the opinions of the writer of this article, as well as for those 

 of Mr. Murray, whose objections follow. 



The article continues, " It is also very properly objected by Mr. 

 Ivlurray, that of different surfaces which at a given temperature 

 radiate dilferent 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 th(^ reduction of its 

 temperature." It then goes on to quote Mr. Murray as saying, that 

 if Mr. Prevost's explanation were just, a blackened surface (which 

 it is proved by Mr. Leslie's experiments radiates the largest quantity 

 of caloric) ought to produce the least cooling effect on the thernio- 

 nittcr, and a nieiallie surface (which radiates the least) ought to 

 produce the greatest." 



i shall beg leave to answer this objection with the preceding one. 



'I'lie Kncyclopiedia continues, " Rejecting then this explanation 



of Ml, I'revost's, we shall proceed to consider that of Pictet; " to 



which also they ol»ject, and therefore leave the phenomena entirely 



unaccounted tor. 



Tiie ssriier ol the article, however, docs not infer from tin's 

 j)lienuu^enon the absolute existence ol" a i'rigoiilic ])rinciple, but 

 only argues " that the objections aguinst Mr. Pievosi's and Mr. 



y J 



