Remarks on 

 the experi- 

 Hicnt with 



272 °^ HEAT. 



distant from the heated surface, did acquire any increase of 

 temperature; but I assure you, if so, I could not discover it by 

 a very delicate air thermometer : so that I conclude, that the 

 effect produced on the thermometer, in the focus of the reflec- 

 tion, was by the calorific rays which passed through the 

 screen. 

 Experiment 1 consider Mr. Leslie's experiment with the sheet of ice as 



with a sheet of establishing nothing whatever; for here the rays are not only 

 arrested, but absorbed ; and thougii I am inclined to believe 

 that some of the rays are transmHted, yet that the " frigorific 

 rays," if I may be allowed the expression, for the sake of being 

 understood, are more than able to counteract any effect which 

 they could produce. 



In the last experiment which I shall at present notice, and 

 which Mr. L. regards as the expert' mentum crucis, I think he 

 does more to establish the fact, that part of the radiant caloric 

 does pass through glass, than to make good his own theory ; for 

 here we see the effect produced, when there is a certainty 

 that none of the radiant caloric can pass, and we find that this 

 effect is less cecieris paribus, by two degrees, than when there 

 was a possibility that some part of it might pass ; and if we 

 compare that with the quantity which passed through the glass 

 in the former experiment, we shall find that they are nearly 

 equal; and as metallic surfaces reflect the whole of the radiant 

 caloric, I conceive there is but Httle difficulty in accounting for 

 the striking difference which he observes took place when the 

 tin coatings of the panes of glass were on the outer side. I 

 admit, with Mr. Leslie, that the calor fie emanation is inca- 

 pable of permeating solid substances which are opaque; but 

 when light can pass through, I am inclined to believe that 

 radiant caloric is also capable of finding its way, or, in other 

 words, that radiant caloric is capable of passing through trans- 

 parent solid substances. 



Sir, I have ventured to trouble you with these rather puerile 

 observations, with the view of drawing some of your corres- 

 pondents to the subject. It is a field in which much may yet 

 be done. 



I am, Sir, 



Your very obedient Servant, 



ANDREW HALLIDAY, M.D. 



General 



elusion. 



Con- 



III. 



