ON RADIANT HEAT. 107 



If, as Mr. Leslie affirms, caloric never passes through a Caloric passes 

 medium without first raising its temperature, blackened *J^"f J^^" 

 glass, used as a skreen, should affect the thermometer in a without raising 

 greater degree than plain glass: for by blackening the sur- x ™ m tem P era * 

 faces of the glass both their absorbing and radiating powers 

 are increased: but we find by Exp. 2d and 4th, that the 

 thermometric rise with plain glass is more than twelve times 

 higher than with blackened glass. Exp. 5th and 8th, how- 

 ever, afford yet more satisfactory evidence of the calorific 

 rays passing through diaphanous bodies independently of 

 heating their substance. Another argument in opposition 

 to Mr. Leslie's opinions, if more were requisite, might be 

 drawn from the results given in Exp. 9th. If his position 

 were weJl founded, since the effect with plain glass is to the 

 effect with painted glass as 7 to 5 ; and since, when the hot 

 flask is removed, the painted glass produces an effect as 15 ; 

 the plain glass should produce an effect as 21. But when 

 the primary source of heat is removed, the effect of the 

 plain glass is to the effect of the painted glass only as 13 

 to 15. 



It is an indubitable fact, that, when a transparent sheet 

 of glass is brought near to a hot body, it will have its tem- 

 perature augmented; but the fairest inductions from my 

 experiments warrant us in concluding, that the use of the 

 thermometric fluid is not solely, or principally, referrible to 

 this secondary source of heat. 



Mr. Leslie's portion, however, holds perfectly true with but not 

 respect to the action of opaque skreens. It is rendered pro- l J2Ji!| media 

 bable by every experiment, in which they were employed; 

 and is satisfactorily proved by Exp. 6th, 7th, and 9th. 

 This leads me to notice the results of Exp. 2nd and 3rd. 

 When an opaque skreen is interposed between the thermo- 

 meter and the hot body, one sur lace of the skreen absorbs 

 caloric from the hot body, the other surface radiates to the 

 thermometer. The most considerable effect, therefore, 

 should be produced on the thermometer when a skreen is 

 used, the surfaces of which are best adapted for ab-orbing 

 and for radiating, and the substance of which is the best 

 conductor of caloric. In the two former qualifications the 

 painted tin and the painted glass are on a par; in the lat- 

 ter 



