ON RADIANT HEAT. 77 



glass. In two minutes the thermometer was up to 2^*. t 

 removed the hot flask ; the thermometer did not experience 

 any farther rise, and in two minutes it had fallen 7 . 



Exp. 9. A 7 oz. flask of boiling water being used as the Exp. 9. 

 source of heat, I found, that plain glass transmitted caloric 

 as 7 to painted glass as 5 ; and that, after the hot flask was 

 removed, the effect of the black glass skreen was to that of 

 the plain glass as 15 to 13. 



Exp. 10. Having painted one side of a pane of glass Exp. 10, 

 black, I placed it on the skreen stand with its blackened 

 surface next to the candle. In two minutes the thermome- 

 ter rose 2 '3°. 



Exp. 11. I opposed the clean surface of the same skreen Exp. II* 

 to the hot body. In two minutes the thermometer stood at 

 3-8°. 



Exp. 12. I removed one of the reflectors, and found, E X p, 12. 

 that, without the skreen was very near to the source of heat, 

 no sensible effect was produced on the thermometer in two 

 minutes. The skreen stand being situate one inch from the 

 hot body, glass did not appear to transmit caloric better 

 than any other medium. 



Before I attempt to draw any conclusions from the pre- Objections** 

 ceding experiments, I think it necessary to anticipate some sweMd - 

 objections, which may possibly be urged to the manner, in 

 which the most striking were made. 



The inequality of neat produced by the combustion of inequality of 

 a candle may be thought a very fertile source of errour : the heat of* 

 and I am ready to allow, that the flame of a candle could 

 not with propriety be employed to determine minute differ- 

 ences in the temperature of the thermometer. But the 

 differences I observed were very far from being minute, 

 and the experiments, as often as repeated, afforded similar 

 results. 



The burning candle will be deemed particularly objec- Thatligtitii 



tionable by all those who contend, that light itself is pos- supposed to i« 



, R r heating, 



sessed of a heating property. It would be entirely foreign 



to my present purpose, to attempt a refutation of this purely 

 hypothetical opinion. I would however avail myself of the 

 excellent experiments of Dr. Herschel, which show, that 

 the heat excited by the different prismatic rays is in no de- 

 gree 



