on the terrestrial Rays that occasion Heat. 525 



Let us now examine what information we may draw from 

 the facts which are recorded in this table. The first that must 

 occur is, that a candle which emits light, is also a copious source 

 of invisible heat. If this should seem to require a proof, I give 

 it as follows. 



That the candle emits heat along with light, the thermometer 

 has ascertained ; and, that a considerable share of this at least 

 must be invisible, follows from comparing together the quantity 

 of light and heat which are stopped by different glasses. The 

 bluish-white one, for instance, stops 86 rays of light, and 625 of 

 heat. Hence, if only visible rays of heat came from the candle, 

 a glass stopping more light, as for instance the dark-red glass, 

 which stops 999,8, ought to stop all heat whatsoever ; but the 

 fact is, that it even stops one hundred rays less than the former. 



This instance alone shews plainly, that the existence of invi- 

 sible terrestrial heat in the flame of a candle, is proved ; while, 

 on the contrary, heat derived from rays that are visible, remains 

 yet to be established, by those who would maintain that there 

 are any such. But, for the sake of argument, let us endeavour 

 to explain how visible rays of heat may be reconciled with the 

 contents of our 6th table. 



" Now although we must allow," it may be said, " that there 

 " is a certain quantity of candle-heat which cannot be seen, we 

 " are however at liberty to assign any ratio that this may bear to 

 " its visible heat-rays. Let us therefore begin with the bluish- 

 " white glass, and make the most favourable supposition we can, 

 " in order to explain its phenomena. Visible or invisible, it stops 

 " 625 rays of heat, and also 86 of light. Now, as in the last 

 " column of the table we have likewise the proportional quantity 

 *' of invisible heat it intercepts, which is 700 out of a thousand, 



MDCCC 3 Y 



