Otf HEAT AND LIGHT. \Q$ 



When we confider the prodigious intenfity of the heat ex- it does not fol- 



cited in the focus of a burning mirror or a fefcs, we are tempted !° w [j^ t ™ f °£ 



to believe, that the concentration and condenfation of the folar f i ute power ef 



rays increafe their power of exciting heat; but, if we examine *}* ra Y st0 P r0 * 

 , f ,- , ,i. r r i ■ r i duce heat can be 



the matter more eroiely, we are obliged to cornels', that loch i ncr eafed by 



an augmentation would be inexplicable. It would be 'equally condenfing 

 fo on both the hypothefes, which natural philofophers have 

 formed of the nature of light: for, as it has been proved both 

 by calculation and experiment, that tw^p undulations in an 

 elaftic fluid may approach and even crofs each other, without 

 deranging either their refpective directions or velocities, if 

 light be analogous to found, we do not fee how the concen- 

 tration or condenfation of thefe undulations can increafe their 

 force of impulfe : and if light be a real emanation, as its ve- 

 locity is not altered, either by the change of direction it un- 

 dergoes in paffing through a lens, or by its reflection from the 

 furface of a polimed body, it feems to me, that the power of 

 each of thefe particles to excite or impart heat, mutt necefiarily 

 be the fame after refraction or reflection as before ; and con- 

 fequently, that the heat communicated or excited muft be, in 

 in all cafes, as the quantity of light abforbed. 



I have juft made Come experiments, which appear to me to Experimental 

 eflablim this fa& beyond queflion. inveftigatkm. 



Having procured from the optician Lerebours two Ienfes Tw0 convex 

 perfectly equal, and of the fame kind of glafs, four inches in fi m iiar were 

 diameter, and of eleven and a half focus, I expofed them at the ufed, 

 fame time to the fun, fide by fide, about noon, when the ffcy 

 was very clear; and by means' of two thermometers, or refer- 

 voirs of heat, of a peculiar conftruciion, I determined the re- 

 lative quantities of heat, that were excited in given times by 

 the folar rays at different diftances from the foci of the lehles. 



The two refervoirs of heat are a ibrt of flat boxes of brafs to throw the 

 filled with water. Each of thefe refervoirs is three inches ten £ un ' s . ,i f ht upon 



v i i ir ,- .- . flat tin boxes 



lines and a half in diameter, and fix lines thick, well polifhed containing water 



externally on all fides except one of its two flat faces, which ai>d blad " ned 



l i i i i i /- i r > . on their f«r# 



was blackened by the lmoke of a candle. On this face the faces. 



folar rays were received in the experiments. 



Each of thefe refervoirs - of heat weighs when empty 6850 



grains, poids de marc, (near a pound troy), and contains 1210 



grains of water (about 2 oz. 2 dwts. 



Taking 



