764 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



ring the time of the experiment : for, notwithstanding the largeness of the screen, 

 some heat will find its way round and over it ; and this acting as a general cause, 

 its effect must be allowed for. 



Exper. 117. Having tried the apparatus sufficiently to 

 find that the thermometers exposed to the transmitting holes 

 would generally receive 20 or more degrees of heat, without 



differing more than sometimes \ or at most J of a degree, I Min. Fire. Bluish-white glass. 

 now placed the bluish- white glass of the 24th experiment on a 66 66 



support prepared for the purpose, so as closely to cover one of 5 86 71 ... 20 : 5 = .250. 



the transmitting holes. A small spring, moveable on its cen- 

 tre, is always turned against the upper part of the transmit- 

 ting glasses, to keep them in their situation. 



This glass stops 750 rays of fire-heat. By looking through it, at the same place in the fire, after the 

 screen was removed, in order to cool the apparatus for the next experiment, I found that this glass can 

 hardly be said to stop any of the light of the fire. 

 Min. Fire. Flint glass. 

 Exper. 118. 67 67 



5 87 72 . . 20 : 5 = .250. It stops 750 rays of fire-heat. 



Min. Fire. Crown glass. 

 Exper. 119. 67 67 



5 86| 7*h • • • 191 : H — .278. It stops 722 rays of fire-heat. 



Min. Fire. Coach glass. 

 Exper. 120. 67h W\ 



5 86| 73 . . • 191 : H = »28?>. It stops 71 i rays of fire-heat. 



Min. Fire. Iceland crystal. 

 Exper. 121. 6*8 68 



5 90h 73£ . . . 22£ : 5\ = .244. This substance stops 756 rays of fire-heat. 



Exper. 122. I took now the piece of talc used in the 

 30th experiment, and, . placing it over the transmitting 

 hole, I had the following result. But, as the unexpected 

 event of a calcination, which took place, was attended 

 with circumstances that ought to be noticed, I shall in- 

 stead of the usual abridgement of the experiments, give 

 this at full length. 



This substance stops 713 rays of fire heat. 



I am now to point out the singularity of this experiment ; which consists, as we 

 may see by the above register of it, in the apparently regular continuance of its 

 power of transmitting heat, while its capacity of transmitting light was totally de- 

 stroyed. For, when I placed this piece of talc over the hole in the screen, it was 

 extremely transparent, as this substance is generally known to be ; and yet, when 

 the experiment was over, it appeared of a beautiful white colour ; and its power of 

 transmitting light was so totally destroyed,that even the sun in the meridian could 

 not be perceived through it. Now, had the power of transmitting heat through 

 this substance been really uniform during all the 5 minutes, it would have been quite 

 a new phenomenon ; as all my experiments are attended with a regular increase of 

 it ; but since by calcination, the talc lost much of its transmitting power, we may 

 easily account for this unexpected regularity. 



Fire. Very dark red glass. 



Exper. 123. 66 06 



894 75 ... 23 4 : 9 = 3b7. This glass stops 6l3 rays of fire-heat. 



Min. Fire Dark-red glass. This glass, which stops 999-3 rays of 



Exper. 124. 67 67 candle-light, stops only 573 rays of fire- 



5 92g 78 . . . 25| : 1 1 = .427. heat ; whereas my piece of thick flint 



glass, which stops no more than 91 rays of that light, stops no less than 750 of fire-heat. It does not 



