VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 767 



Min. Invis. rays. Pale blue glass. 



Exper. 159. 5H 5l£ 



5 53-jj- 51-|- . . . \\ : -f == .417. It stops 583 invisible rays of heat. 



Min. Invis. rays. Dark blue glass. 



Exper. 160. 52-L 5 14 



5 52-| 52| ... I : i = .833. It stops 167 invisible rays of heat. 



Min. Invis. rays. Indigo glass. 



Exper. 161. 52| 52| 



5 54f 53 . . . I5 : f = .500. It stops 500 invisible rays of heat. 



Min. Invis. rays. Pale indigo glass. 



Exper. 162. 52| 52-L 



5 53f 52i ... 1 : I = .750. It stops 250 invisible rays of heat. 



Min. Invis. rays. Purple glass. 



Exper. 163. 5l£ 50-f. 



5 52-g- 51-3. . . . 1-J : 1 = .727. It stops 273 invisible rays of heat. 



Min. Invis. rays. Violet glass. 



Exper. 164. 53£ 52f 



5 54J 53 l . . . 1 : £ = .750. It stops 250 invisible rays of heat. 



,,. T . Crown glass: one side rubbed on emery, 



Min. Invis. rays. P .,' , J> 



* rough side exposed- 



Exper. 165. 49| 48£ This glass, so prepared, stops 6*00 



5 50| 49| . . . I5 : \ — .400. scattered invisible rays of heat. 



,.. , . Coach glass; one side rubbed on 



Mm. Invis. rays. u -j j 



1 emery, rough side exposed. 



Exper, 166. 54 53-f It stops 500 scattered invisible rays 



5 55\ 54 . . . l£ : -I = .500. of heat. 



Min. Invis. rays. Crown glass ; both sides rubbed on emery. 



Exper. 167. 50 49^ It stops 600 scattered invisible rays 



5 5lJ 49| . . . 14 : \ = .400. of heat. 



Min. Invis. rays. Coach glass 3 both sides rubbed on emery. 



Exper. 168. 54| 54 £ It stops 714 scattered invisible rays 



5 55% 54f . . . i : J = .286. of heat. 



Min. Invis. rays. Calcined talc. 



Exper. 169. 51| 50^ This substance stops 889 scattered 



5 53 51 . . . 1£ : •£■ == .111. invisible rays of heat. 



Transmission of Invisible Terrestrial Heat. — This is perhaps the most extensive 

 and most interesting of all the articles we have to investigate. Dark heat is with 

 us the most common of all; and its passage from one body into another, is what 

 it highly concerns us to trace out. The slightest change of temperature denotes 

 the motion of invisible heat ; and if we could be fully informed about the method 

 of its transmission, much light would be thrown on what now still remains a mys- 

 terious subject. It must be remembered, that in the following experiments, I only 

 mean to point out the transmission of such dark heat as I have before proved to 

 consist of rays, without inquiring whether there be any other than such existing. 



My apparatus for these experiments is as follows. A box, fig. 4, pi. 14, 12 

 inches long, 54- broad, and 3 deep, has a partition throughout its whole length, 

 which divides it into 2 parts. At one end of each division is a hole ^ inch in dia- 

 meter; and each division contains a thermometer, with its ball exposed to the hole, 

 and at 1 inch distance from the outside of the box. Four inches of the box, next 

 to the holes are covered ; the rest is open. In the front of it is a narrow slip of 

 wood, on which may rest any glass to be tried; and it is held close to the wood at 

 the top, by a small spring applied against it. Two screws are planted on the front, 



