490 Dr. Herschei/s Experiments on the solar, and 



169th Experiment. 



Invisible rays. Calcined talc. 



9' 5H 5°i 



5 53 51 ...i|:| = ,in 



This substance stops 889 scattered 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 trans- 

 mission, much light would be thrown on what now still remains 

 a mysterious subject. It must be remembered, that in the fol- 

 lowing 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 

 12 inches long, 5^- broad, and 3 deep, has a partition through- 

 out its whole length, which divides it into two parts. At one 

 end of each division is a hole !■ inch in diameter ; and each di- 

 vision contains a thermometer, with its ball exposed to the hole, 

 and at one 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 upon the front, one on each side, which may be 



* See Plate XXV. Fig. i. 



