38 ABSORPTION OF RADIANT HEAT PARTI. 



intervening ether ; this motion is immediately commu- 

 nicated to the ether outside, and is thus lost for the 

 purposes of conduction.' 6 



Melloni had investigated the laws of the radiation and 

 absorption of radiant heat in solid and liquid matter ; 

 but its radiation and absorption by gases and vapours 

 was unknown previous to the experiments of Mr. Tyndall. 



The apparatus employed was a horizontal brass tube 

 four feet long, between two and three inches in diameter, 

 polished inside, and closed air-tight at each end by a 

 plate of rock-salt, which transmits more heat than any 

 other substance. The air could be pumped out of the 

 tube by one pipe, and the gas or vapour for the experi- 

 ment introduced by another. Close to one end of the 

 brass tube there was a thermo-electric pile connected 

 ^^^with its goniometer. On each side of this arrangement 

 there was a vessel of water kept at the boiling point. 

 These two vessels were so placed that when the rays of 

 heat from one of them passed through the exhausted 

 tube, and fell upon one face of the thermo-electric pile, 

 their effect was so neutralized or balanced by the rays 

 of heat falling on the opposite face of the pile from the 

 , thus the needle of the goniometer was steadily 

 maintained at zero, and its deflection instantly showed 

 the absorbent effect produced by any gas or vapour that 

 was admitted into the exhausted tube. 



Since aqueous vapour has a very exalted absorbent 

 power, a gas or vapour was rendered perfectly dry 

 before its absorbent capacity was determined. For that 

 purpose the pipe that introduced it into the brass 

 experimental tube was so constructed that the gas had 

 first to pass . over fragments of pumice-stone wet with 

 strong sulphuric acid, which absorbed its moisture and 

 dried it. Common atmospheric air, however, was not 



6 Tyndall on Heat. 



