GENERAL ACCOUNT OF RADIATION. 



235 



made a great number of researches on the transparency and opacity of 

 gases and vapours, his principal mode of experiment consisting in passing 

 a beam of radiation through a tube about 4 feet long and 2 or 3 inches 

 in diameter, with its two ends closed by rock-salt plates, from a source 

 which, in many of the experiments, was a Leslie cube filled with hot 

 water, as represented in Fig. 138. The beam fell on one face of a thermo- 

 pile, and the heating effect was compensated for by the radiation from 

 a second cube falling on the other face of the pile. A chamber which 

 could be exhausted intervened between the cube and the nearer rock- 

 salt plate, so that no absorption could occur by the air before the radia- 

 tion reached the gas. The air could be pumped out of the tube, and the 

 dry gas to be experimented on could be introduced from a gasholder. 

 When vapours were experimented on, a flask containing some of the liquid 

 to be vaporised replaced the gasholder. By these experiments Tyndall 

 showed that air, oxygen, nitrogen, and hydrogen exercised only a very 

 slight effect, but that compound gases showed decided absorption. When 

 the source was a copper plate heated by a Bunsen burner, Tyndall found 

 the comparative results given below. It will be seen that at a low 

 pressure the differences are still more striking than at the pressure of 

 the atmosphere. In the latter case, probably, the absorption was in 

 some instances so complete that practically all the radiation which could 

 be absorbed was sifted out of the beam before it had traversed the entire 

 thickness, so that the whole length of the tube was not effective for 

 the gases lower on the list. 



The effect of vapours was, in general, very great in some cases 

 enormously greater than that of the simple gases, and he found also 

 that they followed the same general order as the liquids from which 

 they evaporated. 



Tyndall also investigated the emissive powers of gases and vapours, 



