280 Prof. Tyndall on the Absorption and 



gen, and nitrogen, sent in a similar manner over the ball, were 

 equally ineffective. The other gases, however, not only exhibit a 

 marked action, but also marked differences of action. Their 

 radiative powers follow precisely the same order as their powers 

 of absorption. In fact, the deflections actually produced by 

 their respective absorptions at 5 inches tension are as follow : — 



degree 



Air . . . . 



A fraction of 



Oxygen . . 

 Nitrogen 





Hydrogen . . 

 Carbonic oxide 



. '! . 18° 



Carbonic acid . 



. . .25° 



Nitrous oxide . 



. . .44° 



Olefiant gas 



. . .61° 



It would be easy to give these experiments a more elegant 

 form, and to arrive at greater accuracy, which 1 intend to do on 

 a future occasion ; but my object now is simply to establish the 

 general order of their radiative powers. An interesting way of 

 exhibiting both radiation and absorption is as follows : — When 

 the polished face of a Leslie's cube is turned towards a thermo- 

 electric pile the effect produced is inconsiderable, but it is 

 greatly augmented when a coat of varnish is laid upon the po- 

 lished surface. Instead of the coat of varnish, a film of gas may 

 be made use of. Such a cube, containing boiling water, had its 

 polished face turned towards the pile, and its effect on the gal- 

 vanometer neutralized in the usual manner. The needle being 

 at 0°, a film of olefiant gas, issuing from a narrow slit, was 

 passed over the metal. The increase of radiation produced a de- 

 flection of 45°. When the gas was cut off, the needle returned 

 accurately to 0°. 



The absorption by a film may be shown by filling the cube 

 with cold water, but not so cold as to produce the precipitation 

 of the aqueous vapour of the atmosphere. A gilt copper ball, 

 cooled in a freezing mixture, was placed in front of the pile, and 

 its effect was neutralized by presenting a beaker containing a 

 little iced water to the opposite face of the pile. A film of ole- 

 fiant gas was sent over the ball, but the consequent deflection 

 proved that the absorption, instead of being greater, was less 

 than before. The ball, in fact, had been coated by a crust of ice, 

 which is one of the best absorbers of radiant heat. The olefiant 

 gas, being warmer than the ice, partially neutralized its absorp- 

 tion. When, however, the temperature of the ball was only a 

 few degrees lower than that of the atmosphere, and its surface 

 quite dry, the film of gas was found to act as a film of varnish ; 

 it augmented the absorption. 



