THE SCIENTIFIC WORK OF TYNDALL. 663 



which made him averse from overloading his pages with technical 

 experimental details. 



The controversy above referred to I think we may now con- 

 sider to be closed. Nobody now doubts the absorbing power of 

 aqueous vapor. Indeed, the question seems to have entered upon 

 a new phase ; for in a recent number of Wiedemann's Annalen, 

 Paschen investigates the precise position in the spectrum of the 

 rays which are absorbed by aqueous vapor. 



I can not attempt to show you here any of the early experi- 

 ments on the absorption of vapors. But some years later Tyndall 

 contrived an experiment, which will allow of reproduction. It is 

 founded on some observations of Graham Bell, who discovered 

 that various bodies became sonorous when exposed to intermit- 

 tent radiation. 



The radiation is supplied from incandescent lime, and is fo- 

 cused by a concave reflector. In the path of the rays is a re- 

 volving wheel provided with projecting teeth. When a tooth 

 intervenes, the radiation is stopped ; but in the interval between 

 the teeth the radiation passes through, and falls upon any object 

 held at the focus. The object in this case is a small glass bulb 

 containing a few drops of ether, and communicating with the ear 

 by a rubber tube. Under the operation of the intermittent radia- 

 tion the ether vapor expands and contracts ; in other words, a vi- 

 bration is established, and a sound is heard by the observer. But 

 if the vapor were absolutely diathermanous, no sound would be 

 heard. 



I have repeated the experiment of Tyndall which allowed him 

 to distinguish between the behavior of ordinary air and dry air. 

 If, dispensing with ether, we fill the bulb with air in the ordinary 

 moist state, a sound is heard with perfect distinctness, but if we 

 drop in a little sulphuric acid, so as to dry the air, the sound dis- 

 appears. 



According* to the law of exchanges, absorption is connected 

 with radiation ; so that while hydrogen and oxygen do not radi- 

 ate, from ammonia we might expect to get considerable radiation. 

 In the following experiment I aim at showing that the radiation 

 of hot coal gas exceeds the radiation of equally hot air. 



The face of the thermopile, protected by screens from the 

 ball itself, is exposed to the radiation from the heated air which 

 rises from a hot copper ball. The effect is manifested by the 

 light reflected from a galvanometer mirror. When we replace 

 the air by a stream of coal gas, the galvanometer indicates an 

 augmentation of heat, so that we have before us a demonstration 

 that coal gas when heated does radiate more than equally hot air, 

 from which we conclude that it would exercise more absorption 

 than air. 



