220 Lord Bayleigh [March 16, 



was at once sanguine and sceptical — u combination necessary for 

 success in any branch of science. The experimentalist who is not 

 sceptical will be led away on a false tack and accept conclusions 

 which he would find it necessary to reject were he to pursue the 

 matter further ; if not sanguine, he will be discouraged altogether by 

 the difficulties encountered in his earlier efforts, and so arrive at no 

 conclusion at all. One criticism, however, may be made. Tyndall did 

 not at first describe with sufficient detail the method and the precau- 

 tions which he used. There was a want of that precise information 

 necessary to allow another to follow in his steps. Perhaps this may 

 have been due to his literary instinct, which made him averse from 

 overloading his pages with technical experimental details. 



The controversy above referred to I think we may now consider 

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

 vapour. 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 vapour. 



I cannot attempt to show you here any of the early experiments 

 on the absorption of vapours. 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 intermittent radia- 

 tion. 



The radiation is supplied from incandescent lime, and is focussed 

 by a concave reflector. In the path of the rays is a revolving wheel 

 provided with projecting teeth. When a tooth intervenes, the radia- 

 tion 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 radiation the ether vapour expands and 

 contracts ; in other words a vibration is established, and a sound is 

 heard by the observer. But if the vapour were absolutely diather- 

 manous, no sound would be heard. 



I have repeated the experiment of Tyndall which allowed him to 

 distinguish between the behaviour 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 disappears. 



According to the law of exchanges, absorption is connected with 

 radiation ; so that while hydrogen or oxygen do not radiate, 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 



