1894.] on the Scientific Work of Tyndall. 221 



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. 



I come now to the second division of my subject, that relating to 

 Sound. Tyndall, as you know, wrote a book on Sound, founded on 

 lectures delivered in this place. Many interesting and original dis- 

 coveries are there embodied. One that I have been especially 

 interested in myself, is on the subject of sensitive flames. Professor 

 Leconte in America made the first observations at an amateur con- 

 cert, but it was Tyndall who introduced the remarkable high-pressure 

 flame now before you. It issues from a pin-hole burner, and the 

 sensitiveness is entirely a question of the pressure at which the gas 

 is supplied. Tyndall describes the phenomenon by saying that the 

 flame under the influence of a high pressure is like something on the 

 edge of a precipice. If left alone, it will maintain itself ; but 

 under the slightest touch it will be pushed over. The gas at high 

 pressure will, if undisturbed, burn steadily and erect, but if a hiss is 

 made in its neighbourhood it becomes at once unsteady, and ducks 

 down. A very high sound is necessary. Even a whistle, as you see, 

 does not act. Smooth pure sounds are practically without effect 

 unless of very high pitch. 



I will illustrate the importance of the flame as a means of investi- 

 gation by an experiment in the diffraction of sound. I have here a 

 source of sound, but of pitch so high as to be inaudible. The waves 

 impinge perpendicularly upon a circular disc of plate glass. Behind 

 the disc there is a sound shadow, and you might expect that the 

 shadow would be most complete at the centre. But this is not j so. 

 "When the burner occupies this position the flame flares ; but when by 

 a slight motion of the disc the position of the flame is made eccentric 

 the existence of the shadow is manifested by the recovery of the 

 flame. At the centre the intensity of sound is the same as if no 

 obstacle were interposed. 



The optical analogue of the above experiment was made at the 

 suggestion of Poisson, who had deduced the result theoretically, but 

 considered it so unlikely that he regarded it as an objection to the 

 undulatory theory of light. Now, I need hardly say, it is regarded 

 as a beautiful confirmation. 



It is of importance to prove that the flame is not of the essence of 

 the matter, that there is no need to have a flame, or to ignite it at the 

 burner. Thus, it is quite possible to have a jet of gas so arranged 

 that ignition does not occur until the jet has lost its sensitiveness. 

 The sensitive part is that quite close to the nozzle, and the flame is 

 only an indicator. But it is not necessary to have any kind of flame 

 at all. Tyndall made observations on smoke-jets, showing that a jet 

 of air can be made sensitive to sound. The difficulty is to see it, and 



