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 discoveries are there embodied. One, that I have been especially interested in myself, is on the subject of sensitive flames. Prof. Le Conte in America made the first observations at an amateur concert, but it was Tyndall who introduced the remarkable high-pressure flame now before you. It issues from a pinhole 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 neighborhood 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 investigation 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 disk of plate glass. Behind the disk there is a sound shadow, and you might expect that the shadow would be most complete at the center. But this is not so. When the burner occupies this position the flame flares; but when by a slight motion of the disk the position of the flame is made eccentric, the existence of the shadow is manifested by the recovery of the flame. At the center 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 to operate successfully upon it; because, as Tyndall soon found, a smoke jet is much more difficult to deal with than flames, and is sensitive to much
