176 Professor Tyndall [April 8, 



upon by the beam. But if this be the case, and if gases and vapours 

 really absorb radiant heat, they ought to produce sounds more intense 

 than those obtainable from solids. I pictured every stroke of the beam 

 responded to by a sudden expansion of the absorbent gas, and con- 

 cluded that when the pulses thus excited followed each other with 

 sufficient rapidity, a musical note must be the result. It seemed 

 plain, moreover, that by this new method many of my previous results 

 might be brought to an independent test. Highly diathermanous 

 bodies, I reasoned, would produce faint sounds, while highly adiather- 

 manous bodies would produce loud sounds ; the strength of the sound 

 being, in a sense, a measure of the absorption. The first experiment 

 made with a view of testing this idea, was executed in the presence of 

 Mr. Graham Bell ; * and the result was in exact accordance with what 

 I had foreseen. 



The inquiry has been recently extended so as to embrace most of 

 the gases and vapours employed in my former researches. My first 

 source of rays was a Siemens' lamp connected with a dynamo-machine, 

 worked by a gas-engine. A glass lens was used to concentrate the 

 rays, and afterwards two lenses. By the first the rays were rendered 

 parallel, while the second caused them to converge to a point about 

 seven inches distant from the lens. A circle of sheet zinc provided 

 first with radial slits and afterwards with teeth and interspaces cut 

 through it, was mounted vertically on a whirling table, and caused to 

 rotate rapidly across the beam near the focus. The passage of the 

 slits produced the desired intermittence,t while a flask containing the 

 gas or vapour to be examined received the shocks of the beam imme- 

 diately behind the rotating disk. From the flask a tube of indiarubber, 

 ending in a tapering one of ivory or boxwood, led to the ear, which 

 was thus rendered keenly sensitive to any sound generated within the 

 flask. Compared with the beautiful apparatus of Mr. Graham Bell, 

 the arrangement here described is rude : it is, however, very 

 effective. 



With this arrangement the number of sounding gases and vapours 

 was rapidly increased. But I was soon made aware that the glass 

 lenses withdrew from the beam its most effectual rays. The silvered 

 mirrors employed in my previous researches were therefore invoked ; 

 and vriih them, acting sometimes singly and sometimes as conjugate 



* On November 29 : see ' Journal of the Society of Telegraph Eugineers,' 

 December 8, 1880. 



t When the disk rotates the individual slits disappear, forming a hazy zone 

 through which objects are visible. Throwing by the clean hand, or better still by 

 white paper, the beam back upon the disk, it appears to stand still, the slits 

 forming so many dark rectangles. The reason is obvious, but the experiment is a 

 very beautiful one. 



I may add that when I stand with open eyes in the flashing beam, at a 

 definite velocity of recurrence, subjective colours of extraordinary gorgeousness 

 are produced. With slower or quicker rates of rotation the colours disappear. 

 The flashes also produce a giddiness sometimes intense enough to cause me to 

 grasp the table to keep myself erect. 



