308 Prof. Tyndall. Action of an Intermittent Beam [Jan. 13, 



From tlie first, I entertained the opinion that these singular sounds 

 were caused by rapid changes of temperature, producing correspond- 

 ing changes of shape and volume in the bodies impinged 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 concluded that 

 when the pulses thus excited followed each other with sufficient 

 rapidity, a musical note must be the result. It seemed plain, more- 

 over, 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 athermanous 

 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 7 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 immediately 

 behind the rotating disk. From the flask a tube of india-rubber, end- 

 ing in a tapering one of ivory or box wood, 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. 



* On the 29th November : see "Journal of the Society of Telegraph Engineers," 

 December 8, 1880. 



f "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 pro- 

 duced. 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. 



