ACTION OF RADIANT HEAT. 35 



reasoned, would produce faint sounds, while highly atherraanous 

 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 Avith. what I had 

 foreseen. 



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

 the gases and vapors 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 afterward 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 afterward 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 pro- 

 duced the desired intermittence, f while a flask containing the gas or 

 vapor 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 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 ar- 

 rangement here described is rude ; it is, however, very effective. 



With this arrangement the nu^nber of sounding gases and vapors 

 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 with them, acting sometimes singly and sometimes as conjugate 

 mirrors, the curious and striking results which I have now the honor 

 to submit to the Society were obtained. 



Sulphuric ether, formic ether, and acetic ether, being placed in 

 bulbous flasks,J; their vapors were soon diffused in the air above the 

 liquid. On placing these flasks, whose bottoms only were covered by 

 the liquid, behind the rotating disk, so that the intermittent beam 



* On the 29th Noveniber : 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 ve- 

 locity of recurrence, subjective colors of extraordinary gorgeousness are produced. "With 

 slower or quicker rates of rotation the colors disappear. The flashes also produce a 

 giddiness sometimes intense enough to cause me to grasp the table to keep myself erect. 



X I have employed flasks measuring from eight inches to three-fourths of an inch in 

 diameter. The smallest flask, which had a stem with a bore of about one eighth of an inch 

 in diameter, yielded better effects than the largest. Flasks from two to three inches in 

 diameter yield good results. Ordinary test-tubes also answer well. 



