RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
333 
It was obvious, however, that the arrangement of Mr. Bell —a truly beautiful one 
—was not suited to bring out the maximum effect. He had employed a series of glass 
lenses to concentrate his beam, and these, however pure, would, in the case of trans¬ 
parent gases, absorb a large portion of the rays most influential in producing sound. 
This may be illustrated by comparing a rocksalt lens, in my collection, with a glass lens 
of the same focal length prepared in the workshop of M. Duboscq. Transmitted 
through the former, the radiation from an incandescent platinum spiral produced a 
galvanometric deflection of 5 5°, possessing, according to the table of calibration, a 
value of more than 100. Transmitted through the latter, the deflection fell to 10°, or 
to less than i~ 0 -th of the radiation transmitted by the rocksalt. The iurths here shown 
to be intercepted by the transparent glass, consist of heat on which transparent gases 
and vapours would exert a specially absorbent power. Hence the desirability of 
maintaining this important factor, in the radiation employed to test the sonorous 
power of such substances. 
It was with the view of preserving intact these powerful calorific rays that I employed 
in my experiments on calorescence* small concave mirrors silvered in front; and to 
these mirrors I now resorted. My more intense sources of heat comprised a Siemens’ 
lamp connected with a dynamo machine ; an ordinary electric lamp connected with a 
voltaic battery; and a lime light, produced sometimes by the combustion of oxygen and 
hydrogen, and sometimes by oxygen and coal gas. The lime light (which was used by 
me in 1859) is so handy, steady, and otherwise effective, that I have applied it almost 
exclusively throughout this part of the inquiry. Sources of heat, however, of much 
lower temperature than the lime light, have proved competent to evoke musical sounds. 
A candle flame, a red hot coal, a red hot poker, the same poker at the temperature 
of boiling water, and an incandescent platinum spiral, have all been proved effective, 
though of course far less so than the concentrated lime light.+ 
To produce the required intermittence I first employed a circle of sheet zinc 16 
inches in diameter provided with radial slits. This was afterwards exchanged for a 
second disc of the same diameter, but furnished with circumferential teeth and inter¬ 
spaces. The disc was mounted vertically on a whirling table, and caused to rotate 
across the beam near the focus of the concave mirror. Immediately behind the disc 
was placed the flask containing the gas or vapour to be examined, while an india- 
vol. xi., p. 519. With reference to what occurred on the 29th of November he writes thns:—“Professor 
Tyndall at once expressed the opinion that the sounds were due to rapid changes of temperature in the 
body submitted to the action of the beam. Finding that no experiments had been made at that time to 
test the sonorous properties of different gases, he suggested filling one test-tube with the vapour of sul¬ 
phuric ether (a good absorbent of heat), and another with the vapour of bisulphide of carbon (a poor 
absorbent); and he predicted that, if any sound was heard, it would be louder in the former case than in 
the latter. The experiment was immediately made; and the result verified the prediction.” 
* Philosopical Transactions, 1866, Yol. 156, p. 1. 
t These earlier experiments will be found recorded in the Proceedings of the Royal Society, vol, 31, 
pp. 307, 478. 
