A. G. Bell—Production of Sound by Light. 309 
Sale and Draper were soon able to corroborate the statements 
that had been made by Willoughby Smith. 
Sale * presented his researches to the Royal Society on the 
8th of May, 1873, and in the following November, Draper + 
presented his results to the Royal Irish Academy in the shape 
of a joint paper by himself and Richard J. Moss 
Draper and Moss gave in their paper an admirable summary 
of the condition of our knowledge regarding selenium at that 
time. They confirmed Hittorff’s aciscamerrts that the tempera- 
ture of minimum resistance of granular selenium was some- 
where about 210° C., and that at 217° C. (the fusing point), the 
resistance suddenly increased. They carried the temperature 
to a still higher point than Hittorff had done, and found that 
the resistance again diminished, reaching a macnn minimum 
at 250° C. 
During the course of their experiments they produced a 
variety of granular selenium not different in appearance from 
other specimens but having different electrical properties. In 
this form the resistance became greater instead of less when the 
temperature was raised. 
They also used thin plates of selenium instead of the cylin- 
drical bars formerly employed, and found great advantage from 
the increased sensitiveness of the former to light. 
Sale found upon exposing selenium to the action of the solar 
spectrum that the maximum effect was produced just at or out- 
side the extreme edge of the red end of the ey at a point 
nearly coincident with the maximum of the heat rays, thus 
rendering it uncertain whether the effect was tee to light or to 
radiant heat. 
In the winter of 1873-4 the Earl of Rosse} attempted to de- 
waar be no effect ; whereas, a thermopil sneer similar condi- 
tions gave abundant indications of a cu 
He also cut off the heat rays of ity coat tig from 
luminous bodies by the interposition of glass and alum n 
the selenium and the source of light without materially affecting 
the result; but when the thermopile was employed the greater 
portion of the heat-effect was cut off. 
* Proc. Roy. Soc., xxi, 283; see also Pogg. Ann., cl, 333; Phil. Mag., IV, xlvii, 
viii, 1 
¢ Proc. Roy. Irish Acad., II, Nov. 10th, 1873, 1, 529; see alsoa Lakapron aang or 
from Richard J. Moss to Nature, Aug. 12t th, 1835, xii, 291; being an answer 
letter from J. E. H. Gordon upon the “ Anomalous beha vior of Selenium,” wise 
lished in that journal on the 8th of July, 1875; po vol. xii, p. 1 
¢ Phil. Mag., IV, March, 1874, xlvii, 161; see, also, thle forest, TH, vii, 512. 
