516 



Mr. W. H. Preece. On the Conversion of [Mar. 10,. 



40. Zinc disk (0*7 millim. thick) blacked on one side. 



a. Black ontside, sounds = 10. 



b. ,, inside, ,, = 3. 



It tlius appears that transparent bodies behave in an opposite way 

 to opaque bodies. Grlass and mica can be rendered athermanous and 

 silent by making the carbon deposit sufficiently thick. Zinc, copper, 

 and ebonite can produce sonorous effects by a proper disposition of 

 carbon. The effect in these latter cases may be due either to molecular 

 pressure, in fact, to a radiometer effect, though very feeble in intensity ; 

 or it may be the result of conduction through the mass of the dia- 

 phragm, that is, radiant heat is reduced to thermometric heat bv 

 absorption by the carbon deposit on the outside of the disk, this heat 

 is transmitted through the disk and radiated to the absorbent gases in 

 the interior. 



41. To test these questions, a delicate microphone was fixed on 

 a zinc disk (0*7 millim. thick) without any air-cavity in front of or 

 behind it. 



a. The disk was blackened ; no measurable trace of sound was 

 perceptible. 



b. The disk was cleaned, with precisely the same result. 



42. The zinc disk was now replaced on the case, fig. 5, with the air- 

 cavity, and sounds =10 were emitted as before. 



a. The zinc was neatly covered with white paper — a highly 

 athermanous substance; the sounds were nil. 



b. The paper was blackened ; the sounds were still nil. 



c. The zinc was covered with clear mica ; faint but unmeasur- 

 able sounds were heard. 



d. The mica was blackened ; no perceptible sounds were 

 obtained. 



e. The zinc disk was blackened slightly in the usual way; 

 sounds = 10 were obtained. 



f. The zinc was thickly blackened ; all sounds disappeared. 



43. Ebonite disks of different thicknesses were used, and layers of 

 carbon of different thicknesses were deposited on the thinnest of them. 

 The sounds became fainter and fainter as the thickness of the layer of 

 carbon and the thickness of the ebonite increased, until they dis- 

 appeared altogether. 



These experiments fully establish the inference that the effect is one 

 of conduction, and that the blackened surface of an opaque body like 

 zinc acts as though the source of heat were transferred to the outside 

 surface of the disk. 



44. Tubes of various sizes and dimensions were now tried, to con- 

 firm Messrs. Bell and Tainter's observations on tubes. They invariably 

 gave out satisfactory sounds when the intermittent rays were directed 



