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



and Tainter's experiments must be diathermanous, and the better their 

 character in this respect the greater the effect ; remove them, and the 

 effect is greater still. Messrs. Bell and Tainter* showed that the 

 sounds maintained their timbre and pitch notwithstanding variation 

 in the substance of the disk, and M. Mercadier found that a split or 

 cracked plate acted as well as when it was whole. These facts are- 

 consistent with the expansion of the contained air, but not with any 

 mechanical disturbance of the disks. Moreover, M. Mercadier showed 

 that the effect was improved by lampblack, but he applied it in the 

 wrong place. 



The disks may, and perhaps do under certain conditions, vibrate, 

 but this vibration is feeble and quite a secondary action. The sides of 

 the containing vessel must possess the power to reduce the incident 

 rays to thermometric heat, and impart it to the vapour they confine, 

 and the more their power in this respect, as when blackened by 

 carbon, the greater the effect. The back of the disk may alone act 

 in this respect. Cigars, chips of wood, smoke, or any absorbent sur- 

 faces placed inside a closed transparent vessel will, by first absorb- 

 ing and then radiating heat rays to the confined gas, emit sonorous 

 vibrations. 



The heat is dissipated in the energy of sonorous vibrations. In all 

 cases, time enters as an element, and the maximum effect depends on 



* " Journal of Society of Telegraph Engineers," December 8, 1880. 



