the Contact of Bodies having different Temperatures, 9 



stal, as it readily flies to pieces on the sudden communication of 

 heat. With proper precautions, feeble, but well-established 

 vibrations were obtained. 

 Onyx* — A distinct tone. 



Tourmaline. — After many trials I obtained a continuous 

 vibration and low tone. 



Fossil Wood, — Two different specimens were examined, and 

 distinct tones obtained with both of them. 



.i/ Banded Agate, — Strong and continuous vibrations, when the 



"rocker was very hot. 



Chalcedony. — Loud and long- continued knockings on a knob 

 of this mineral. 



Glass, — Decided vibrations on the smooth rounded edge of 

 the foot of a drink ing-glass. Mr. Trevelyan believed that he 

 once obtained vibrations upon glass, but the fact is doubted by 

 Professor Forbes. This is the only experiment on non-metallic 

 bodies, as far as I am aware of, hitherto on record. 



''-•- Earthenware, — A feeble tone, which soon ceased, was obtained 



.^^n the edge of a dinner plate. 



to Flint, — A decided tone, though not so strong as that obtained 



-i:firom rock-ciystal. 



ro^ Lydian Stone, — Permanent vibrations. 



-fi! Heliotrope. — A durable tone. 



Iceland-spar, — A lighter rocker than any of those hitherto 

 described was found necessary in experimenting with this cry- 

 stal. The mass is soft, and is readily bruised by the rocker 

 when the latter is heavy. With a suitable instrument a con- 



- tinuous feeble tone was obtained. 

 s Red Hamatite. — Distinct tones were produced by several spe- 

 '^cimens of this mineral. 



lu' Arseniacal Cobalt. — A strong tone. 



\&:i Meteoric Iron from Mexico. — A low musical tone. 



f o;^ This list might be readily extended. The substances men- 



kiioned in it were chosen on account of their accidentally pre- 

 senting the conditions favourable to experiment. The principal 

 condition is a clean even edge. Several of the minerals pos- 

 sessed such edges cut artificially ; others possessed them natu- 

 rally. In the case of chalcedony, the rocker was placed upon a 

 rounded knob; in the case of tourmaline, one of the ridges, 

 which usually run along the surface of the prism, served as a 

 support ; with glass and earthenware, the surfaces were smooth 

 and rounded. As a general rule, however, I have found an even 

 edge best. With such an edge, and rockers similar to those 

 described, no difficulty will be experienced in repeating and ex- 

 tending these experiments. 



It is usual to permit the knob at the end of the handle of the 



