8 
DR. TYNDALL ON THE VIBRATIONS AND TONES PRODUCED BY 
Glass. — Decided vibrations on the smooth rounded edge of the foot of a drinking- 
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 on the edge of a 
dinner plate. 
Flint. — A decided tone, though not so strong as that obtained from rock-crystal. 
Lydian Stone. — Permanent vibrations. 
Heliotrope. — A durable tone. 
Iceland-spar. — A lighter rocker than any of those hitherto described was found 
necessary in experimenting with this crystal. The mass is soft, and is readily bruised 
by the rocker, when the latter is heavy. With a suitable instrument a continuous 
feeble tone was obtained. 
Red Hematite. — Distinct tones were produced by several specimens of this mineral. 
Arseniacal Cobalt. — A strong tone. 
Meteoric Iron from Mexico. — A low musical tone. 
This list might be readily extended^ The substances mentioned in it were chosen 
on account of their accidentally presenting the conditions favourable to experiment. 
The principal condition is a clean even edge. Several of the minerals possessed 
such edges cut artificially; others possessed them naturally. In the case of chalce- 
dony, 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 extending these 
experiments. 
It is usual to permit the knob at the end of the handle of the rocker to rest upon 
a flat surface, while the instrument itself leans slantingly against the bearer. In 
delicate experiments I think a knife-edge is a better support for the handle, the 
rocker being placed horizontal, or nearly so. 
Omitting the last three substances, which might, perhaps, with some justice be 
regarded as metallic, we find a number of exceptions to the law under consideration 
which far exceeds the number of bodies mentioned in the paper of Professor Forbes. 
These exceptions demonstrate that the second law also is untenable. 
The third general law runs as follows : — 
“ The vibrations take place with an intensity proportional ( within certain limits) to 
the difference of the conducting powers of the metals for heat , the metal having least 
conducting power being necessarily the coldest .” 
The evidence adduced against the validity of the first law appears to destroy this 
one also ; for if the vibrations are to be ascribed to a difference in the conducting 
powers of the rocker and bearer, then when there is no such difference there ought 
to be no vibrations. But we have shown, in half-a-dozen cases, that vibrations 
