6 
DR. TYNDALL ON THE VIBRATIONS AND TONES PRODUCED BY 
shape, I did not think it necessary to strengthen the action by forming it into a 
regular rocker. 
The number of metals capable of this action might, I doubt not, be greatly ex- 
tended. Thus far we have obtained vibrations with 
Iron upon iron, 
Copper on copper, 
Brass on brass. 
Silver on silver. 
Zinc on zinc, 
Tin on tin, 
and these, I think, are sufficient to show that the first general law of Prof essor Forbes 
does not stand the test of experiment. 
Seebeck indeed had already proved the untenableness of this law. His method of 
experimenting has been followed in one or two of the cases above described. The 
placing of the heated rocker upon pointed wires is his idea. Rockings are very 
readily obtained in this way ; but when tones are required, the sharp edge will, I 
think, in general be found preferable.. 
The second general law of action stated by Professor Forbes is as follows : — 
“ Both substances must be metallic .” 
This is the case which first excited my attention ; for even granting the final ex- 
planation given by Professor Forbes to be the true one, the necessity of the law 
before us does not at all follow. Previous to entering upon the present subject I had 
found that rock-crystal and rock-salt possessed conducting powers not much, if at 
all, inferior to some of the metals ; and this led me to suppose that either, or both of 
these substances, might possibly be made to exhibit the action which the above law 
restricts to metals. 
My first attempts failed through want of delicacy, as first attempts generally do. 
But a little practice suggested the means of imparting to the rocker the requisite 
degree of mobility. Crystals of quartz were cut in such a manner that when the 
rocker was laid upon them a very slight force was sufficient to cause it to oscillate. 
By this means I had the satisfaction of obtaining distinct vibrations from a brass 
rocker placed upon rock-crystal. 
I refrain from entering into a more exact statement of the manner in which the 
crystals were cut ; for subsequent experience proved that there is no difficulty in ob- 
taining the effect, without any artificial preparation whatever. 
I shall now proceed to describe the results obtained with non-metallic bodies. 
1. Roch-crystal . — The brass rocker already described was heated and placed upon 
the natural edge of the prism ; the stem was supported by a knife-edge, so that the 
rocker lay nearly horizontal : a strong tone was thus obtained. 
Vibrations also followed when the rocker was laid upon the edge of the pyramid 
which caps the hexagonal prism. 
