8 Dr. Tyndall on the Vibrations and Tones produced by 



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. 



The experiments were repeated with fumy quartz, and the 

 same result was obtained. 



2. Fluor-spar. — A smaller brass rocker than that last used 

 was found to answer best with this crystal. The dimensions 

 corresponding to AC, AB, fig. 1, were 3*8 in. and 1*25 in. 

 respectively ; while the depth was the same, or nearly so. This 

 rocker having been placed upon the natural edge of the crystal- 

 line cube, a clear and melodious note was instantly produced. 

 Forcible vibrations were also obtained with the larger rocker, 

 but not so clear a tone. 



The angle of the cube was cloven off so as to expose the edge 

 of the octahedron ; on this edge also vibrations were obtained. 



Fortification Agate. — Distinct vibrations and tones were ob- 

 tained with the large brass rocker. I found it sometimes con- 

 venient to increase the time of oscillation by laying a thin brass 

 bar, with small knobs at the ends, across the rocker — a mode of 

 experiment due, I believe, to Mr. Trevelyan. With this precau- 

 tion, in the case before us, the rocker continued swinging for 

 nearly half an hour ; and when it ceased, it was under the tem- 

 perature of boiling water. 



Rock-salt. — The rocker used in the foregoing experiment was 

 laid aside, and a piece of rock-salt was prepared for trial. The 

 mass was cloven so as to exhibit the surfaces of the primitive 

 cube, and was so placed that the straight line formed by the in- 

 tersection of two of the surfaces of the cube was horizontal. 

 Previous to heating the rocker, I laid it, according to practice, 

 upon the mass, merely to ascertain whether the arrangement 

 was likely to answer. To my astonishment a deep musical 

 sound commenced immediately. The temperature of the rocker 

 was at this time far below that of boiling water, and when it had 

 ended its song it was scarcely above a blood-heat. 



The heated rocker was laid upon a large boulder- shaped mass 

 of the salt ; it commenced to sing immediately. I scarcely know 

 a substance, metallic or non-metallic, with which vibrations can 

 be obtained with greater ease and certainty than with this mine- 

 ral. To the remarkable properties which the researches of Mel- 

 loni have shown to belong to rock-salt, a new one may now be 

 added. • 



Aventurine. — I was tempted to try this mineral from having 

 met a piece of it possessing a clean sharp edge. The large 

 brass rocker, placed hot upon this edge, gave a decided tone. 



Sulphate of Potash, — Care is required with this artificial cry- 



