Royal Institution. 225 



which remained untouched was, however, of sufficient interest to 

 induce him to ])rosecute his original intention. 



The general laws of Professor Forbes were submitted in succession 

 to an experimental examination. The first of these laws affirms 

 that " the vibrations never take place between substances of the same 

 nature." This the lecturer found to be generally the case when the 

 hot rocker rested upon a block, or on the edge of a thick plate of the 

 same metal ; but the case was quite altered when a thin plate of 

 metal was used. Thus, a copper rocker laid upon the edge of a 

 penny-piece did not vibrate permanently ; but when the coin was 

 beaten out by a hammer, so as to present a thin, sharp edge, con- 

 stant vibrations were obtained. A silver rocker resting on the edge 

 of a half-crown refused to vibrate permanently, but on the edge of 

 a sixpence continuous vibrations were obtained. An iron rocker on 

 the edge of a dinner knife gave continuous vibrations. A flat brass 

 rocker placed upon the points of two common brass pins, and having 

 its handle suitably supported, gave distinct vibrations. In these 

 experiments the plates and pins were fixed in a vice, and it was 

 found that the thinner the plate within its limits of rigidity, the 

 more certain and striking was the effect. Vibrations were thus 

 obtained with iron on iron, copper on copper, brass on brass, zinc on 

 zinc, silver on silver, tin on tin. The list might be extended, but 

 the cases cited are sufficient to show that the proposition above cited 

 cannot be regarded as expressing a " general law." 



The second general law enunciated by Professor Forbes is, that 

 " both substances must be metallic." This is the law which first 

 attracted the lecturer's attention. During the progress of a kindred 

 inquiry, he had discovered that certain non-metallic bodies are en- 

 dowed with powers of conduction far higher than has been hitherto 

 supposed ; and the thought occurred to him that such bodies might, 

 by suitable treatment, be made to supply the place of metals in the 

 production of vibrations. This anticipation was realized. Rockers 

 of silver, copper and brass, placed upon the natural edge of a prism 

 of rock-crystal, gave distinct tones ; on the clean edge of a cube of 

 fluor-spar the tones were still more musical ; on a mass of rock-salt 

 the vibrations were very forcible. There is scarcely a substance, 

 metallic or non-metallic, on which vibrations can be obtained with 

 greater ease and certainty than on rock-salt. In most cases a high 

 temperature is necessary to the production of the tones, but in the 

 case of rock-salt the temperature need not exceed that of the blood. 

 A new and singular property is thus found to belong to this already 

 remarkable substance. It is needless to enter into a full statement 

 regarding the various minerals submitted to experiment. Upwards 

 of twenty non-metallic substances had been examined by the lecturer, 

 and distinct vibrations obtained with every one of them. 



The number of exceptions here exhibited far exceeds that of the 

 substances which are mentioned in the paper of Professor Forbes, 

 and are, it was imagined, sufficient to show that the second general 

 law is untenable. 



The third general law states, that " The vibrations take place 



