MODERN SCIENCE. 227 



monstrate, also by experiments (1798), that HEAT is a 

 KIND OF MOTION, and not a substance as it had been 

 surmised even after Lavoisier's complete and satisfactory 

 theory of combustion ; and Rumford showed also that heat 

 can be produced by friction, actually boiling-water by heat 

 so produced. By his laborious experiments and results he 

 brought the attention of scientific men to heat a power 

 relatively neglected until then. 



1756 1827. Chladni, who may be called the precursor of 

 Helmholtz as regards at least a certain series of the latter 

 physicist's researches, guided by the investigations of Sauveur, 

 Berriouilli, Stancari, and Euler, on musical vibrations, dis- 

 covered the longitudinal vibrations of strings and the 

 MUSICAL VIBRATIONS OF SOLID BODIES, going in that 

 direction much further than Galileo and those just mentioned. 

 His experiments, which are fully described in his Acoustic 

 Figures and his Die Akustik, are as instructive as they 

 are interesting, showing as they do, numerous phenomena of 

 sound unsuspected till then. He invented the clavicylinder 

 and the euphone two instruments fashionable for a time 

 the latter especially on account of its great sweetness and 

 power. Marloye's harp is a fine application of Chladni's 

 elucidation of longitudinal vibrations of rods ; musical boxes 

 are examples of musical vibrations of metallic plates ; the 

 harmonicon an example of those of glass plates. The 

 vibrations of plates are governed by the law : In plates of 

 the same kind and shape, the number of vibrations per 

 second is directly as the thickness of the plates, and in- 

 versely as their areas. In elastic rods of the same kind, 

 " the number of longitudinal vibrations is inversely as their 

 length, whatever be the diameter and form of their transverse 

 section" Helmholtz has worked out this branch with 

 incomparable fulness and skill. 



1758 1815. Nicholson DECOMPOSED WATER by accident 

 by the VOLTAIC BATTERY (1801) a discovery which Davy 

 considered as " the foundation of all that has since been done 

 in electro-chemical science." 



1766 1828. Wollaston invented the CAMERA LUCIDA, 

 and, like Malus, the reflecting GONIOMETER, for measuring 



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