502 Prof. Tyndall on the Action of Free Molecules on 



It was obvious, however, that the arrangement of Prof. Bell 

 — a truly beautiful one — was not suited to bring out the 

 maximum effect. He had employed a series of glass lenses 

 to concentrate his beam; and these, however pure, would, in 

 the case of transparent gases, absorb a large portion of the 

 rays most influential in producing sound. This may be illus- 

 trated by comparing a rock-salt lens, in my collection, with a 

 glass lens of the same focal length prepared in the workshop 

 of M. Duboscq. Transmitted through the former, the radia- 

 tion from an incandescent platinum spiral produced a galva- 

 nometric deflection of 55°, possessing, according to the table 

 of calibration, a value of more than 100. Transmitted through 

 the latter, the deflection fell to 10°, or to less than ^ of 

 the radiation transmitted by the rock-salt. The -f^ here 

 shown to be intercepted by the transparent glass, consist of 

 heat on which transparent gases and vapours would exert a 

 specially absorbent power. Hence the desirability of main- 

 taining this important factor, in the radiation employed to test 

 the sonorous power of such substances. 



It was with the view of preserving intact these powerful 

 calorific rays that I employed in my experiments on calores- 

 cence * small concave mirrors silvered in front ; and to these 

 mirrors I now resorted. My more intense sources of heat 

 comprised a Siemens lamp connected with a dynamo-ma- 

 chine ; an ordinary electric lamp connected with a voltaic 

 battery; and a lime-light, produced sometimes by the com- 

 bustion of oxygen and hydrogen, and sometimes by oxygen 

 and coal-gas. The lime-light (which was used by me in 

 1859) is so handy, steady, and otherwise effective, that I have 

 applied it almost exclusively throughout this part of the in- 

 quiry. Sources of heat, however, of much lower temperature 

 than the lime-light have proved competent to evoke musical 

 sounds. A candle-flame, a red-hot coal, a red-hot poker, the 

 same poker at the temperature of boiling water, and an incan- 

 descent platinum spiral have all been proved effective, though 

 of course far less so than the concentrated lime-light f. 



To produce the required intermittence, I first employed a 

 circle of sheet zinc, 16 inches in diameter, provided with radial 

 slits. This was afterwards exchanged for a second disk of the 



vapour of sulphuric ether (a good absorbent of heat), and another with the 

 vapour of bisulphide of carbon (a poor absorbent) ; and he predicted that, 

 if any sound was heard, it would be louder in the former case than in the 

 latter. The experiment was immediately made ; and the result -verified 

 the prediction." 



• Philosophical Transactions, 1866, vol. clvi. p. 1. 



t These earlier experiments will be found recorded in the Proceedings 

 of the Koyal Society, vol. xxxi. pp. 307, 478. 



