1881.] 



Radiant Energy into Sonorous Vibrations. 



511 



3. * A similar ebonite disk equally well whitened, gave slightly less 

 intense sounds estimated at 18. 



4. A zinc disk blackened gave sounds =8. 



5. A similar disk polished gave only sounds =2. 



6. Arnica disk blackened gave scarcely any sounds at all. 



7. A clean mica disk gave sounds =5. 



These effects were produced many times, and on different occasions, 

 and they were so unsatisfactory as to throw doubts on the accuracy of 

 the radiometer explanation. They were not so decided as theory led 

 one to anticipate. The effects produced by the zinc disk, though 

 very weak, favoured the theory ; those given by the mica disk com- 

 pletely refuted it ; while those given by the ebonite disks were almost 

 of a neutral character. 



It was, however, thought that if D be the disk (fig. 4), and C the 



source of light, then if the excursions of the disks to and fro were due to 

 expansion from the absorption of heat, it would first bulge towards A, 

 since the side towards the source of light would expand first. If, on 

 the other hand, it were due to the radiometer effect, it would first 

 bulge towards B. 



8. An extremely delicate electrical contact arrangement was con- 

 structed to determine this by means of a telephone, which recorded 

 the excursions to and fro of the disk ; but the result was sometimes in 

 one direction, and sometimes in the other. Moreover, the effect was 

 slow, and we failed to obtain more than five distinct vibrations per 

 second. 



This result raised the question whether in Bell and Tainter's ex- j 

 periments the disks vibrated at all. 



intensity of the two sounds, i.e., of the sonometer and the source investigated, and 

 the yalue of the latter is given in terms of the scale of the former. 

 * The consecutive figures indicate the number of the experiments. 



