Prof. A. M. Mayer's Researches in Acoustics. 517 



powerful synchronous vibrations in the molecules of compounds 

 as to shake their atoms asunder ; for we have already seen how 

 very feeble impulses sent through a medium of great tenuity 

 can, when rapidly recurring and of the proper period, produce 

 mechanical effects which at first sight appear incredible. Time 

 is required in both cases to produce an appreciable action. The 

 time required in the case of the sonorous vibrations decreases 

 as their number per second increases; and in the case of the 

 setherial vibrations we have analogous phenomena. In the 

 acoustical experiment, if the fork be much out of tune with the 

 pulses transmitted by the fibre, no motion is produced in the 

 fork ; likewise we may imagine that when the period of vibra- 

 tion of one or more of the constituent atoms of a certain mole- 

 cule is far removed from unison with any of the setherial vibra- 

 tions falling upon it, no motion or chemical decomposition will 

 ensue. 



The analogy between the two classes of phenomena is yet 

 more striking when we remember that the fork selects from the 

 composite vibratory motion which traverses the fibre only that 

 vibration which is in unison (or only slightly removed from 

 unison) with its own proper periodic motion ; so likewise the 

 molecule, or atoms of the molecule, select only those vibrations 

 from the ray which are in unison with their own atomic periods ; 

 and on the tuning of the atom depends whether the result of the 

 action of the ray will evince itself as heat, phosphorescence, che- 

 mical action, or fluorescence. 



The following experiments show that the method of analysis 

 we are now discussing surpasses in delicacy and sharpness of 

 definition any other method in which sympathetically vibrating 

 bodies are employed. As already shown, the forks select from 

 the composite vibratory motion which strikes them only those 

 simple vibrations which are in unison with their own vibratory 

 periods. This remark, however, requires some modification, 

 though the qualification necessary is less than is required when 

 other similar methods of analysis are used. In all cases of co- 

 vibration there is a certain range of pitch, above and below the 

 sound which is in unison with the existing vibration, through 

 which the covibrating body responds. The further the remove 

 from unison the weaker the response*. But in some cases a 

 slight remove of the pitch from unison will cause a great dimi- 

 nution in the intensity of the covibrations, while in others the 

 same departure from unison causes only a slight or even inap, 



* See Helmholtz's Tonempjindwngen, p. 217, for the law connecting the 

 variation of intensity of covibration with the variation of pitch in the exci- 

 ting body. 



