232 



longitudinal, in the production of vvliicli repulsive forces are 

 alone concerned ; whilst^ on the contrary, light and heat vibra- 

 tions are necessarily transverse, and the production of these is 

 solely due to attractive forces. Now, these respective forces 

 obey very different laws, for whilst attractive forces obey gene- 

 rally, and probably universally, the law of the inverse square of 

 the distance between the attracting particles, molecular repul- 

 sion must obviously — at all events, in gaseous matter — obey the 

 law of the inverse cube of the distance, as a corollary to Boyle^s 

 law of the constant ratio (within wide limits) of gaseous pressure 

 to density ; therefore, from the rates of transmission of longi- 

 tudinal vibrations, nothing can be predicated respecting the 

 possible rates of transmission of transverse waves. It has been 

 asserted that molecular repulsion is a dynamic resultant effect 

 of molecular vibration, and therefore incapable of expression 

 by a statical law ; but it is very doubtful whether molecular 

 attraction is not equally a dynamic sequence, and therefore 

 not a whit more entitled to claim a statical law than the 

 former. This view may be illustrated by an experiment, 

 in which a disc of card at the end of a light suspended rod, 

 and placed near a tuning-fork, is attracted or drawn towards 

 the latter, when thrown into vibration by means of a violin- 

 bow. 



40. Sir C. Wheatstone has long since shown that electricity 

 traverses a copper wire at a velocity not less than that of light. 

 Whether electricity be matter or motion, this result shows that 

 the capability of matter to transmit the vibrations of light is by 

 no means improbable. Moreover, it is now generally admitted 

 that when a body becomes heated, its own molecules, and not 

 merely those of the supposed interstitial ether, are thrown into 

 a state of vibratory motion, the amount of heat corresponding 

 probably to the amplitude of the vibrations. If, then, ordinary 

 matter be assumed to be susceptible of heat- vibrations, can any 

 valid reason be assigned for its insusceptibility of light-vibra- 

 tions, when the close relationship, if not the absolute identity, 

 of these two forms of energy is manifested by so many pheno- 

 mena common to both, such as those of reflection and refraction, 

 polarisation, and the reciprocal properties of emission and 

 absorption, whether general or selective. 



41. The reciprocity between the powers of radiating and ab- 

 sorbing both light and heat which exists in all substances, so far 

 as experiment has shown, presents a cogent argument in favour 

 of the hypothesis that the energies of both light and heat are 

 exerted on the molecules of sensible matter, and not on any 

 supposed interstitial medium. It is a well-established fact that 

 those surfaces of bodies which radiate heat most freely also 



