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longitudinal, in the production of which 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 
