402 Correspondence. 
common dynamical origin, a much closer analogy may 
nevertheless be traced through the phenomena of phospho- 
rescence, fluorescence, and calorescence. Is it, in fact, 
generally believed that the transmission of heat-motion is 
effected by interstitial ather, and not by the molecules of 
the medium itself? If not, why should a hypothetical 
medium be assumed for light motion which is not required 
for that of heat, since the rapidity of the undulations and 
their velocity of transmission is so much the same in both? 
While at the same time the converse permeabilities to light 
and heat of crystals of alum, and those of dark, smoky 
quartz, present striking examples of the existing, yet un- 
known, differences of physical constitution which are met 
with in the various kinds of matter. 
It may further be remarked that the dynamical theory 
of electricity, if tenable, presents additional reasons for 
denying the necessity of the presence of zther in ordinary 
matter. For if the molecules of a rod of copper can 
transmit an electric wave at the rate of at least 250,000 
miles in a second, why cannot those of a rod of glass 
transmit a course of light at about three-fourths of that 
velocity ? 
If, then, these premises be established, it must be granted 
that the presence of zther is not essential to the trans- 
mission of any known kind of wave-motion; and if so, is 
it not more in accordance with the true spirit of philosophy, 
in default of posittve knowledge, to abandon unhesitatingly 
the more violent hypothesis, and to adopt provisionally 
the more reasonable one above suggested ? 
The phenomena of phosphorescence present further 
evidence of the intimate relations existing between elec- 
tricity, heat, and light. It has been observed that fluor- 
spar may be rendered phosphorescent by a very moderate 
application of heat, but that it will not again phosphoresce 
under similar circumstances until an electric spark has been 
repeatedly passed over its surface. Is it not probable that 
the phosphorescence by heat of the minerals fluor and 
apatite and ordinary incandescence are similar phenomena, 
differing only in the temperature (amount of vis viva) at 
which heat-motion impressed on the molecules of different 
bodies is imparted as light-motion to the surrounding 
medium, 
~ Our ideas of vzs viva and its relation to “accumulated 
work” are inseparably associated with that of gravitation ; 
is it not, therefore, something like a contradiction in terms — 
