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account for the phenomena of polarisation.. But all observed 
phenomena of light are in perfect harmony with the undulatory 
theory, as now commonly accepted; and not only does this 
theory fit all previous observations, but the appearance that 
will be presented to the eye, when a ray of light is transmitted 
through any hitherto untried arrangement of transparent 
media, may safely be predicted by analysis, as in the remark- 
able case of Airy’s spirals, seen when a polarised ray is 
successively transmitted through two plates cut from a right 
and a left-handed quartz crystal. Caloric was once assumed to 
be the matter or substance of heat ; and the observed radiation 
of cold induced Black to ascribe to cold an independent material 
existence ; but the observed phenomena are completely explic- 
able on the “ theory of exchanges,” which means that every 
body radiates its own temperature, whether high or low, and 
that every surrounding body absorbs the radiations ; conse- 
quently, the radiations of a cold body will lower the heat of a 
warmer body in its vicinity, just as a cistern with two pipes of 
unequal bore will, if fed by the larger and emptied by the 
smaller, become gradually fuller, while if fed by the smaller 
and emptied by the larger, its contents will be diminished : the 
parallel, in absorbed and emitted radiations, is obvious. 
31 . Again, it was formerly taught that there were two electric 
“ fluids” — the “vitreous” and “resinous”; but these were 
subsequently merged into one, and the positive and negative 
aspects of electricity were assumed to be differences in quantity 
only, and not in kind , positively electrified bodies being in 
excess, and those negatively electrified, in defect of the normal 
quantity. Moreover, magnetic properties were supposed to be 
vested in two “fluids” — the “austral” and “boreal” — pos- 
sessing mutually attractive and self-repulsive properties. But 
all these theories are more than probably alike groundless ; they 
are, moreover, utterly inconsistent with the perpetually re- 
curring interchanges of the various presumed forms of energy ; 
for it is impossible to conceive one kind of matter to be con- 
verted into another kind, or matter to be converted into mere 
motion, and vice versa. 
32. In the vibratory motions of the atmosphere and other 
bodies, which convey to the ear the impression of sound, the vibra- 
tions are demonstrably longitudinal; that is, the vibratory motion 
of each particle is in the direction in which the wave is travel- 
ling, as in the wave the wind produces in a field of corn : in the 
vibrations of light and heat, the phenomena of polarisation 
require that they must be transverse, that is, the vibration of 
each particle must be in a plane perpendicular to the direction 
of the wave, as in the ripples on the surface of still water. 
