12 
MR. POWER ON THE ABSORPTION OF THE SOLAR RAYS, ETC. 
tions of heat, may become more or less persistent in the medium ; producing at one 
time the phenomenon of fixed chemical action, at another time that of permanently 
latent heat, at another time that less permanently latent or retarded heat, at another 
time that of coloration and absorption, at another time that of phosphogenic action. 
The remarkable phenomena lately discovered by Professor Stokes seem closely allied 
to the latter, differing however in the circumstance that they cease to exist the mo- 
ment the exciting rays are withdrawn. Guided by analogy, I am inclined to think 
that these phenomena will be found hereafter to possess some slight though insensible 
duration, while I regard all action which is really momentary as expending itself 
upon the passing rays as they emerge in the form of reflected or refracted rays. 
But all these effects, of whatever kind, I regard as due to one and the same cause, 
which can, I conceive, be no other than the expenditure or distribution of the vis viva 
originally derived from the sun, and conveyed by the ether unchanged in amount. 
4. By the term vis viva is here meant, the sum of the vibrating molecules each mul- 
tiplied by the square of its velocity, a quantity which, by the usual dynamical theories, 
is constant, when we neglect the distant attractions (whose effects must be insensible 
on the all-but-iinponderabie ether), and take into account only the mutual actions of 
the molecules upon each other, in all cases, most certainly, when the molecules after 
being put in motion return to the same places which they occupied before their motion 
commenced, and there resume their former state of rest. It is extremely probable 
that this is the case in the propagation of a solar ray through the ethereal spaces, as we 
know that it is the case when a small vibratory pulse is in the course of propagation 
along a stretched wire, as also in every case of propagated undulations in which we 
can examine a the circumstances. We may further argue that, if the particles of an 
ethereal space, originally at rest, after transmitting a state of motion from a preceding 
to a succeeding space, were not again reduced to rest, the space first mentioned will 
continue to originate fresh motions, which would be propagated in one or more direc- 
tions, after the former ivave has passed ; the ethereal space, therefore, after transmitting 
a luminous wave, would either continue for a while to be self-luminous, which is con- 
trary to all we know of light, or else to be the source of vibrations of a different nature 
to those which set it in motion ; a supposition, which is so great a departure from 
simplicity, as to be extremely improbable; it is further, as before stated, opposed to 
all the analogies presented by cases of propagated undulations in which the circum- 
stances are known. There is therefore scarcely room for a doubt that the vis viva of 
the luminous waves is transmitted through the ethereal spaces unchanged in quantity. 
There is yet another way of establishing this principle, which may be more satis- 
factory to some minds. 
Let p, p' denote the vires vivas due to a luminous wave as it spreads out spherically 
with the velocity (a), and crosses successively over the spherical surfaces irr 2 , 4 Tr' 2 
in equal times r, r' being any two distances from the origin of the light regarded 
