239 
manent, as that produced on the salts of gold and silver in 
ordinary photography, but generally as transient as the ray 
which produces it ; and that this chemical action is resolved into 
electric energy, which is transmitted by the optic nerve to the 
brain. That the duration of the impression on the retina is 
proportional to its intensity, any one may convince himself by 
looking at a bright light, and then closing the eyes, when a 
bright image will for a longer or shorter period, according to 
the intensity of the light, remain visible. 
55. The principle of the dissipation of energy, as a corollary 
to that of its conservation, has of course been equally ignored ; 
but it must here suffice to give a familiar illustration, both of 
the conservation and the dissipation of energy, in the action of 
the rifle-ball. This reaches the target with less velocity, and 
consequently with less energy, than it possessed on leaving the 
muzzle ; a portion of its energy has been expended in producing 
heat by friction against the particles of air between which it 
passes, which is dispersed through the surrounding atmosphere, 
and thus becomes dissipated. On reaching the target the pro- 
gressive motion of the mass is arrested, and converted into 
molecular motion, which is cognisable only as heat, by which 
the mass is reduced to the fluid state, and splashes of molten 
metal are scattered in all directions. These again impart their 
heat partly to the air through which they pass, partly by radia- 
tion into space, and partly to the ground on which they fall ; 
and thus the whole energy of the ball becomes dissipated. An 
analogous explanation will apply to all other cases of the dissi- 
pation of energy. 
56. In the two essays above-mentioned, the objections of their 
authors to the validity of the correlation and conservation of 
energy appear to the writer to lie, not against any observed 
facts, or their mutual relations, but exclusively against the vague 
or illogical terms in which the interpretation of them has 
hitherto been expressed by physicists. Mr. Moore having, in 
consequence of a published remonstrance, withdrawn his unfair 
criticism of the writers explanation of “ latent ” heat,* he is 
glad to embrace the present opportunity of withdrawing with 
equal publicity, any imputation he may have made against Mr. 
Moore's literary candour ; the publication of a short letter con- 
taining that withdrawal having been declined by the journal in 
which the remonstrance was published. At the same time it 
cannot be denied that this writer has grossly misrepresented 
the course of philosophic thought pursued in regard to many 
problems in physics, especially those relating to the transmission 
and transformation of energy. 
* Elements of Natural Philosophy , sixth edition, p. 786. 
