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 consequence of a published remonstrance, withdrawn his unfair 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 writer^s 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. 



