1858.] Molecular Impressions by Light and Electricity, 459 



of similar relations between the structure of bodies and their trans- 

 mission of electricity. 



The converse of this class of effects, however, forms more 

 properly the subject of this evening's communication, viz., the 

 changes in the molecular structure of matter produced by Light 

 and Electricity. The effect of light on pl«nts, on their growth 

 and colour, the bleaching effects of light on coloured bodies, the 

 phosphorescence of certain substances by insolation or exposure 

 to the sun, have long been known, and yet do not seem to have 

 awakened in the minds of the ancient natural philosophers any 

 notion of the general molecular effects of light. Leonard Euler 

 alone conceived that light may be regarded as a movement or 

 undulation of ordinary matter ; and Dr. Young, in answer, stated 

 as a most formidable objection, that if this view were correct all 

 bodies should possess the properties of solar phosphorus, or should 

 be thrown into a state of molecular vibration by the impact of light, 

 just as a resonant body is thrown into vibration by the impact of 

 sound, and thus give back to the sentient organ an effect similar to 

 that of the original impulse. 



In the last edition of his Essay on the " Correlation of Physical 

 Forces," (1855, p. 131,) Mr. Grove has made the/ollowing remarks 

 on this question : " To the main objection of Dr. Young that all bodies 

 would have the properties of solar phosphorus if light consisted in 

 the undulations of ordinary matter, it may be answered that so 

 many bodies have this property, and with so great variety in its 

 duration, that non constat all may not have it, though for a time so 

 short that the eye cannot detect its duration ; the fact of the phos- 

 phorescence by insolation of a large number of bodies is in itself 

 evidence of the matter of which they are composed being thrown 

 into a state of undulation, or at all events molecularly affected by 

 the impact of light, and is therefore an argument in support of the 

 view to which objection is taken." The above conjecture has been 

 substantially verified by the recent experiments of M. Niepce de 

 St. Victor, of which the following is a short resum4: — 



An engraving which has been for some time in the dark is 

 exposed to sunlight as to one half, the other half being covered by 

 an opaque screen : it is then taken into a dark room, the screen 

 removed, and the whole surface placed in close proximity to a sheet 

 of highly sensitive photographic paper. The portion upon which the 

 light has impinged is reproduced on the photographic paper, while 

 no effect is produced by the portion which had been screened from 

 light. White bodies produce the greatest effect, black little or 

 none, and colours intermediate effects. 



An engraving exposed as before, then placed in the dark upon 

 white paper, conveys the impression to the latter, which will in its 

 turn impress photographic paper. 



Paper, in a tin case, exposed to sunlight, then covered up by a 

 tin cover will, when opened in the dark, radiate from the aperture 



