no CORRELATION OF PHYSICAL FORCES. 



ments which I have made on the voltaic arc, some of which 

 I have mentioned in this essay, are, to my mind, confirmatory 

 of it. 



If it be admitted that one of the so-called imponderables is 

 a mode of motion, then the fact of its being able to produce 

 the others, and be produced by them, renders it highly diffi- 

 cult to conceive some as molecular motions and others as 

 fluids or undulations of an ether. 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 some for a time so short 

 that the eye cannot detect it. M. E. Becquerel has made 

 many experiments which support this view; the fact of the 

 phosphorescence by insolation of a large number of bodies, 

 and the phenomena of fluorescence afford evidence of dense 

 matter 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 which I am advocating. 

 Dr. Young admits that the phenomena of solar phosphorus 

 appear to resemble greatly the sympathetic sounds of musical 

 instruments, which are agitated by other sounds con- 

 veyed to them through the air, and I am not aware that 

 he gives any explanation of these effects on the ethereal 

 hypothesis. 



Some curious experiments of M. Niepce de St. Victor 

 seem also to present an analogy in luminous phenomena to 

 sympathetic sounds. An engraving which has been kept for 

 some days in the dark is half-covered by an opaque screen, 

 and then exposed to the sun ; it is then removed from the 

 light, the screen taken away, and the engraving placed oppo- 

 site, and at a short distance from, photographic paper ; an 

 inverted image of that portion of the engraving which has 

 been exposed to the sun is produced on the photographic 

 paper, while the part which had been covered by the screen is 

 not reproduced. If the engraving, after exposure, is allowed 

 to remain in contact with white paper for some hours, and the 

 white paper is then placed upon photographic paper, a faint 



