SOURCE OF ORGANIC ENERGY. 31 



is from Professor W. R. Grove's essay on the 

 Correlation of Physical Forces, relating experiments 

 of M. Niepce de St. Victor : — 



"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 opposite, 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 reproduced on the photo- 

 graphic paper, while the part which has been cov- 

 ered by the screen is not produced. If the engrav- 

 ing, 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 

 image of the exposed portion of the engraving is re- 

 produced. Similiar results are produced by mottled 

 marble exposed to the sun ; an invisible tracing on 

 paper by a fluorescent body, sulphate of quinine, is, 

 after insolation, reproduced on the photographic 

 paper. Insolated paper retains the power of pro- 

 ducing an impression for a long period, if it is 

 kept in an opaque tube hermetically closed." 

 The following is from the same source : — 

 " As with heat, light, and electricity, the daily 

 accumulating observations tend to show that each 



