SECT. XXIV.] 



PHOTOGKAPHIC PHENOMENA. 



237 



chemical rays to pass freely. Thus, light and photographic 

 energy may be regarded as distinct and independent pro- 

 perties of the solar beam. 



It is not known whether photographic energy be absorbed 

 by material substances or not, neither is it known whether 

 it be concerned in crystallization, and in producing those 

 changes in the internal structure of crystals when exposed to 

 the sun, already mentioned; but the power is universal 

 wherever the solar beam falls, though the effect only becomes 

 evident in cases of unstable molecular equilibrium. The com- 

 position and decomposition of those solids, liquids, and aeri- 

 form fluids, hitherto attributed to light, are chiefly owing to 

 this energy ; and, as similar chemical changes may be pro- 

 duced by currents of electricity, an occult connexion between 

 these two imponderable influences is shadowed out. 



