SKUT. XXIV. PHOTOGRAPHIC PHENOMENA. ',205 



abundantly through yellow glass, or a solution of chro- 

 mate of potash ; but the greater part of the chemical 

 rays are excluded, and chlorine gas diluted with common 

 air, though highly pervious to the luminous and calorific 

 principles, has the same effect. Sir John Herschel 

 found that a slight degree of yellow London fog had a 

 similar effect with that of pale yellow media : he also 

 remarked that a weak solution of azolitmine in potash, 

 which admits a great quantity of green light, excludes 

 chemical action ; and some years ago, the author, while 

 making experiments on the transmission of chemical 

 rays, observed that green glass, colored byoxyde of cop- 

 per, about- the 20th of an inck thick, excludes the pho- 

 tographic rays, and as M. Melloni has shown that sub- 

 stance to be impervious to the most refrangible calorific 

 rays, it has the property of excluding the whole of the 

 most refrangible part of the solar spectrum, visible and 

 invisible. Green mica, if not too thin, has also the same 

 effect, whereas amethyst, deep blue and violet-colored 

 glasses, though they transmit a very little light, allow 

 the chemical rays to pass freely. Thus light and pho- 

 tographic energy may be regarded as distinct and inde- 

 pendent properties of the solar beam. 



It is not known whether photographic energy be ab- 

 sorbed by material substances or not, neither is it known 

 whether it be concerned in crystalization, and in pro- 

 ducing those changes in the internal structure of cfystals 

 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 mo- 

 lecular equilibrium. The composition and decomposi- 

 tion of those solids, liquids, and ae'riform fluids hitherto 

 attributed to light, are chiefly owing to this energy ; and 

 as similar chemical changes may be produced by cur- 

 rents of electricity, an occult connection between these 

 two imponderable influences is shadowed out, 

 8 



