214 PHOTOGRAPHIC PHENOMENA. SECT. XXIV. 



obscure part beyond the violet ; but the whole number of those 

 inactive lines, especially in the dark spaces, is so great that it is 

 impossible to count them. 



Notwithstanding this coincidence in the inactive lines of the 

 two spectra, photographic energy is independent of both light and 

 heat, since it exerts the most powerful influence in those rays 

 where they are least, and also in spaces where neither sensibly 

 exist ; but the transmission of the sun's light through coloured 

 media makes that independence quite evident. Heat and light 

 pass abundantly through yellow glass, or a solution of chromate 

 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 re- 

 marked 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, coloured 

 \)y oxyde of copper about the 20th of an inch thick, excludes the 

 photographic rays ; and, as M. Melloni has shown that substance 

 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-coloured glasses, though they transmit a very little light, 

 allow the chemical rays to pass freely. Thus light and photo- 

 graphic energy may be regarded as distinct parts of the solar 

 beam, and both being propagated by vibrations of the etherial 

 medium they are merely motion. Excellent images have been 

 obtained of the moon in its different phases by Professor Secchi, 

 at Rome ; candlelight is nearly deficient of the chemical rays. 

 How far they may influence crystallization and other molecular 

 arrangements is unknown, but their power is universal wherever 

 the solar beam falls, although their effect only becomes evident 

 in cases of unstable molecular equilibrium. 



It is not by vision alone that a knowledge of the sun's rays is 

 acquired : touch proves that they have the power of raising the 

 temperature of substances exposed to their action. Sir Williani 

 Herschel discovered that rays which produce the sensation of heat 



