134 PERIHELION MOTIONS. 



to a beam which had passed through the red sulpho-cyanate of iron, the paper became 

 of a brick-red colour ; if to a beam which had passed through a solution of sulphate of 

 copper and ammonia, it became of a blue-brown ; and, lastly, on exposing a piece in a 

 box, which I shall presently mention, for five days, to light which had been acted on by 

 bichromate of potassa, it became perceptibly of a faint yellowish green. 



513. It is very probable that there exist in the sunlight, rays having particular chem- 

 ical powers. 



514. A beam which has passed through bichromate of potassa does not appear to 

 cause the union of a mixture of chlorine and hydrogen. I kept such a mixture for 

 several hours in it, and could not perceive any change. 



515. But this same beam can, nevertheless, enable vegetable leaves to effect the de- 

 composition of carbonic acid. I took a wooden box, about a cubic foot in dimensions, 

 and having removed its bottom, replaced it with a pair of parallel plates of glass, so ad- 

 justed that there was an interstice between them of half an inch, or thereabout. Into 

 the trough thus formed I poured a solution of bichromate of potassa, or any other salt 

 under trial, and the box being raised on one end, served as a closet in which bodies 

 could be exposed to the action of beams that had passed through any given medium. 



516. In this little chamber, its trough being filled with a solution of the bichromate, 

 I placed a matrass containing water slightly impregnated with carbonic acid, and a 

 few vegetable leaves; after a little while, air bubbles were copiously given off; there 

 had been placed, similar in all respects, another matrass in the direct rays of the sun, 

 and when a quantity of gas sufficient for analysis was evolved, it was found that car- 

 bonic acid had in both cases been decomposed, though, as might have been expected, 

 in the latter more energetically. The result gave a mixture of carbonic acid, oxygen, 

 and nitrogen : the uniform appearance of this latter body was subsequently traced to 

 the leaves employed. 



517. Plants also become green in light that has been submitted to the action of these 

 yellow salts, and, therefore, deprived of the rays that blacken chloride of silver. I took a 

 number of pea plants out of the garden, in May, 1837, and caused them to vegetate in 

 light modified in this way, and also in light which had passed through sulpho-cyanate 

 of iron, and sulphate of copper and ammonia, &c., but in every instance the leaves 

 became green. It may also be mentioned that seeds of common cress were caused to 

 germinate and grow under these circumstances ; the young plants, after reaching a cer- 

 tain size, were always green, but those which had grown in the dark had yellow leaves 

 and white stalks. 



518. Professor SILLIMAN states, in one of the early numbers of his Journal, that he 

 witnessed an explosion of hydrogen and chlorine caused by the light of a common fire. 



519. RITTER was the first who asserted that the opposite extremities of the spectrum 

 possess opposite powers of chemical action ; he states that phosphorus will emit fumes 

 in the red ray, but if the violet be thrown on it, it ceases to smoke ; this experiment I 

 repeated often, and under favourable circumstances, but could not make it succeed. 



520. I could succeed, however, in showing very beautifully the interference of that 

 class of chemical rays which blacken chloride and bromide of silver, but failed in trying 



