of the Sun made m Virginia. 83 



trogen : the uniform appearance of this latter body was sub- 

 sequently traced to the leaves employed. 



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. 1 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 sul- 

 phate 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 certain 

 size were always green, but those which had grown in the 

 dark had yellow leaves and white stalks. 



Professor Silliman states, in one of the early numbers o* 

 his Journal, that he witnessed an explosion of hydrogen and 

 chlorine, caused by the light of a common fire. 



4. Ritter was the first who asserted, that the opposite ex- 

 tremities of the spectrum possess opposite powers of chemi- 

 cal 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 cir- 

 cumstances, but could not make it succeed. 



5. 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 to pro- 

 duce their polarization, for want of proper apparatus. An 

 electric current circulating in a wire does not seem to have 

 any influence on these chemical rays ; I found that the same 

 neat magnified image of the wire was obtained, on chloride 

 paper, when it was placed in a beam diverging from a lens, 

 whether the current was made to pass or was stopped. 



So much for chemical actions ; let me now ask your attention 

 to a mechanical result of solar light, which is very curious. 



{a). Having made a large air-pump jar very clean and 

 dry, place a few pieces of camphor on the plate of the pump, 

 and exhaust. Carry the pump with its receiver into the sun- 

 shine, and very soon you will see all that side which is nearest 

 the sun covered with crystals, but there will be few or none 

 on the side which is furthest from him. With the brilliant 

 sun of Virginia, I have seen this effect take place, and beauti- 

 ful stellated crystals appear in four rninutes, literally covering 

 the whole of the upper parts of the jar nearest the sun. 



{b). Or, make in a tube of half an inch or more in diame- 

 ter, and upwards of thirty inches long, a torricellian vacuum ; 

 pass up through the mercury a fragment of camphor. The 



G 2 



