83 
of the Sun made in Virginia. 
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 minutes^ literally covering 
the whole of the upper parts of the jar nearest the sun. 
(6). 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 
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