84? Experiments on the Light of the Sun made in Virginia, 
tube may now be kept for any length of time in the dark, 
without anything happening; but bring it into the beams of 
the sun, and in a few minutes crystallization will happen, on 
the side next the luminary, 
(c). Again, paste on the inside of an air-pump jar, a piece 
of tinfoil an inch in diameter, and having operated as in ex- 
periment [a) expose this side towards the sun. Crystals will 
soon form, but the tinfoil will protect the glass in its vicinity, 
and none will be found within a certain space round the me- 
tallic circle. 
{d). Crystallization is not necessarily connected with these 
results : the vapour of mercury in a torricellian void is con- 
densed towards the light ; so also the dew which settles on 
the inside of a jar containing water is always on the side 
nearest the window. The rays of the sun have also the power 
of decomposing a solution of chloride of gold : the metalline 
spangles are deposited on that side of the glass which is 
nearest the light. 
Artificial light gives none of these results. 
[e) . Having removed the piece of tinfoil used in experiment 
(c), place it on a little stand in front of the receiver; it will 
hinder the crystallization taking place in the parts on which 
its shadow is cast, and also for a certain space in the vicinity. 
[f) . Take ajar that has already been coated with crystals, 
place the tinfoil before it, and it will remove all those crystals 
which are within its shadow. 
{g). Instead of using a piece of tinfoil as in experiment 
(c), make the receiver hot, and rub upon it a piece of resin, 
so as to leave a transparent circle of that substance ; expose 
to the light, and it will be found that the resin cannot protect 
the glass. 
{h). If along the inside surface of a vessel, about to be ex- 
posed to the sun, a glass rod be rubbed, rows of crystals will 
be deposited on the lines which were described by the end 
of the rod, but the vessel must be very dry for this experi- 
ment to succeed.* 
Now, can we explain these singular results on any other 
knomi principle than this; that the side of the jar nearest the 
sun radiates freely the heat that it receives, back again, 
whilst radiation is interfered with at the other side; that in 
pviint of fact the anterior side is the colder, and the other the 
hotter? Yours respectfully, 
University, New York, Nov. 28, 1839. John W. Draper. 
• 'Hiis result would appear to point to some change of tlie mechanical 
condition of the glass, affecting either the radiation from its surface, or 
that through its substance, or both. — E dit, 
