84< Experiments On the Light of the Sun 7nade 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 a jar 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 

 known 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 

 point of fact the anterior side is the colder, and the other the 

 hotter? Yours respectfully, 



University, New York, Nov. 28, 1839. JoHN W. DRAPER. 



* This result would appear to point to some change of the mechanical 

 condition of the glass, affecting either the radiation from its surface, or 

 that through its substance, or both, — Edit. 



