THESE PHENOMENA PROBABLY DUE TO RADIANT HEAT. 135 



to produce their polarization for want of proper apparatus. An electric current circu- 

 lating 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. 



521. 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 sunshine, and very soon you will see all that side which is nearest to the sun cov- 

 ered with crystals, but there will be few or none on the side which is farthest from him. 

 With the brilliant sun of Virginia I have seen this effect take place, and beautiful stel- 

 lated crystals appear in four minutes, literally covering the whole of the upper parts 

 of the jar nearest the sun. 



(Z.) Or make a tube of half an inch or more in diameter, and upward of thirty 

 inches long, a torricellian vacuum ; pass up through the mercury a fragment of cam- 

 phor. The 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 tin foil an inch in diam- 

 eter, and having operated as in experiment (), expose this side towards the sun. 

 Crystals will soon form, but the tin foil will protect the glass in its vicinity, and none 

 will be found within a certain space round the metallic circle. 



(J.) Crystallization is not necessarily connected with these results : the vapour of 

 mercury in a torricellian void is condensed 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 to the light. 



Artificial light gives none of these results. 



(e.) Having removed the piece of tin foil 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. 



(/) Take a jar that has already been coated with crystals, place the tin foil before 

 it, and it will remove all those crystals which are within its shadow. 



(g.) Instead of using a piece of tin foil, 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. 



(A.) If along the inside surface of a vessel about to be exposed 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 experiment to succeed. 



522. 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, while radiation is interfered with at the other side ; that, in point of fact, 

 the anterior side is the colder, and the other the hotter ? 



