MEMOIR XIII.] EXPERIMENTS MADE IN VIRGINIA. 



203 



placed in a beam diverging from a lens, whether the 

 current was made to pass or was stopped. 



NOTE. These were instances of the application of 

 photography to the solution of physical problems long 

 before the announcement of the discoveries of Daguerre 

 or Talbot. 



The following are some mechanical results of solar 

 light: 



(a.) Having made a large air-pump jar clean and dry, 

 a few pieces of camphor were placed on the plate of the 

 pump, and the jar exhausted. The pump and its re- 

 ceiver were then set in the sunshine, and very soon all 

 that side towards the sun was covered with crystals ; but 

 few or none were on the farther side. 



(&.) A torricellian vacuum having been made in a 

 tube half an inch or more in diameter, and upwards of 

 thirty inches long, a fragment of camphor was passed up 

 through the mercury into it. The tube might be kept 

 for any length of time in the dark without anything 

 happening, but on bringing it into the beams of the sun 

 crystallization in a few minutes took place on that side 

 of the tube next the luminary. 



(tf.) On the inside of an air-pump 

 jar a circle of tinfoil an inch in diam- 

 eter was pasted ; and having oper- 

 ated as in experiment (a), that side 

 was exposed to the sun. Crystals 

 soon formed, but the tinfoil protected 

 the glass in its vicinity, and none 

 were found within a certain space 

 round the metallic circle. Fig. 32. Fig. 32. 



