MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 447 



absorbing trough was not used. Though the sun's rays 

 were not brilliant, the chloride at once melted, forming a 

 reddish -looking liquid. It was kept in that condition 

 all day. When cool it proved to be in the state of 

 horn-silver, easily cut by a knife. When the rays first 

 touched it a fume was disengaged, due probably to the 

 escape of vapor of water. It seems, therefore, that this 

 substance when perfectly dry is not decomposable by 

 sunlight, though so sensitive at common temperatures 

 when moist. 



In Fig. 95, a is the platinum crucible ; &, the place of the 

 material experimented upon, re- 

 ceiving at the focus the converg- 

 ing rays, c. 



I must refer to the original 

 Memoir for the detail of numer- 

 ous experiments on many metal- 

 lic compounds, the general re- 

 sult of these being that, no mat- 

 ter how brilliant a ray may be, it cannot carry a decom- 

 position farther than a feeble one acting for a corre- 

 sponding longer period of time could do. Compounds 

 that can resist the force of an ordinary ray cannot be 

 broken down by the intense illumination of the focal 

 point of a burning -lens. That instrument cannot do 

 what the voltaic pile has done effect decompositions 

 which had never been effected before. 



To reduce the disturbing effect of heat as far as pos- 

 sible, and give every advantage to the condensed lumi- 

 nous focus, I received the cone of rays coming from the 

 twelve -inch burning -lens on a glass globe (Fig. 96) six 

 inches in diameter, filled with water. This increased 

 the converging of the rays, and brought them more 

 quickly to a focus. Then through the neck of the globe 

 was introduced to the focus, in a matrass, spoon, or other 



