446 



ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



(I 



Fig. 94. 



In the focus of blue light thus formed there was ex- 

 posed for two and a half 

 hours (from 7| to 10J A.M., 

 June 13, 1848) an inverted 

 half- inch bulb containing 

 iodine water, with a few 

 particles of iodine. Tem- 

 perature in the shade, 64 ; 

 in the sun, 86. At the 

 end of that time there was 

 found an insignificant bub- 

 ble of air, estimated at one 

 thirtieth of an inch in diameter. It could, of course, be 

 nothing but atmospheric air.- 



The absorbing medium was now removed, and the 

 full rays of the sun permitted to converge on the mat- 

 rass. The* temperature of the water quickly ran up to 

 the boiling-point, and the bulb was filled with steam 

 and the purple vapor of iodine. Everything seemed 

 favorable for the decomposition of the water to take 

 place, if the iodine could accomplish it under so intense 

 a radiation. At first I thought that the experiment had 

 succeeded, for the color of the bulb became paler a 

 result that ought to have ensued if hydriodic acid 

 was forming and oxygen being eliminated. The action, 

 therefore, was kept up for four hours ; but as soon as 

 the sun was screened from the lens and the bulb began 

 to cool, the water returned and filled it almost entirely. 

 This, therefore, shows that under a most intense radia- 

 tion iodine cannot decompose water. 



A similar experiment was tried with bromine, and 

 with the same result. It failed to decompose water. 



Some silver chloride, carefully purified, was exposed 

 in a little crucible of platinum foil (Fig. 95) so inclined 

 that the cone of rays could come in at its mouth. The 



