374 Prof. Draper on the Chemical Action of Light. 



access to the water ; for on submitting the same specimeil to 

 another exposure in the focus for three hours^ after the gas had 

 been decanted from it^ a little bubble, the diameter of which was 

 estimated at y'^th of an inch, was all that could be procured. 



In like maimer I endeavoured to decompose water by iodine, 

 and with the same negative result, even when the exposure to 

 the focal point lasted for four hours. When proper care had 

 been taken to remove from the solution all traces of air, no gas 

 was evolved. 



To reduce the heating effect of the lens, and allow the more 

 refrangible rays to act, there was interposed between the lens 

 and its focus a stratum of a solution of sulphate of copper and 

 ammonia, one-third of an inch thick, and included between two 

 flat plates of glass suitably arranged, and carried along with the 

 other parts by the movement of the clock. The cone of solar 

 rays now passed through this absorbing medium. 



In the focus of blue light thus formed, there was exposed for 

 2i hours (from 7| to 10| a.m., June 13, 1848) an inverted half- 

 inch bulb, containing iodine water, with a few particles of iodine. 

 Temperature in the shade 64°, in the sun 86°. At the end of 

 that time there was found an insignificant bubble of air, estimated 

 at 3\jth of an inch in diameter. It could of course be nothing 

 but atmospheric aii'. 



The absorbing medium was now removed, and the full rays of 

 the sun permitted to converge on the matrass. The tempera- 

 ture of the water quickly ran up to the boiling-point, and the 

 bulb was filled with steam and the purple vapour of iodine. 

 Everything seemed favourable for the decomposition of the water 

 to take place, if the iodine could accomplish it under so intense 

 a radiation. x\t first I thought that the experiment had suc- 

 ceeded ; for the colour of the bulb became paler, a result that 

 ought to have ensued if hydriodic acid was forming, and oxygen 

 being eliminated. The action was therefore kept up for four 

 hom-s ; 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, proving that it was nothing but steam that had filled 

 it, the iodine having been expelled into the air by the high tem- 

 perature. When cvei'ything was quite cold, an insignificant 

 bubble of gas remained. 



This, therefore, shows that, under a most intense luminous 

 radiation, iodine cannot decompose water. 



Some water was boiled until the air was thoroughly expelled 

 from it ; when cool, it was saturated with bromine. At lOi a.m., 

 July 6, 1848, it was exposed to the focus, and kept there till 

 1 P.M. About YHo cubic inch of air was collected. 



By gently inclining the matrass this gas was decanted, the 



