172 CHLORINE AND HYDROGEN. 



We have evidence which appears to prove that this 

 chemical agent may be absorbed by simple bodies, and 

 that by this absorption an actual change of condition is 

 produced, in many respects analogous to those allotropic 

 changes which we have previously considered. Chlorine, 

 in its ordinary state, does not combine with hydrogen in 

 the dark. If we employ the yellow medium of chlorine 

 gas, for the purpose of analyzing the sun's rays pre- 

 viously to their falling upon some chemical compound 

 which is sensitive to actinic power, we shall find that 

 the chlorine obstructs all this actinism, and, however 

 unstable the compound, it remains unchanged. But the 

 chlorine .gas which has interrupted this wonderful agent, 

 appears to have absorbed it, and it is so far altered in its 

 constitution that it will unite with hydrogen in the 

 dark* In like manner, if, of two portions of the same 

 solution of sulphate of iron, one is kept in the .dark and 

 the other exposed to the sunshine, it will be found that 

 the solution which has been exposed will precipitate 

 .gold and silver from their combinations much more 



lay filtration, is mixed with lime-water in the dark, no precipita- 

 tion to any considerable extent takes place for a long while, 

 indeed, none whatever, though after very long standing a slight 

 flocky sediment is formed, after which the action is arrested 

 entirely. But if the mixture, either freshly made or when cleared 

 by subsidence of this sediment, is exposed to sunshine, it in- 

 stantly becomes milky, and a copious formation of a white preci- 

 pitate (or a pale yellow one, if the platinic solution be in excess) 

 takes place, which subsides quickly and is easily collected. The 

 same takes place more slowly in cloudy daylight." On the action 

 of light in determining the precipitation of Muriate of.Platinum by 

 Lime water ; being an extract from a letter from Sir John F. W. 

 Herschel, K.H., F.R.S., &c., to Dr. Daubeny. Phil. Mag. 1832. 



* On a change produced by Exposure to the Beams of the Sun, 

 in the properties of an elementary substance, by Professor Draper; 

 On the changes which bodies undergo in the dark, by Robert Hunt : 

 lieport of the Thirteenth Meeting of the British Association, vol. 

 xii, Description of the Tithonometer, an instrument for measuring 1 

 the chemical force of the Indigo-tithonic rays: by J. W. Draper, 

 M.D. Philosophical Magazine, Dec. 1843, vol. xxiii. 



