When light produces a chemical change. it naturally loses inure or less of its original in tensity, part of its energy being transformed into chemical energy. When, therefore, light passes through a medium in which it causes mane chemi cal change, its intensity is diminished, first through purely optical absorption, and secondly through 'photochemical extinction."l'hus, for example, if light is ',asset] through a given vol ume of chlorine gas, it is partly converted into heat and its intensity is found to have diminished by it corresponding amount. If now, in a second experiment. light of the same original intensity is passed through a mixture of chlorine and hy drogen, the intensity is forind to have diminished by a greater amount than in the first experiment, offing to the photochendeal combination of rine and hydrogen into hydrochloric acid. The absorption of light by the hydrogen gas present in the second experiment is small and may be left entirely out of aeconnt. In either experiment, the diminution of intensity may of course be measured by means of one of the aetinometers de scribed above.
In (qiche:inn, it may be pointed out that if a given substance is capable of undergoing a chemi cal change both under the influence of light and Without this agency. the change in the two eases
need not necessarily be quite the same. Thus it ha•, been proved that the decomposition of hy driodie acid takes place, under the influence of light, according to the equation.
111=1-1+r, i.e. molecule after molecule of hydriodie add is primarily decomposed into single atoms of hy drogen and iodine. On the other hand, if by driodie acid is decomposed by heat, without the intervention of light, the decomposition takes place according to the equation 2111 i.e the molecules are decomposed in pairs, each pail' yielding a molecule of hydrogen gas and a lecide of iodine.
Consult: Nernst, Thcorctische Cheinie (Stutt gart, I011(/) ; Eder, Handbuch der Photographic (Mille, 1884) ; Niewenglowski. La photographic el la pholochiptic (Paris. 1898) ; Bunsen and Roscoe, Pholoehemisehe rntersuchungcn (re printed in Ostwald's K1assik4r der card:ten !wnschafte», Leipzig. 1892) : Draper, Nricatific Memoirs, Being Experimental Vont ributionR to a Knowledge of Radiant Energy (New York, 1878). See PlioTOGRAPHY; PHOTOSYNTHESIS; PHOSPHO RESCENCE; AC"I'INoGRAPII, etc.