THE ACTION OF LIGHT 555 



catalysts are produced by the action of light. In these cases the action of light 

 leads to the same products which appear in the dark under the same conditions of 

 temperature and solvent. Light merely accelerates the process by causing the 

 formation of a catalyst for the reaction, which then obeys the usual laws of 

 catalysis. 



It is found that the catalyst may be formed from the reagents, or one of them, 

 by the action of light, and may then disappear on the removal of the light. Or, in 

 other cases, the catalyst may continue to exist and exert its action for some time 

 after the light has been taken off. 



4A. Reactions with Loss of Energy, in which a Catalyst is Formed by Light, the 

 Catalyst lasting only as long as the Illumination, and vanishing in the Reaction. 

 One of the best known of all catalytic light reactions, namely, the combination of 

 hydrogen and chlorine under the action of ultra-violet light, belongs to this group. 

 A great number of investigations have been made on this reaction since the first 

 exact research by Bunsen and Roscoe (1855-1859). Details of these will be found 

 in Weigert's monograph (1911, pp. 44-56). Under the usual conditions of 

 experiment, the effect is found to have a latent period, the so-called " Induction 

 Period" during which no combination takes place. Subsequent investigation 

 showed that this was due to the presence of impurities, especially on the walls 

 of the vessel used. These impurities use up the catalyst for a time. What is the 

 catalyst? From what was said above, it is clear that light energy is used up 

 to produce it, and it appears to be chlorine in an " activated " form of some kind. 



The fact that the drier the gases are, the more slowly does the reaction proceed, suggested 

 to Mellor (1902) that there is formation of an intermediate compound (a;CL 2 , yH 2 0, zH, 2 ), as 

 in certain other cases of catalysis, such as that of molybdic acid on hydrogen peroxide and 

 hydriodic acid, as described above (page 324). The investigations of Burgess and Chapman 

 (1906) directed attention to the cloud formation, due to the production of condensation nuclei in 

 the illuminated gases. Whether these nuclei are identical with the hypothetical compound 

 of Mellor seems doubtful, and it is more probable that they do not differ essentially from 

 other cloud nuclei formed by radiations. 



Chlorine is made " active " by light for other reactions also, such as for 

 combination with carbon monoxide, sulphur dioxide, hydrocarbons, etc. What- 

 ever the nature of the catalyst may be, it must consist of chlorine plus light 

 energy, and therefore act chemically as chlorine itself. Accordingly, it disappears 

 in the reaction. In such reactions it appears that the primary action of light is 

 to form nuclei, which start a reaction in a way analogous to that in which they 

 cause condensation of water vapour to drops of liquid. Weigert calls them 

 " reaction nuclei," and points out that their mode of action is like that of other 

 heterogeneous catalysts. The reacting substances are condensed on their surfaces 

 by adsorption, and the reaction proceeds there more rapidly as a consequence of 

 mass action. 



Compared with the reactions in which light energy is stored up, and often in 

 considerable amount, these catalytic reactions require little energy to form the 

 catalyst, and are, as a rule, very sensitive. 



The phenomena of optical sensitisation belong to the present category of 

 reactions. Light cannot act unless absorbed, and the question naturally arises 

 whether the addition of some substance, such as a dye, to a system which is not 

 affected by light of a particular wave length, is able to make it sensitive if the dye 

 absorbs rays of this wave length. In point of fact, such is the case, although at 

 first sight the reason is not obvious. The light is absorbed by the sensitiser, and 

 must therefore produce changes in this, not necessarily in other, parts of the system. 

 The key is given by the formation of catalysts from the sensitiser, which appear to 

 be heterogeneous in nature and, at all events in many cases, require the presence of 

 oxygen, "activating" it so that potassium iodide is oxidised as by ozone. 



A simple experiment given by Wager (1914) shows this fact. Strips of paper containing 

 starch are soaked in a solution of methyl violet, methyl green, eosin, fuchsin, or fluorescein, 

 exposed to light, and then moistened with potassium iodide solution. Iodine is liberated, and 

 stains the starch blue. It is interesting that cyanin, although bleached by light, does not give 

 rise to active oxygen. 



The mode of action of optical sensitisers seems to be of a somewhat general 



