AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 31 



not begin as soon as the vessel is exposed to the light. There is a " lost time " at 

 the beginning; two hours, three hours, are necessary for the solution to put itself 

 in action. During this whole time the work is wliolly interior and betrays itself 

 by no diminution of the; acidimetric titre. 



This "dead time" at the beginning of the reaction should not surprise us. 

 When we study the different reactions of chemistry, from this point of view, we 

 become aware that there ai-e few which begin immediately upon realization of the 

 exterior conditions of production, even in cases when tlie energy which comes into 

 play is altogether internal as regards the mixture. The formation of a pi'ecipitate 

 of barium sulphate is not instantaneous ; that of calcium sulpliate, or of calcium 

 tartrate, is still less so ; a mixture of fui-mate and ot" permanganate of potassium 

 remains apparently inert for some seconds, after which begins an abundant, and, to 

 some extent, an explosive liberation of carbonic acid, [)r()ceeding from the c(>ml)us- 

 tion of formic acid. 



Here the heat produced by the reaction intervenes to increase its activity. We 

 can say the same of the phenomenon which Bunsen and Roscoe discovei'ed and in- 

 vestigated under the name of "photochemical induction," in the cond:)ination, in the 

 sunlight, of chlorine and hydrogen. This reaction also requires a certain time to 

 commence, but it accelerates afterwards, because it is exothermic. The same 

 remark applies to the reduction of cliloride or bromide of silver in the presence of 

 an organic substance, which also shows a "dead time" at the beginning, and be- 

 comes more energetic afterwards. The same i-eraark ap[)lies, moi-eovei', to almost 

 all photographic operations, whether we wish to obtain luminous impressions, to 

 develop images, or to produce positive prints. 



If we observe a "dead time " when the forces are internal and accelerating, it is 

 not surprising that we should find a Hke one also in the solar combustion of oxalic 

 acid, whei-e the impulse is to come from without and where the reaction is so feebly 

 exothermic. But this vei'ification presents here an interest which it has not else- 

 where, for we connect it intimately with the phenomena of sensibilization, which 

 we discovered pi-eviously in the solutions of oxalic acid. In botli cases a molecular 

 preparation is evidently involved, the mechanism of which is still unknown to us, 

 but which results in placing the molecule upon a kind of inclined plane, down 

 which it may be made to roll by the slightest impulse. As a confirmation of this 

 idea, I have ascertained that in fact the "dead time" at the beginning is less pro- 

 tracted with solutions which have been made sensitive, than with new solutions, so 

 that if the latter do not undergo in the light of the sun, as we have seen before, 

 the same degree of combustion as the others, it is [tai'tly because the "dead 

 time " at the beginning is shoi'ter. But I say " pai'tly " because there is still another 



