AND THR ACTINIC CONSTITUTION OF THE ATMOSPHERE. 37 



Thus a previous insolation increases the rapidity of combustion, not only in di- 

 rect liglit but also in diffused light. The experiment of September •2d shows, how- 

 ever, when compared with that oi' tlie day before, that this dift'used light must 

 have a certain intensity to make its effect measurable after some hours. But, 

 viewed by itself, this experiment shows that even a dark day still has an accelerat- 

 ing effect upon a solution which has not seen the sky since the second day before. 

 The insolated liquid was fi'om that moment kept in diff'used light at the back 

 of a room with but one window, faciug the north. It was found that after a few 

 days it had not sensibly changed in titre, l)ut variation began to show itself at the 

 end of a month. We thus see how here also, in spite of good conditions of preser- 

 vation, the phenomena of slow combustion appear which have lieen observed since 

 Wettstein in solutions of oxalic acid. Not insolated, this solution presei'ved in the 

 same manner had remained much more stable, which shows that it is necessary to 

 avoid exposui'e to light, even temporarily, or even in a carefully closed flask, of 

 solutions of oxalic acid which are intended for processes of titration. The lumi- 

 nous impression, once received, persists and makes them much less stable — it con- 

 tinues, as we shall presently see, even after the lic[uid solution has been placed in 

 darkness. 



Exp. — Another experiment was begun identical with those that have just been 



described except that the flask which contained the insolated solution, 



sheltered from the air, was kept for three nights and two days in a cupboard 



of the laboratory before being distributed into vessels on September 6th. 



Unfortunately the day of the 6th was disturbed by cirri and cloudlets. 



The insolated liquid lost 20 per cent in the sun, 



Non-insolated liquid lost 7 per cent in the sun. 



The oxidation in diff'used light was insignificant. 

 The proportion of oxalic acid burnt in sunlight is, therefore, still, after 60 

 hours of obscurity, three times greater in the insolated solution than in the other. 

 But the sensitiveness decreases afterwards and the difference soon ceases to be 

 measurable after a day's exposure. We here meet again with that retrogradatiou 

 which we have pointed out earlier, and which brings us back to nonnal sensitive- 

 uess. 



1 a(hl, in order to close the subject, that this solar impression, which disap- 

 pears sloA\dy, is on the other hand produced very rapidlj'^, and that, when investi- 

 gating comi)arati\ely, with respect to the combustion which they undergo, the 

 solution which I had exposed to the sun, in three flasks, and 1, 2 and 3 days, 

 respectively, I ha\e not Ijeen able to show that there ^vas any essential difference 

 between them ! 



