AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 33 



Everything, then, goes on .is if the sensitiveness which, as we have seen, a solu- 

 tion of oxalic acid attains if left to itself in darkness, wei'e not by any means a 

 maximum sensitiveness, and might be greatly inci'eased in the light. I have in fact 

 ascertained — and we shall presently see an example of it — tliat a I'ecent solution of 

 oxalic acid may be made very sensitive by a few hours' exposure to the sun, and so 

 be brought up to the level of an old solution, or even beyond it. But then an un- 

 foreseen consequence appears : the sensitiveness acquired in darkness is persistent 

 — might it not be perhaps the same with sensitiveness acquired in the light, so that 

 the accelerating effect of a fine day might spread, as it were, in its totality, or at 

 least in part, over the following day ? 



The following experiment proves in fact that a solution left foi' a whole day in 

 the sun, and which has not been entirely oxidized, will retain for the next day a 

 greater sensitiveness than another [)art of the same solution which was not pre- 

 viously insolated. 



Ejiyp. — There were exposed every day to the light four identical vessels, two of 

 which were carefully examined at the close of each day, while the other 

 two were left in reserve for the day following; on this day they were 

 again exposed to the sun at the same time with the four new vessels of a 

 second expei'iment. The total of combustion in the vessels which were 

 exposed for two days was then compared with the sum of combustions in 

 the vessels which were each exposed only one day. Some of the i-esults 

 which I thus obtained are as follows: 



The day of September 2d, combustion 1° I^ I , . ^ i^i all 



" " " " ,^d, " 24 ^f-^"^ 



Both days together, 68 </„. 



The amount of combustion has therefore doul)led. Here ai-e the results 

 of another experiment : 



The day of September 4th, combustion 12 ^ I ,, ^ ;„ all 



" " '■ " 5th, " 11 fo) 



Both days together, " 38 f- 



The difference points in the same direction as the pi'eceding experiment; only 

 it is not quite as great, because the two days, September 4th and 5th, were both 

 quite indifferent days (page 28), whilst the day of September 3d in the first experi- 

 ment was very fine. 



To sum up, we see that the insolated vessel of the fii-st experiment under- 

 went on the second day a combustion of 68 - 10 = 58 per cent, while a new vessel 

 suffered oidv an oxidization of 24 per cent. As to the second experiment, the cor- 



