AND THR ACTINIC CONSTITUTION OF TIIK ATM08PIIERE. 15 



several organic substances undergo in the solar liglit, and wliiL-li offer a greater 

 resistance than oxalic acid. These I had been compelled to leave in the light for 

 weeks and for mouths before the attack was complete. They thus sununed up tlie 

 influence to which they liad been subjected during the length of the exposure. 

 Now this phenomenon required, generally, for its termination, nuudi more time, 

 sometimes three or four times more, in Paris than in the countiy. 



Among the facts of this kind I can only quote one which I iiud 1 hail by 

 chance inserted in a work intended for the examination of anothei' question. A 

 deci-normal solution of tartaric acid which was every day exposed from 10 a.m. till 

 2 p.nr. to the sun in Paris, had lost Ijy combustion in seven months and a half only 

 10 per cent of its acid, while in the Oantal an identical solution had lost in two 

 months 47 per cent. This involves a combustion about fifteen times more rapid, 

 and although the length of exposure was a little greater every day in the Cantal 

 than in Paris, and although the quantum of solar combustion increases moi'e rapidly 

 than the length of exposure, this is not enough to make up for the difference. In 

 another case involving the combustion of glucose in an alkaline liquid, I found 

 that to take two years in Paris which had required only three mouths in the 

 Cantal. 



Finally, this experiment teaches us also that average years do not resemble 

 each other and that, if thei'e are some which are I'ich in chemical i-adiatious, there 

 are poor ones also. These differences between one year and another, from 

 this point of view, appeared to me more marked than in any other respect. We 

 have shown above the most striking inequalities between consecutive days of the 

 same season. They recur, less markedly, for consecutive years. 



These statements which, I repeat, I regret not being able to support by 

 figures, suggest a number of problems for which I have begun to seek a solution. 



In the first place, the fact that active combustion is stronger on fine days in 

 spring than in summer and in autumn, shows that there must be another cause of 

 action than the influence of temperature, or the height of the sun above the horizon. 

 We are naturally led to think of the influence of the volatile organic pi'oducts 

 which vegetation scatters in the air during summer, and which, if they are capable 

 of beino- oxidized, absorb and utilize for their own benefit the chemical radiations 

 of the light which passes through the atmosphere, preventing them from reaching 

 the soil. We are confirmed in this view by what has been said before (page 6) 

 concerning the relative poverty of solar light, at the time when it reaches us, in 

 radiations able to oxidize oxalic acid. 



In the second place, the diffei-ence between the sum total of the annual radia- 

 tions at Paris and at the Cantal, or on the high table-lands of the Puy-de-Dome, 



