86 



ATMOSPHERIC ACTINOMETRY 



action of liirlil. enables the solution to uiuleriro in iliffused lii^ht a combustion 

 which is out of question as long as it has only its normal degree of sensibility, that 

 is, the (legi'ee obtained by kee|)ing for some weeks in a diffused light. 



bjxp. — In August, 1889, at Noalhac near Aurillac (Cautal), at an altitude of 

 about 700 metres, I prepared a solution of oxalic acid, part of which was 

 left in a flask, exposed to a very feeble liglit, while another portion was 

 exposed to the sun in a stoppered bottle. This was in order to see if the 

 process of rendering the solution sensitive was necessarily accompanied 

 by a i)rocess of combustion, or whether it could be accomplished without 

 the latter. The experiment showed that the two effects are independent 

 of each other. The solution contained in the closed flask had at its dis- 

 posal only a very small quantity of dissolved oxygen, \vlii(li it consumed, 

 moreover, with but a slight diminution of its titre, but in one day of inso- 

 lation it reached an iiiten.se sensitiveness, which was maintained for 

 seveial days at the same rate by pi'eserving it in a diffused light. 



On August 30th, I exposed to the sun two vessels containing some of 

 this insolated solution, and at the same time, two vessels of the same 

 non-insolated i)ieparation. The figures for solar combustion are : 

 18 and 19 per cent for the non-insolated, and 

 92 and 92 " " " insolated liquid. 



The day was a veiy fine one ; the flask with the insolated solution 

 remained in the sun, but closed. ( )n August 3lst, two groups of two 

 vessels each weie ])repared, one containing non-insolated, the other inso- 

 lated solutiim. One of the.se groui)S was exposed to the sun, the other 

 on a window-ledire, facinsr the north, where it received no licrht but that 

 coming from the sky and dimmed somewhat by a slight dry fog. The 

 figui'es found after 8 hours' exposure, for combustion in the sun and in 

 iliffu.sed light, were the following: 



Insolated liquid lost 63 per cent in the sun. 



Non-insolated " " 2-1 " " " " 

 Insolated " "19 " in diffused light. 



Non-insolated " " 6 

 The next day, September 1st, <luring a dark, threatening day, tlic 

 same arrangement gave the following results: 



Insolated liipiid lost 50 per cent in the sun. 



Non-insolated " " 13 ' " 



Insolated " " 6 " in diff'used light. 



Non-insolated " " 3 " " " ' " 



