32 ATMOSPHERIC ACTINOMETRY 



phenninenoii. We shall see that coml)ustioii,once be<;iin, does not t;o on with regu- 

 lar and equal steiis, but is made to proceed faster and fastei-. In other words, every- 

 thing goes on as if the sensitiveness were increasing during the oxidation. To i)Ut 

 it still differently, the quantity of burnt oxalic acid, which amounts to little or next 

 to nothing during the first moments of the ex^wsure to the sun, starts out and in- 

 ci'eases, from that instant, quicker than time, so that there is no proportion between 

 the lenitth of the insolation and its consuming effects. 



In order to take account of effect of insolation upon an oxalic solution, let us 

 slightly modify the conditions of an ex[)eriment which I have just described. Let 

 us expose in the morning a dozen similar vessels to the sun, and let us withdiaw 

 every othei- hour two of them, which will give us the sum total of combustion up to 

 that moment. It will be easy liy this means to ascertain the progress of combustion 

 during the whole day. The following experiment I cite, not as being the most 

 complete of those which I have made, l)ut because it was pei'formed with a solution 

 of the same sensitiveness as that used in other experiments which I shall (piote 

 presently, so that all of them ai-e comparalile. 



Exp. — On September 6, 1888, at 8.30 a.m., on Mont Dore, I exposed to the sun 

 four vessels, which I withdrew at varioiis intervals, and in which I meas- 

 ured the proportion of oxalic acid burnt. 



We see at the start the " dead time " of the beginning. We see, moreover, 

 that from the fourth to the eighth hour, that is to say fi-om 12.30 p.m. to 

 4.30 P.M., the combustion was twice as lapid as from 10.30 a.m. to noon, in 

 spite of the gradual descent of the sun towards the horizon. During the 

 last two hours, and notwithstanding the obliquity of the solar rays, which 

 is ali-eady great at this time of the year, the conibustion was still two 

 thirds of what it had been between 10 a.m. and noon. 

 It is always the same, whether the total combustion be feeble, as it was at 

 Mont Dore, or active, as I have at times found it in Paris. From the sum total of 

 my results I think I may conclude that the progress of solar coml)Ustiou does not 

 remain constant during the whole of the <lay, and that instead of increasing towards 

 noon, in order to decrea.se afterwards in proportion as the sun ai)]>roaches the hori- 

 zon, it, on the contrary, experiences a progressive acceleration wliirh does not cease 

 till the sun is near its setting. 



