30 



ATMOSPHERIC ACTINOMETRY 



of illuiniiiatioii was, everytliing flse hi'ing t'qiial, proportional to its (liuation. We 

 shall see that this is not so, auil that the effect increases much more rapidly than 

 the increase of tiiiie, so that all the notions which we entertain on this subject 

 stand in need of reviNion. 



INir.UENCE OF TlIK DUKATIOX Ol' ILLU.MIXATIOX. 



So far we have taken as a measure of tiie total actinic eft'ect during the period 

 of exposuiCj the sum total of the oxalic acid consumed. The conclusions which 

 we have thus leached, subsist, whatever may be the law which connects the com- 

 bustion with the actinic eifect ; it has been enough for us to expose, duiiug the 

 same length of time, solutions equally sensitive, in two different places and to pro- 

 ceed always by comparison. 



l)iil the law of the increase of actinic effect, with the time of insolation, does 

 not the less merit attentive investigation. To begiu, let us ask first, if the total 

 effect of combustion, observed at the close of a day, upon a solution of oxalic acid 

 exjiosed to the sun, rei)resents the sum of the divers actinic effects produced at the 

 different hours. 



One way to answer the question is to expose in the morning, side by side, two 

 vessels holding the same quantity of the same solution. One is to be examined at 

 the end of the day, and this will give us the sum total of the effect. The other is 

 to be examined at the end of an houi-, and then to be replaced by another vessel like 

 the first, but containing new ]i(|ui(l, to Ijc likewise studied after anothei- hour's ex- 

 posure. In like manner, we shall renew the study at successive hoiii-s. If the 

 actinic effects accumulate, without loss or encroachments in the liquid of the vessel 

 which has been exposed to the sun all day long, the quantity of acid which we 

 shall find has disa})peared must ecpial the sum of the (piantities of acid which have 

 vanished in the vessels that were exposed for an houi' each. 



The ex})eriment, rei)eated again and agoin, shows that it is never so. The sum 

 of the (piantities of acid burnt in the vessels which have been exposed each one 

 hour <)nly, is always insignificant in comparison witii that consumed in a vessel 

 w iiich has spent the whole day in the sunlight. The difference varies from one day 

 to aiiotliti-, ami iiicrea.ses with the intensity of combustion. It decreases slightly if 

 we carry the exposure of the successive vessels to two liours, and still more if we 

 extend it to three (»r four lioui's, as should be e.\[»ected. Hut, even if we divide a 

 day of ten hours into two equal periods, one from 7 a.m. to noon, anil the other 

 from noon to o i».m., the sum total of acid consumed in the two vessels that corresjHUul 

 to the two periods of exposure sometimes does not e.xceed half the acid consumed 

 in the vessel which spent ten hours in the sun. The combustion, therefore, does 



