4 ATMOSPHERIC ACTINOMETRY 



therefore, fur the present, to kuow that our solution of oxalic acid is peculiarly 

 affected hy the action of the luminous and the chemical parts of the spectrum. 

 We shall soon find I'easous to believe that it is the chemical part alone which acts. 

 But we have fii-st to investigate the manner by which combustion is produced 

 befoi-e we can detei'mine what influences cause it. 



Study of the Pkooess. 



Oxalic acid, dissolved in water and exposed to light, aV)soi'bs oxygen, and 

 changes almost entirely into carbonic acid. There appears also a little formic acid, 

 but in almost infinitesimal quantities. Hence it follows, as we have seen, that we 

 can ascei-tain the quantity of oxidized acid by a simple acidiinetric determination. 



INFLUENCE OF CONCENTKATION. 



In order to study the actinometric process, the first thing to discover is the 

 degree of concentration which gives the largest amount of sensil)ility. In order to 

 know this I exposed to the sun, under precisely the same conditions, during three 

 fine days from June 4th to June (Stli, including about 36 hours of insolation, foni- 

 liquids, containing, respectively, per litre : 



grm. grm. grm. grm. 



63 31.5 12.6 6.3 of oxalic acid; 



that is, I 1 J ,',, of an equivalent, 



per liti'e. At the end of this tiine, an acidinictric analysis gave nic tin- (juaiitities 



of acid which had been burnt, and I computed from this the proportion i>f acid 



which had disappeared from each of the vessels. 



The figures were the following, counted in milliLCianunes per litre: 



1 E(iuiv. i E(|uiv. I Eqiiiv. j\ Eijuiv. 



Quantity of acid burnt, 2,500 2,800 4,700 3,300 



rr()|)ortion, 4 ^ 9 '■^ 38^ 521?: 



To I'each llie niaxiinuni in the aljsolutc ipiantity of acid consumed we must, 

 therefore, operate with solutions neither too concentrated nor too weak. Solutions 

 which are too concentrated oxidize slowly, and I lie varialion in t-licmical value is 

 often noted with difficulty. It is, on the other hand, very ea.sy to measure this 

 variation with solutions wliicli are rather weak, because it represents a notable 

 fraction of the primitive value. But, on the otlu r hand, w lien the litjuid has been 

 weakened by the sun, the last poi'tions burn quite slowly. There are, therefoie, 

 two dangers to avoid. 



After various trials I decided upon a solution whose variation of title during 

 the most favorable days should not exceed one half of the initial value. This is a 



