AND TTIE ACTINIC CONSTITUTION OF THE ATMOSPHIORK. 7 



Exp. — I procured two cyliiidiical vessels witli flat bottoms, of Bolieniiau glass, 

 tlie bottom surfaces of wliicli were as 1 to 2. 1 exposed tliciii to the sun, 

 one with 10 c. c, the other with 20 c. c. of one and the same half-deci-norraal 

 solution of oxalic acid. The quantities of burnt acid have always been in 

 the proportion of 1 to 2, in a long series of experiments, with an approxi- 

 mation equal to that wliich the process of analysis should demand. 

 We shall have to avail oui'selves of all these results when we try to ascertain 

 the cause of atmospheric absorption. Let us Vje content, for the time being, with 

 dravN'ing a practical conclusion fi'om them, namely, that it is desirable always to 

 work with vessels of the same dimensions, and with ccpial <[uantities of solution, if 

 we wish to obtain figures that can be compared with each other. 



I employ small blown vessels of Bohemian glass, with flat bottoms, such as are 

 found in trade. I choose them of tlie same dimensions, or nearly so, which can 

 easily be ascertained by fitting together their edges and noting whether they have 

 nearly the same external diameter. It is not necessary to carry accuracy any 

 fai-ther, considering all the inevitable irregularities connected with measurement. 



Those which I have used measured about 4.5 centimetres in diameter, and 

 10 c. c. of liquid had there a thickness of about 6 millimeti'es. When I was at work 

 in the country, in the Cantal or in the Puy-de-Dome, whei-e the clouds of atmos- 

 pheric dust are not calcareous, I left them freely exposed to the air upon a small 

 table, so placed as to face the south and to expose them to the sun all day long. 

 The heating which takes place in them is never veiy great, as the following 

 experiments show ; although they Avere not made in a flat vessel, but in a cone- 

 shaped glass with a foot. The heating is less by 4" or 5" C. when working witli 

 a flat vessel. 



Jj^xj). The same glass with a foot and containing 10 c. c. of a half-deci-normal 



solution of oxalic acid was exposed from the 15th to the 27th of August, 

 1885, daily to the sun. Every day the mean pressure, the maximum 

 tempei-ature of the liquid, and the aspect of the sky were carefully noted. 

 Here follow the pi'oportions of acid bui'iit on the difl:"ei-ent days, during 

 which the weather was very fine. The experiments were made at Fan, in 

 the Cantal, at an altitude of about 700 metres. 



