AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 5 



solution containing about j^ an (u^uiv alcnt or about ^ gnininies of oxalic acid per 

 litre. Ten cubic centimetres of this liquid are saturated by about an ecjual volume 

 of common lime-water, so tluxt the daily variation of the titre amounts to 4 or 5 c. c. 

 of lime-water, a (piantity which can be measured down to yj-^, liy means of a 

 burette. The accuracy which we thus obtain is more than sufficient, as we shall 

 presently see. 



INFLUENCE OF THK DEPTH OF SOLUTION. 



In discussing the (juestion of oxidation, we nuist consider the part played by 

 the ease with which oxygen [)eneti'ates into the depths of the solution. We can 

 easily calculate that the 10 c. c. of oxalic solution, which are used in each one of 

 the experiments, require foi' complete combustion about 3 c. c. of oxygen, a quantity 

 greatly superior to that which is already dissolved. Whatevei- the facility may be 

 with which this gas penetrates into a liquid, which is subjected for 8 to 10 hours 

 to insolation in free contact with the air, we may well ask if a solution of oxalic 

 acid oxidizes in the sun in the same way in a vessel of shallow depth, in which its 

 thickness is small, as in a cone-shaped glass or in a round tube. The following 

 experiment furnishes an answer to this question : 



J^Jxp. — On August 16th, 17th, and 18th, I exposed to the sun 10 c. c. of a -^\ 

 uormal solution of oxalic acid, as follows : 



(a) In a cone-shaped glass, 



(b) In an ordinary test tube, 



(c) In a Bohemian glass mattrass with flat bottom. 



To secure unifoimity of temperature, the cylindrical tube b was placed 

 upright in the mattrass c ; the exposure continued from 8 o'clock a.m. till 

 3.30 in the evening. The following proportions of acid were consumed : 



Thus, everything else being equal, the proportion of acid consumed is much 

 greater in a vessel with a flat bottom than in a cylindiical tube. The difference is 

 indeed so very striking, that the difficulty with which oxygen penetrates the solu- 

 tion does not suffice to explain it. 



A combustion of 13 per cent, produced in 7 houi's in the 10 c. c. of liquid, 

 contained in the matti-ass b, has not required more than 0.4 c. c. of oxygen ; in 

 other words, about 6 times the normal quantity dissolved in the solution. When 



