G ATMOSPIIEKIC ACTINOMETRY 



we tliiiik of the lajtidity with whieh de-aerated water aerates itself anew, it is Iiard 

 to believe that it was the oxygeu which was wanting, and we are thus led tt> be- 

 lieve that the chemical action was at fault. If t'he incident ray does not bring with 

 it an excess of chemical energy, the superficial layers absorb as much as is available, 

 ami there is none left for the lower strata, even though all the needful oxygen 

 sh()uld be at hand to burn the acid whicli is present. 



The ijuestion is of some importance, because it teaches us the quantum of 

 chemical action which may be expected from light in the vit-iiiity of the soil, and 

 consequently, also, the degree of atmospheric aljsorptioii. In older to get informa- 

 tion on this subject, let us operate with shallow, cylindrical vessels, which are at 

 most a centimetre high at the rim, so that there can be no stagnation of air above the 

 liipiid, and that the oxygen always has easy access to the latter. If the actinic 

 influence is deficient in the incident light, we must be able to put in evidence the 

 influence of the surface and the depth of the liquid. For ecjual depths the com- 

 bustion will have to be piopoitional to the surface. Foi' C(jual surfaces with dif- 

 ferent depths, combustion, if limited to the superficial layers, should not inciease 

 with the volume and the depth of the solution, or at least not increase so rapidl\-. 

 This is exactly what experience shows. 



Exp. — Into two cylindi-ical, very shallow vessels, having the same surface, I poured 



10 and 20 c. c. of a half-deci-normal solution of oxalic acid. After a rather 



dark and somewhat stoi-my day, I find that 28 per cent, of the acid has 



been burnt in the vessel that held 10 c. c, and only 23 per cent, in the other. 



As it held twice as much li(|uid as the other, the absolute quantities of 



acid burnt are relatively 28 and 4(i, while the ilepths of solution were in 



the I'atio of 1 to 2. Cond)ustion, therefore, incieases less (juickly than 



dei)th. As the latter has not exceeded a centimetre in the vessel in 



which it was greatest, and as, moieover, the total combustion was very 



slight, we cannot admit that oxygen was wanting. Rut the solar rays, 



deprived of theii- chemical radiations, which were rendered active by their 



])assage through the superlicial strata, leached the lower layers very nuich 



weakened, although the luminous transpai'ency of the two liipiids was 



perfect. 



There exists then a kiml of shifting of the actinic lays during the passage of 



the \\\sS\i tIirou!.rh the first layers which it encounters; and, wlicthcr these rays are 



not abundant m- whether the absorptioii lu- \ei'y cfiicicnl and tlic nicciiuni very 



opaijuc for thcni, the weakening process is very I'apid. In return, when the surface 



alone is allowed to varv, while the heii^ht of the li(|uid I'emains unchaiitred, the 



eltect of combustion is [iioportional to the surface, and conse(]Uently to the volume. 



