THE NITRIC ACID OF THE SOIL. 263 
free nitrogen is based upon the effects of a reducible 
body, like sesquioxide of iron or sulphate of lime, to 
which attention has been already directed. 
In a very carefully conducted experiment, Cloez * trans- 
mitted atmospheric air purified from suspended dust, and 
from nitric acid and ammonia, through a series of 10 large 
glass vessels filled with various porous materials. Vessel 
No. 1 contained fragments of unglazed porcelain; No. 2, 
calcined pumice-stone; No. 3, bits of well-washed brick. 
Each of these three vessels also contained 10 grms. of car- 
bonate of potash dissolved in water. The next three vessels, 
Nos. 4, 5, and 6, included the above-named porous materials 
in the same order; but instead of carbonate of potash, they 
were impregnated with carbonate of lime by soaking in 
water, holding this compound in suspension. The vessel 
No. 7 was occupied with Meudon chalk, washed and 
dried. No. 8 contained a clayey soil thoroughly washed 
with water and ignited so as to carbonize the organic 
matters without baking the clay. No. 9 held the same 
earth washed and dried, but not calcined. Lastly, in No. 
10, was placed moist pumicé-stone mixed with pure car- 
bonate of lime and 10 grms. of urea, the nitrogenous princi- 
ple of urine. Through these vessels a slow stream of puri- 
fied air, amounting to 160,000 liters, was passed, night and 
day,for 8 months. At the conclusion of the experiment, 
vessel No. 1 contained a minute quantity of nitric acid, 
which, undoubtedly, came from the atmosphere, having 
escaped the purifying apparatus. The contents of Nos. 
2,4, and 5, were free from nitrates. Nos. 8 and 6, con- 
taining fragments of washed brick, gave notable evidences 
of nitric acid. Traces were also found in the washed 
chalk, No. 7, and in the calcined soil, No. 8. In No. 9, 
filled with washed soil, niter was abundant. No. 10, 
* Recherches sur la Nitrification—Lecons de Chimie professées en 1861 ala 
Société Chimique de Paris, pp. 145-150. 
