THE NITRIC ACID OF THE SOIL. 259 
face charged with sulphides (sulphur-springs). Water 
containing sulphates in solution often acquires an odor of 
sulphuretted hydrogen by being kept bott!ed, the cork or 
other organic matters deoxidizing the sulphates. The 
earth just below the paving-stones in Paris contains con- 
siderable quantities of sulphides of iron and calcium, the 
gypsum in the soil being reduced by organic matters. 
(Chevreul.) These sulphides, when exposed to air, speed- 
ily oxidize to sulphates, to suffer reduction again in con- 
tact with the appropriate substances, and under certain 
conditions, operate continuously, to gather and impart 
oxygen. One of the causes of the often remarkable and 
inexplicable effects of plaster of Paris when used as a fer- 
tilizer may, perhaps, be traced to this power of oxidation, 
resulting in the furmation of nitrates. This point requires 
and is well worthy of special investigation. 
c. Lastly, the free nitrogen of the atmosphere appears 
to be in some way involved in the act of nitrification—is 
itself to a certain extent oxidized inthe soil, as has been 
‘maintained by Saussure, Gaultier de Claubry, and others 
(Gmelin’s Hand-book of Chemistry, IT, 388). 
The truth of this view is sustained by some of Bous- 
singault’s researches on the garden soil of Liebfrauenberg 
(Agronomie, etc., T., 1, 318). On the 29th of July, 1858, 
he spread out thinly 120 grammes of this soil in a shallow 
glass dish, and for three months moistened it daily with 
water exempt from compounds of nitrogen. At the end 
of this time analysis of the soil showed that while a small 
proportion of carbon (0.825°|,) had wasted by oxidation, 
the quantity of nitrogen had slightly increased. The 
gain of nitrogen was but 0.009 grm. = 0.008"|,,. 
In five other experiments where plants grew for several 
months in small quantities of the same garden soil, either 
in the free air but sheltered from rain and dew, or ina 
confined space and watered with pure water, analyses 
