The Nitrlftihui Ferraentst of the Soil. 70o 
found the nitrate in traces only, or in very small quantities, in 
some forest and meadow soils and soils with growing crops ; in 
very small quantity after very wet weather in autumn, and 
immediately after the growth of a crop ; in larger quantities in 
fallow soils in a dry autumn, and in largest quantities after a 
long spell of dry weather during the summer. In one case he 
gives under 2 parts of nitrate per million in the soil of a hop field 
in September after heavy rains, 600 parts per million in the same 
soil in the following July after a long spell of dry weather, and 
33 parts again in the following October. Whether his figures 
are strictly accurate or not, the great fluctuation in this floating 
capital of the soil was evidently quite familiar to him, and 
subsequent observers have but confirmed the general tenour of 
his results. Greedily absorbed from the soil by a growing crop, 
easily washed out of it by the winter's rains, and accumulating 
or being foiirned in the soil during warm and not too dry weather, 
and especially in fallows — these were obviously the main deter- 
mining circumstances of the fluctuations. 
It is only natural that the mode of formation of such 
a valuable substance should be an interesting and promising 
, field of inquiry. To the natural process of nitriji cation, as 
it occurs in the nitre-producing villages of India, Europe has 
, been, and still is, largely indebted for a supply of nitrate of 
potash wherewith to make gunpowder. The heaps of nitre- 
earth found near the sites of former habitations consist of house 
I refuse mixed with porous soil, ashes from the fires, urine, &c. 
. After long-continued exposure to Indian warmth, lixiviation of 
; this nitre-earth with water furnishes a solution from which 
saltpetre is extracted by evaporation and crystallisation. 
, In 1777, when France could not import saltpetre, the 
f Government caused to be printed " Instructions for the Esta- 
blishment of Nitre-heaps," which Boussingault makes the sub- 
ject of one of his essays, and his observations and drawings 
make it plain that before his time the practical conditions of 
nitrification were well known. Heaps of soil mixed with ashes 
and animal refuse, aiTanged in layers separated by loose straw 
kept under cover, freely exposed to air, and watered as often as 
possible with urine, turned and removed once or twice if practic- 
able, furnished in the course of some months a notable supply of 
nitre. If treated after the manner prescribed, we learn that about 
i 450 tons of material would in two years furnish about 4^ tons of 
' crude saltpetre. The watering with urine was to be stopped some 
months before the final lixiviation. Though earth was regarded 
as a purely mechanical agent, and any earth not too compact 
VOL. n. T. s. — 8 3 A 
