e is found in the form of an efflorescence on the
surface of different parts of the soil. The occurrence of nitre under
such conditions is due, partly to the natural richness of the soil in
nitrogen, and partly to its artificial enrichment through receiving the
nitrogenous excrements of the inhabitants of the villages and their
cattle. The constant process of evaporation going on in such a warm
climate has the effect of inducing an upward tendency of the soil-water,
the result being a concentration of all the nitre the soil contains in
its surface layer. This goes on until a regular incrustation is formed,
and the soil is covered by a white deposit of nitre. Whenever this
becomes apparent, the surface portion of the soil is scraped off by the
_sorawallah_, or native manufacturer, and collected and treated for the
purpose of recovering, in a pure state, the saltpetre.

_Saltpetre Plantations._

The large demand for saltpetre, larger than could be supplied by these
nitre soils, soon gave rise to the semi-artificial method of production,
formerly so largely practised in Switzerland, France, Germany, Sweden,
and in many other parts of the Continent, by means of the so-called
"nitre beds," "nitraries," or "saltpetre plantations." Previous to the
introduction of this method of manufacture, the demand for saltpetre for
gunpowder had become so great, that every source of nitre was eagerly
sought for. Thus, when it was discovered that the earth from the floors
of byres, stables, and farmyards were particularly rich in nitre, and
when mixed with wood-ashes formed an important source of it, the right
to remove these in France was vested in the Government under the
Saltpetre Laws, which obtained till the French Revolution. This great
scarcity soon led, however, to a careful investigation being made into
the conditions under which potassium nitrate was formed in nitre
soils.[101] These conditions, which included the presence of rich
nitrogenous matter, warmth, free aeration of the soil, and a certain
proportion of moisture, became, in the course of years, more and more
thoroughly understood, and the result was the institution of numerous
"saltpetre plantations." These generally consisted of heaps of mould,
rich in nitrogen, mixed with decomposing animal matter, rubbish of
various kinds, manurial substances, ashes, road-scrapings, and lime
salts.[102] The heap was interlaid with brushwood, and was watered from
time to time with li