_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 liquid manure from stables, consisting chiefly of dilute urine. In forming the heap care was taken to keep the mass porous, so as to admit of the free access of air. The heap was further protected from the rain by covering it with a roof. In course of time considerable quantities of nitrates were developed, and the nitre was occasionally collected by scraping it from the surface, where it became concentrated just as in the nitre soils. In all cases, however, the heaps, when considered rich enough in nitre, were treated from time to time with water which, by subsequent evaporation, yielded the nitre in a more or less pure condition.[103] This mode of obtaining nitre is no longer practised to any extent, since it is now more conveniently obtained from the treatment of nitrate of soda with potassium chloride. _Cause of Nitrification._