quid 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._

We have adverted to these nitre plantations as showing how the
conditions most favourable for the development of nitrification were
recognised long before anything was known as to the true nature of the
process. It was only in 1877 that the formation of nitrates in the soil
was proved to be due to the action of micro-organic life,[104] by the
two French chemists, Schloesing and Muentz, who discovered the fact when
carrying out experiments to see if the presence of humic matter was
essential to the purification of sewage by soil. In these experiments
sewage was made to filter slowly through a certain depth of soil (the
time occupied in this filtration being eight days). It was found that
nitrification of the sewage took place. By treating the soil with
chloroform[105] it was found that it no longer possessed the power of
inducing the nitrification of the sewage. When, however, a small portion
of a nitrifying soil was added, the power was regained. From this it was
naturally inferred that nitrification was effected by some kind of
ferment. This conclusion was soon confirmed by subsequent experiments by
Warington at Rothamsted, who showed that the power of nitrification
could be communicated to media, which did not nitrify, by simply
seeding them with a nitrifying substance, and that light was
unfavourable to the process. Since then the question has formed the
subject of a number of researches by Mr Warington at Rothamsted, as well
as by Schloesing and Muentz, Munro, Deherain, P. F. Frankland,
Winogradsky, Gayon and Dupetit, Kellner, Plath, Pichard, Landolt, Leone,
and others. From these r