theory of nitrification.  If it is to be shown that nitrification will occur in the absence of any ferment, it is clear that all ferments must be rigidly excluded during the experiments; the solutions must be sterilized by heat, the apparatus purified in a similar manner, and all subsequent access of organisms carefully guarded against.  It is only experiments made in this way that can have any weight in deciding the question.

Leaving now the theory of nitrification, I will proceed to say a few words, first, as to the distribution of the nitrifying organism in the soil; secondly, as to the substances which are susceptible of nitrification; thirdly, upon certain conditions having great influence on the process.

The Distribution of the Nitrifying Organism in the Soil.—­Three series of experiments have been made on the distribution of the nitrifying organism in the clay soil and subsoil at Rothamsted.  Advantage was taken of the fact that deep pits had been dug in one of the experimental fields for the purpose of obtaining samples of the soil and subsoil.  Small quantities of soil were taken from freshly-cut surfaces on the sides of these pits at depths varying from 2 inches to 8 feet.  The soil removed was at once transferred to a sterilized solution of diluted urine, which was afterward examined from time to time to ascertain if nitrification took place.  These experiments are hardly yet completed; the two earlier series of solutions have, however, been examined for eight and seven months respectively.  In both these series the soil taken from 2 inches, 9 inches, and 18 inches from the surface has been proved to contain the nitrifying organism by the fact that it has produced nitrification in the solutions to which it was added; while in twelve distinct experiments made with soil from greater depths no nitrification has yet occurred, and we must therefore conclude that the nitrifying organism was not present in the samples of soil taken.  The third series of experiments has continued as yet but three months and a half; at present no nitrification has occurred with soil taken below 9 inches from the surface.  It would appear, therefore, that in a clay soil the nitrifying organism is confined to about 18 inches from the surface; it is most abundant in the first 6 inches.  It is quite possible, however, that in the channels caused by worms, or by the roots of plants, the organism may occur at greater depths.  In a sandy soil we should expect to find the organism at a lower level than in clay, but of this we have as yet no evidence.  The facts here mentioned are in accordance with the microscopical observations made by Koch, who states that the micro-organisms in the soils he has investigated diminish rapidly in number with an increasing depth; and that at a depth of scarcely 1 meter the soil is almost entirely free from bacteria.