from which explosive compounds are made contains nitrogen in the form in which it can be used by plants.  Saltpetre, for example, is equally good as a fertilizer and as a basis for gunpowder.  The products of the explosion are gases no longer capable of use by plants, and thus every explosion of nitrogen compounds aids in this gradual dissipation of nitrogen products, taking them from the store of plant foods and throwing them away.

All of these agencies contribute to reduce the amount of material circulating in the food cycle of Nature, and thus seem to tend inevitably in the end toward a termination of the processes of life; for as soon as the soil becomes exhausted of its nitrogen compounds, so soon will plant life cease from lack of nutrition, and the disappearance of animal life will follow rapidly.  It is this loss of nitrogen in large measure that is forcing our agriculturists to purchase fertilizers.  The last fifteen years have shown us, however, that here again we may look upon our friends, the bacteria, as agents for counteracting this dissipating tendency in the general processes of Nature.  Bacterial life in at least two different ways appears to have the function of reclaiming from the atmosphere more or less of this dissipated free nitrogen.

In the first place, it has been found in the last few years that soil entirely free from all common plants, but containing certain kinds of bacteria, if allowed to stand in contact with the air, will slowly but surely gain in the amount of nitrogen compounds that it contains.  These nitrogen compounds are plainly manufactured by the bacteria in the soil; for unless the bacteria are present they do not accumulate, and they do accumulate inevitably if the bacteria are present in the proper quantity and the proper species.  It appears that, as a rule, this fixation of nitrogen is not performed by any one species of microorganisms, but by two or three of them acting together.  Certain combinations of bacteria have been found which, when inoculated in the soil, will bring about this fixation of nitrogen, but no one of the species is capable of producing this result alone.  We do not know to what extent these organisms are distributed in the soil, nor how widely this nitrogen fixation through bacterial life is going on.  It is only within a short time that it has been demonstrated to exist, but we must look upon bacteria in the soil as one of the factors in reclaiming from the atmosphere the dissipated free nitrogen.

The second method by which bacteria aid in the reclaiming of this lost nitrogen is by a combined action of certain species of bacteria and some of the higher plants.  Ordinary green plants, as already noted, are unable to make use of the free nitrogen of the atmosphere It was found, however, some fifteen years ago that some species of plants, chiefly the great family of legumes, which contains the pea plant, the bean, the clover, etc, are able, when growing in soil that is