changes, the nitrogen products of animal life are broken frequently into compounds as simple as ammonia (NH3), or into compounds which the chemists speak of as nitrites (Fig. 25 at D).  Now these compounds are not ordinarily within the reach of plant life.  The luxuriant vegetation of the globe extracts its nitrogen from the soil in a form more complex than either of the compounds here mentioned; for, as we have seen, it is nitrates chiefly that furnish plants with their nitrogen food factor.  But nitrates contain considerable oxygen.  Ammonia, which is one of the products of putrefactive de-composition, contains no oxygen, and nitrites, another factor, contains less oxygen than nitrates.  These bodies are thus too simple for plants to make use of as a source of nitrogen.  The chemical destruction of the food material which results from the action of the putrefactive bacteria is too thorough, and the nitrogen foods are not yet in condition to be used by plants.

Now comes in the agency of still another class of micro-organisms, the existence of which has been demonstrated to us during the last few years.  In the soil everywhere, especially in fertile soil, is a class of bacteria which has received the name of nitrifying bacteria (Fig. 26).  These organisms grow in the soil and feed upon the soil ingredients.  In the course of their life they have somewhat the same action upon the simple nitrogen cleavage products just mentioned as we have already noticed the vinegar-producing species have upon alcohol, viz., the bringing about a union with oxygen.  There are apparently several different kinds of nitrifying bacteria with different powers.  Some of them cause an oxidation of the nitrogen products by means of which the ammonia is united with oxygen and built up into a series of products finally resulting in nitrates (Fig. 26).  By the action of other species still higher nitrogen compounds, including the nitrites, are further oxidized and built up into the form of nitrates.  Thus these nitrifying organisms form the last link in the chain that binds the animal kingdom to the vegetable kingdom (Fig. 25 at 4).  For after the nitrifying organisms have oxidized nitrogen cleavage products, the results of the oxidation in the form of nitrates or nitric acid are left in the soil, and may now be seized upon by the roots of plants, and begin once more their journey around the food cycle.  In this way it will be seen that while plants, by building up compounds, form the connecting link between the soil and animal life, bacteria in the other half of the cycle, by reducing them again, give us the connecting link between animal life and the soil.  The food cycle would be as incomplete without the agency of bacterial life as it would be without the agency of plant life.