Gluten and fibrine are distinguished from starch and fat by containing nitrogen. The nitrogen forms four fifths of the air. It exists also in ammonia, and in aqua fortis, or nitric acid. These two compound bodies exist and are found in the soil; and from the soil the nitrogen is taken up by the plants, and gluten is formed. When the animal consumes the gluten, it builds up and renews the waste of its several parts. The gluten of the plant is transformed into the flesh and tissues of the living animal. ' Thus the nitrogen of the soil through the plant has reached the dignity of being a part of breathing, intellectual man.

Having reached this form, this restless element, nitrogen, grows weary of inactivity and moves on. Not only the living body as a whole is in constant motion, but even its minutest parts are continually active. "They are like the population of a great city moving to and fro, coming and going continually, weeded out and removed every hour by deaths and departures, yet as unceasingly kept up in numbers by new in-comers, changing from day to day so insensibly as to escape observation, yet so evidently that after a few years scarcely a known face can be discovered among the congregated thousands." Scarcely has the gluten of the plant been fitted comfortably to its place in the muscle, the skin, or the hair of the animal, when it begins to be dissolved again and is removed.

The living animal absorbs much oxygen from the air by its lungs. One part of it converts the carbon of the food into carbonic acid; another portion is built into the substance of the body; a large part is employed in dissolving and removing the waste in urea, uric acid, etc. This urea and uric acid return to the soil from which the nitrogen they contain originally came, and where they are reconverted into ammonia, nitric acid, and other substances, and are now ready to enter into new roots and go the rounds again.

The undigested gluten is rejected in the animal droppings, mingles with the soil, and is changed to ammonia and nitric acid. The vegetables which die undergo natural decay, and again enter the soil and air as elements. Animal bodies themselves die at last, and the nitrogen they contain is made to assume the forms in which plants are able to take it up. The ammonia and volatile compounds of nitrogen, produced by animal and vegetable decay, rise in the form of gas or vapor, and escape into the air. The rains of heaven wash the ammonia out and bring it back to the earth. Part of it is, however, resolved into elementary nitrogen, and is thus lost to plants. To make up for this loss, nitric acid is formed in the air in small quantities.

Through the agency of the electric currents the oxygen and nitrogen of the air unite to form nitric acid. Ammonia is given off by volcauoes, and is returned to the earth by the rains and snows.