of the plant, has attained to its highest dignity as a part of the body of breathing and intellectual man.

But having attained this most perfect form, the restless elements soon grow weary, so to speak, of their new dignity. Not only is the living body in constant movement as a whole, but all its parts, even the minutest, are in perpetual motion. They are like the population of a great city, moving to and fro, coming and going continually, weeded out and removed hour after hour by deaths and departures, yet as unceasingly kept up in numbers by new incomers;changing from day to day so insensibly as to escape observation, yet so evidently, that after the lapse of a few years, scarcely a known face can be discovered among congregated thousands. And so rapid is the tear and wear of the animal machine, to change our figure, in consequence of this incessant movement, that the repairs which are constantly called. for are said to renovate the whole frame-work in less than a month. Every wheel in that short space is renewed. New materials are brought in for the purpose, while the old are thrown away and rejected. Scarcely has the gluten of the plant been comfortably fitted into its place in the muscle, the skin, or the hair of the animal, when it begins forthwith to be dissolved out again-to be decomposed and removed. from the body. Restlessness, beyond our control, is thus inherent in the very matter of which we are formed.

A brief summary will show how and in what forms this taking down and removal of the bodily substance is so rapidly effected.

The living animal absorbs much oxygen from the air by its lungs. One portion of this oxygen is employed to convert the carbon of a certain part of its food into carbonic acid; another portion is built into the substance of the body itself (p. 565); but a large proportion also is employed in

*See WHAT, HOW, AND WHY WE DIGEST.

CIRCULATION OF NITROGEN.

637

dissolving out and removing the waste, and now worthless, matter of the muscles and other tissues. This inhaled oxy.

gen is, in fact, the agent through which the change of matter is effected. The muscle, for example, combines with oxygen, and after several intermediate transformations, is finally changed into substances called urea, uric acid, &c., which pass away through the kidneys. This urea and uric acid return to the soil, from which the nitrogen they contain originally came. There they are gradually converted into ammonia, nitric acid, and other substances such as the plant roots originally took up, and which, now re-formed, are ready again to enter into new roots, and thus to recommence the same round of change.

But the animal does not extract and work up all the gluten of the vegetable food it eats. A part of it escapes digestion, and is rejected in the animal droppings. This mingles with the soil, and there, like the urea, &c., is changed into ammonia and nitric acid. The same happens to the gluten of vegetables which die, and, without entering the stomach, undergo direct natural decay in the air or in the soil. Animal bodies themselves die also at last, and, like the vegetable gluten, pass through those successive changes. which we call putrefaction and decay. As the result of these changes, the nitrogen they contain is again made to assume those forms in which plants are able to take it up, and to convert it into their own substance.

Thus, after various turns of the wheel, all the nitrogen. that entered the plant in the form of ammonia, nitric acid, and similar available compounds, returns again to the soil in one or other of the same states. Some of the matter revolves a time or two less, returning at once from the plant to the soil without passing through the animal at all, or at once from the muscle to the soil without undergoing the ordeal of the kidneys-but whether it runs one, two, or

three heats, all arrives, sooner or later, at the same goal, ready to start again on the same race. A bird's-eye view of this circulation is presented in the following scheme :

Thus we end where we began-the soil, the plant, and the animal being involved in one never-ceasing, mutuallydependent revolution. We need scarcely concern ourselves, therefore, for the destiny of the organic part-the tissues and blood of our bodies. Its fate is decided by fixed and unerring laws. When it has served our purpose, new and immediate uses await it. We attempt in vain to detain it from pre-determined labours, or, by the arts of the embalmer, to compel it to perpetuate a loved and honoured form. We need not wait even, as in Hamlet's supposition, for the body to crumble into dust. The fluids and tissues decompose rapidly, and are quickly dissipated, so that what is now part of the body of a Cæsar or a Venus, may literally within a week become part of a turnip or of a potato.

Even here, however, or in respect to this organic form of matter, we obtain occasional glimpses of a still wider circle. While the same portion of matter, on the whole, goes round and round unceasingly, as we have described, a certain portion of the ammonia and other volatile compounds of nitrogen, which are produced by decaying animal and vegetable substances, rises in the form of gas or vapour, and escapes into the air. It rises also in unknown quantity from the lungs and skins of animals, in their breath and perspira

A WIDER CIRCLE.

639 tion. This ammonia the rains of heaven wash out and bring back again to the earth-thus restoring it to the soil from which it originally came, and to the wants of vegetable life. But these very rains also carry down a portion of it directly into the sea, and, through the rivers, sweep it from the land. Yearly, also, a part of the ammonia, nitric acid, and other similar compounds, is by natural operations resolved into elementary nitrogen, and is thus lost to living plants.

To make up for this waste, nitric acid is continually formed in the air in minute quantity. The nitrogen and oxygen of the atmosphere unite to form this acid through the agency chiefly of electric currents, which are continually passing through the air. Ammonia also is given off into the atmosphere from all living volcanoes; and both of these compound substances the falling rain dissolves and carries earthward, so that the failing supplies of nitrogen, in an available form of combination, are continually kept up. Thus, from the great atmospheric reservoir a stream of nitrogen of unknown bulk flows down yearly to the earth in the forms of nitric acid and ammonia, while a similar stream returns again yearly to the air in the form of elementary gas, after having probably many times gone through the cycle of changes in which gluten and fibrin take a part. Within what conceivable time could the nitrogen of the whole atmosphere take part in this slow circulation?

CHAPTER XXXIII.

THE CIRCULATION OF MATTER,

A RECAPITULATION.

Circulation of mineral matter.-General form of this circulation from the soil through the plant into the animal, and thence to the soil again.-Special form.-Circulation of phosphoric acid and of saline matter.-Shedding of leaves and annual decay of vegetable productions.-Course of mineral matter through the animal body.Waste and death of the body, and its return to the soil.-General view of this circulation. Its constancy and rapidity.-Vain attempts to preserve human dust apart.-Mummies, pyramids, and Etruscan tombs.-The Valley of Hinnom.— Customs in Thibet and the Himalayas.-How the natural diminution of mineral plant-food is replaced.-Interference of slow geological revolutions.-Lessons taught by all this.-Sinall quantity of matter on which all life depends.-Lesson of constant, intelligent activity with a view to a definite end.-Purposes served by every movement of matter in living bodies.-How the plant waits upon and serves the animal. Small change in the condition of things which would banish life from the world.-Man forms no part of the scheme of the universe.-His insignificance the crowning lesson.

IV. THE CIRCULATION OF MINERAL MATTER.-We must now trace the revolutions through which the dust also -the earthy, inorganic, incombustible, or mineral part of the animal-passes.

When a portion of a plant is burned in the air, the organic or combustible part is dissipated, and disappears; but a small quantity of ash or mineral matter remains behind. The wood-ash left when trees are burned is a familiar example of this. In like manner, when any part of an