THE BODY WE CHERISH. 333 with the chemistry of our everyday life. It is sufficient for my present purpose, however, to have shown that the molecular mechanism, so to call it, of the body we cherish, is not less wonderful than its anatomical structure-and that though a little more profound and difficult to comprehend, it is not less worthy of being studied by the intelligent, the cultivated, or the reverential mind. CHAPTER XXXI1. THE CIRCULATION OF MATTER, A RECAPITULATION. Employment of matter for successive uses; popular ideas regarding-Shakespeare 3 Hamlet.-Human saltpetre.-The circulation of water.-Ascent of vapour in tropical regions.-Evaporation from the leaves of plants.-Expulsion from the lungs and skin of animals.-Chemical circulation of water.-Circulation of carbon. -Quantity of carbon in the atmosphere; how it is continually renewed.-Decay of shed leaves and bark, and yearly ripening herbage.-Breathing of animals.—— Relations of air, plant, and animal, as regards this carbon.-Burying of carbon in the earth; restoration to the air by the burning of coal.-Carbon confined in limestone rocks; how the earth breathes this out again.-Circulation of nitrogen.— Gluten of plants.-Forms in which nitrogen exists in plants, in the soil, and in animals.-Restlessness of matter within the animal body.-Rapid waste of the tissues; agency of oxygen in this waste.-Production of urea; change of this in the soil.-General scheme of the circulation of nitrogen; we cannot restrain it.— How part of the nitrogen escapes, and revolves in a wider circle. THAT the same portion of matter may, in the operations of nature, be employed for various successive purposes, living and dead, has long been familiar to the popular mind. Philosophers of almost every age have speculated on the changes of matter, and poets have found scope for their imaginations on a subject at once so interesting and so indefinite. It is only from the results of modern scientific investigation, however, that clear and positive ideas have been obtained as to the nature, the necessity, and the connection of these natural changes. We now know not only that matter does constantly change, but that it constantly circulates in a round of unceasing change. It has been shown that the transformations it undergoes are necessary to the existing condition of things; that they take place in a fixed and predetermined order; and that they are again and again renewed in an endlessly revolving succession. There is a degree of rude sublimity in the curious reasoning of Hamlet, when he says: "Alexander died; Alexander was buried. Alexander returneth into dust; the dust is earth; of earth we make loam; and why of that loam, whereto he was converted, might they not stop a beerbarrel? 'Imperial Cæsar, dead and turned to clay, O that that earth which kept the world in awe, And yet the matter-of-fact touch of modern knowledge turns the whole of this into an absurd conceit. The body of man crumbles into a handful of loose dust, it is true; but this dust is not earth, of which we can make loam to stop a gap or flaw withal; and thus, in the incorrectness of his facts, we forget the merits of the poet. More might be made by a true poet of the fact related by Mr. Squier, that the Romish priests at Leon, in Nicaragua, sell the burial-ground around their churches, for the use of their occupants, for periods of from ten to twentyfive years; "at the end of which time the bones, with the earth around them, are removed and sold to the manufacturers of nitre." * So that to the unexpected, warlike, and base use of making "villanous saltpetre," are the best and most peaceful of the Nicaraguan citizens yearly converted. The words of Shakespeare and the fact of Squier may both suggest to us many reflections; but there is nothing * SQUIER'S Nicaragua, vol. i. p. 394. positive in either of them, beyond the meagre moral, that what forms part of the living, cherished, almost worshipped body to-day, may be employed for most unexpected, and what appear most vile, purposes to-morrow. This limited truth formed the substance of all the ancients knew, and of all the moderns could say, until very recently, regarding the changes and future fate of the animal body after the living spirit had left it. But this branch of natural knowledge has been so wonderfully illustrated by the researches of the present and passing generations, that we can now follow the same particle of matter through a long series of successive visible transformations. To-day we can see it living in the plant, to-morrow moving in the animal; next floating as a constituent portion of the thin air, or rippling along as an ingredient of the clear brook; then resting for a while in the lifeless soil, waiting till the opportunity arrives for its commencing a new career. It will not, I believe, be without interest to my readers, after perusing the details of the preceding chapters, if I briefly recapitulate in this place the substance of what has been already stated in regard to the changes of matter;what is the nature of the transformations it undergoes; by what agencies they are brought about; and for what important end. I shall begin with the simple, and advance to the more complicated. I. THE CIRCULATION OF WATER.-The simplest form of the circulation of matter is that which is presented by the watery vapour contained in the atmosphere. From this vapour the dews and rains are formed which refresh the scorched plant and fertilise the earth. The depth of dew which falls we cannot estimate. On summer evenings it appears in hazy mists, and collects on leaf and twig in sparkling pearls; but at early dawn it vanishes again un CIRCULATION OF WATER. 337 measured-partly sucked in by plant and soil, and partly dispelled by the youngest sunbeams. But the yearly rainfall is easily noted. In our island it averages about thirty inches in depth; and in Western Europe generally, it is seldom less than twenty inches. Among our Cumberland mountains in some places a fall of two hundred inches a year is not uncommon; while, among the hills near Calcutta, as much as five hundred and fifty inches sometimes fall within six months. Now, as the whole of the watery vapour in the air, were it to fall at once in the form of rain, would not cover the entire surface of the earth to a depth of more than five inches-(Dr. PROUT)-how repeated must the rise and fall of this watery vapour be! To keep the air always duly moist, and yet to maintain the constant and necessary descent of dew and rain, the invisible rush of water upwards must be both great and constant. The ascent of water in this invisible form is often immediate and obvious, depending solely upon physical causes. But it is often also indirect; and, being the result of chemical or physiological causes, is less generally perceptible. Thus 1o. Water circulates abundantly between earth and air through the agency of purely physical causes. We see this when a summer shower, falling upon our paved streets, is speedily licked up again by the balmy winds, and wafted towards the region of clouds, ready for a new fall. But, on the greatest scale, this form of circulation takes place from the surface of the sea in equatorial regions, heated through the influence of the sun's rays. Thence streams of vapour are continually mounting upwards with the currents of ascending air, and with these they travel north and south till colder climates precipitate them in dew rain, or snow. Returned to the arctic or temperate seas by many running VOL. II.-15 |