From the Eclectic Review. THE BALANCE OF NATURE. It is our present design to point out, in some detail, one or two of the great fundamental laws and arrrangements of the physical world, which, although from their familiarity we are too apt seldom to dwell upon their contemplation, are in truth radically connected with the stability of our world and the maintenance of that "balance of Nature," as a consequence of which the face of our globe is maintained in a state of enduring fruitfulness and beauty. It is no paradox to say, that in the present state of things, life could not exist without death, and that the correlative processes of vitality and decay, production and corruption, formation and destruction, are so intimately connected and so mutually dependent, that their reciprocal and compensatory action alone maintains the equilibrium of our natural system and the essential conditions of its permanent well-being. leaf of the oak or the painted wing of the insect-but no atom is lost, no single molecule destroyed; and the very signs and tokens of his dissolution, the corruption or decay of the frame once fair or strong, are, despite the innate sense of dread which they inspire, and which the upward look can alone control, but parts of a gigantic system, wherein death is the very condition of life, in itself most beautiful and wise. Not only matter but even force appears to be indestructible, and when it ceases to be appreciable by the senses which usually take cognizance of it, is simply subdivided or resolved into different modes of action, from which the original and normal force may be in turn reproduced. The subject of the "correlation of the physical forces" has been ably treated by Mr. Grove, who has demonstrated that all the ordinary affections of matter, which form the chief objects of We have used the word destruction in experimental physics-as motion, heat, its ordinary acceptation; but before en- electricity, light and chemical affinitytering upon the main part of our subject, are mutually convertible and reconvertible, let us briefly pursue the train of thought each being capable of generating all the which it suggests. It must not be forgot-rest, and that, although from the extreme ten that in truth we neither know, nor can in any degree conceive of, such a thing as destruction in our universe. In common parlance we speak, indeed, of the destruction of a city, if its buildings are despoiled by an invading army or engulfed in the convulsions of the earthquake, while in reality the materials, the atoms, so to speak, of which it was built up, are but scattered hither and thither to be rearranged in other forms. The marble column or the granite slab may be disintegrated and dispersed, till it becomes, perhaps, the sand of the seashore or the dust of the highway; yet no single particle ceases to exist, or is, in any proper sense, destroyed. "Man decays and ages move," and in the course of years or centuries the very fibers and tissues of his body, the sinews which have toiled, and the brain that has thought, may reappear in the emerald delicacy of the practical researches necessary, and from the mystery which still hangs round many of the occult phenomena of the "imponderable agents," the subject must be regarded as yet in its infancy, science is steadily progressing towards the establishment of direct quantitative relations between them. Taking as a starting-point the simplest affection of matter, motion, the apparent effect of impeded or arrested motion is the statistical condition of rest, while in some cases its real result is merely a subdivision of the force into channels so minute that they cease to be appreciable to the usual modes of observation; but wherever a real cessation of motion results, a new mode of action is generated, in the shape of heat-the intensity of which is proportional to the amount of motion merged, or in other words to the friction. This is the most familiar of all the cases in which one force generates or is converted into another. The savage who lights his fire by rubbing together two dry sticks, illustrates it; the heated axle-tree of a railway carriage is an equally simple instance. Mr. Joule has reduced the correlation of heat and mechanical force to a numerical expression, obtained in experiments upon the boring of metal cylinders, such as cannon, under a known application of force. He arrives at the conclusion that a force equal to the fall of seven hundred and seventy pounds through a space of one foot generates sufficient heat to raise the temperature of one pound of water by one degree of our ordinary thermometrical scale. If, instead of being homogeneous, the bodies between which friction takes place are of different kinds, not only heat, but, more or less, electricity is generated -the principle upon which the common electrical machine is constructed, and the familiar experiment is based, in which a stick of sealing-wax, simply rubbed upon a cloth or the coat-sleeve, acquires the power of lifting or attracting small pieces of paper, or other light material. Through electricity, heat likewise readily produces magnetism; this new force possessing the strange peculiarity that it is always developed in lines perpendicular to the direction of the electrical currents which cause it. It is equally evident that motion may be made to produce light, whether indirectly, through the medium of electri city, or through that of heat, induced by prolonged friction: indeed, heat and light appear to be so intimately connected, and so closely parallel in their phenomena one to the other, that they might, perhaps, be consistently regarded rather as modifications, or, possibly, degrees of the same force, than as distinct and separate forces. All that between masses heat exerts a power of mutual repulsion, involving motion; but it is in its action upon the molecules or ultimate particles of matter, that heat most palpably produces motion. bodies, when heated, expand, whether solids, liquids, or gases; the two former classes of substances being respectively converted by a sufficient degree of heat into the superior forms of liquids and gases. It is true that many solids are not fused, while many others, which can be melted, are not volatilized by any known heat; but there can be little doubt that, could we command a heat of sufficient intensity, the most refractory solids would be forced to assume in turn the condition of liquids and gases. Basalt fuses in the blast-furnace, but we have no means of volatilizing it; yet the intense heat of the electric arc in vacuo converts iron into a vapor-into iron steam, in fact; and were the like method applicable to the former substance, corresponding results would possibly ensue. The motion produced by expansion of liquids is sufficiently exhibited in the ordinary thermometer, where it is made the actual index of the heat which produces it; while the gigantic power of steam, and the conversion of its molecular motion into that of masses, need no comment. Heat, then, produces motion, and through motion all the other forms of action; while electricity is also its immediate and direct result, and magnetism and chemical affinity, if not produced, are modified by it in a striking manner. In a similar manner, Mr. Grove demonstrates that electricity, light, and chemical affinity may each become the source of all the other forms of physical force, and traces the principle through those wonderful phases of action, to which the mutual influences of magnetism and electricity have given birth, as well as through the more familiar phenomena of photogenic and voltaic action. Motion, further, as generating heat and electricity, which in turn produce chemical action, may be regarded as creating this last species of force; and, finally, Space does not permit us to follow him motion itself may be directly reproduced into the details of these interesting subby each of the forces which have them.jects; and we shall only refer, as illustraselves emanated from it. tive of the correlation of electricity and If now we take heat as the starting chemical affinity, to the brilliant discoverpoint, it will be found capable of deter-ies of Davy, who, by means of these forces, mining all the other forms of action. First decomposed the alkalies, and for the first of all, motion is an invariable effect of time demonstrated their true composition heat. Many experiments appear to prove as oxides of those metals; one of which, at least, in connection with the recent | Causation is the will, creation the act of production of aluminium, has assumed a God." high degree of general, as well as purely scientific, interest. An experiment devised by Mr. Grove exhibits in so beautiful a manner the production of all the other modes of force by light, that we are induced to quote his own concise description of it: "A prepared daguerreotype plate is inclosed in a box filled with water, having a glass front, with a shutter over it. Between this glass and the plate is a gridiron of silver wire. The plate is conThe plate is connected with one extremity of a galvanometer coil, and the gridiron of wire with one extremity of a Breguet's helix, an elegant instument, formed by a coil of two metals, the unequal expansion of which indicates slight changes in temperature: the other extremities of the galvanometer and helix are connected by a wire, and the needles brought to zero. As soon as a beam of either daylight or the oxyhydrogen light is, by raising the shutter, permitted to impinge upon the plate, the needles are deflected. Thus, light being the initiating force, we get chemical action on the plate, electricity circulating through the wires, magnetism in the coil, heat in the helix, and motion in the needles." Photography, that new and beautiful art, whose many triumphs already achieved are but the harbingers of more varied and extensive applications, affords striking examples of the intimate, and as yet mysterious, connection of light with chemical action, from which force all the others may be with facility envolved; while the remarkable researches of Faraday upon the effect of heat, upon magnetic and diamagnetic substances, and of magnetism upon polarized light, and the more recent discoveries of Stokes and Becquerel in connection with phosphorescent and fluorescent bodies, forcibly convince us of the littleness of our real knowledge upon these subjects, and remind us that the ground as yet brought within the domain of physical research is but an insignificant spot of the infinite field of unknown territory which still awaits the exploring eye of science. "In all these phenomena," writes Mr. Grove, "the more closely they are investigated, the more are we convinced that, humanly speaking, neither matter nor force can be created or annihilated, and then an essential cause is unattainable. These facts reveal the exquisite balance maintained in the actions of the imponderable agents of the universe, each passing readily into the other, so that each necessary force is sustained in its universality and efficiency. It is, however, to that vast series of phenomena which exhibit the intimate connection or "correlation” which subsists between the three great kingdoms of the mineral, vegetable, and animal world, that we wish particularly to refer; their mutual relations in that great cycle of changes, the interruption of which would reduce the face of our globe to a desert as uninhabitable, at least to the higher tribes of organic life, as when the trilobite and the lingula were the sole tenants of the seas which rolled around a primeval world. Consider, briefly, the chief functions of plants and animals, the sources of their food, and the character of their assimilations and excreta. Thrust any plant into the fire, and observe first the crackling and spitting which indicates the dissipation of the water which has filled its pores and sap vessels: notice then that it takes fire, and burns until nothing is left but a diminutive gray ash. Now this ash consists of the mineral substances which the plant has, during its growth, abstracted from the soil, and which, although minute as compared with the original bulk of the plant, constitute a large amount in the aggregate of a thick and serried crop. Thus, a crop of potatoes withdraws from the soil full two hundred pounds of mineral matter per acre, while a crop of beet-root abstracts five hundred, and one of turnips six hundred and fifty pounds. Now this large amount of mineral material is wholly derived from the soil, of which it has become a constituent part by that long process of degradation, or wearing away, whereby the hardest rocks which formed the surface of our primeval world have been disintegrated and reduced to powder by the ceaseless action of rain, frosts, storms, and currents, during countless ages of past time. The organic constituents of the plant, the parts burnt away in the fire, consist chiefly of carbon, hydrogen, and nitrogen; but it is with the carbon, as by far their largest component, that we are chiefly to do. Whence does the plant derive its supplies of these elements? A moment's Although carbonic acid is one of the most stable substances known to the chemist, and decomposible in the laboratory by only the most potent agents, the leaves of plants are endowed with the wonderful power of effecting this decomposition with the utmost facility, assimilating the carbon with their own tissues, and returning the oxygen to the air. consideration will suffice to show their of our own country may possibly be great origin. Our atmosphere consists essen-repositories of matter which may, in the tially of a definite mixture of the two course of centuries, constitute the staple gases, oxygen and nitrogen. Air has fuel-stores of England? been brought from a hight of twenty-one thousand feet above the sea level, from the summits of Chimborazo and Mont Blanc, from the banks of the Nile and the sandy plains of Sahara, from the most desolate wilds and the most densely peopled cities, from the recesses of the "forest primeval" and the swarming alleys of London and Paris; and analysis has proved it to contain, with very minute local exceptions, the same invariable proportions. In addition, there are present, as unfailing constituents, ammonia in minute quantity, and carbonic acid-that gas which gives their effervescence to sparkling wines, and when respired acts as a deadly poison to animal life, to the extent of three to six parts in ten thousand. Minute as is the quantity of carbonic acid thus distributed through the air, it would, if collected into one layer upon the earth's surface, form a stratum full eight feet deep. Under the influence of the sun's rays, this gas is highly favorable to the growth of plants, and it is in fact from this apparently small amount that the vast mass of vegetation derives nearly its entire supply of carbon. Were the whole surface of the earth a continuous meadow, from each hectare (two and a half acres) of which 100 cwt. of hay was yearly reaped, these meadow plants would, in twenty-one to twentytwo years, exhaust the whole of the carbonic acid in the air, and the whole living creation would at the same time come to an end. The air would no longer support plants—that is, would no longer furnish them with an indispensable condition of life. But the functions of animal are precisely the converse of those of vegetables; for the animal takes oxygen from the air, and burning by its means the carbon of his food, exhales carbonic acid as a product of the combustion, by virtue of the functions of respiration, with which our readers are already familiar. The extent The gigantic trees which adorn the and importance of its results can scarcely forests of tropical regions, with their be overrated, when a simple calculation, serried undergrowth of matted creepers, based upon the average amount of food the dense pine woods of more northern consumed per diem by adults and children zones, and the abundant though less con- respectively, gives the vast sum total of spicuous vegetation of temperate climes, nearly two thousand tons of carbonic acid all derive their stock of carbon from this expired daily from the lungs of the insmall but essential constituent of the at-habitants of London; a quantity sufficient mosphere. It is highly probable that a to bury the entire population six feet far larger proportion of this gas impreg- deep in its poisonous vapors in less than nated the air at that period when the profive years. fuse and remarkable flora of the Carboniferous epoch clothed the earth with its forms of luxuriant beauty. The animal remains of that epoch belong wholly to orders of beings whose existence was compatible with this state of things, while this same circumstance would strongly favor "the accumulation of vegetable débris in extensive beds," such as compose our present coal-fields. May we not reasonably add, that in the gigantic vegetation of existing tropical forests we may perhaps trace a source which, is to compensate in the lapse of ages for the exhaustion of the stock of fuel of other regions; and that the peat bogs A vast addition to this amount is made by the respiration of domestic animals, beasts of burden, etc.; while a quantity, probably far larger than is contributed by the above sources, is evolved from the innumerable fires and furnaces of the metropolis, every one of which is carrying on in itself the same process which, in a less intense form, is incessantly performed in the human lungs. Now the whole of this gigantic system of "expiration" is but a restoration to the atmosphere of the carbon which has been first extracted from it by plants, whether by the pines and tree ferns of the Carboniferous epoch, and since stored 116 up for man's use in the form of coal, or by the grass, corn, and other vegetable products which, directly or indirectly, become the food of all extant animals; again to be extracted from the air by fresh generations of plants, and through them to become the food of a like succession of animals. Thus is perpetuated that endless cycle of metamorphoses which Infinite Wisdom has ordained for the maintenance of perfect equilibrium between all parts of the natural world. In a like manner the ultimate source of the nitrogen which plants consume is invariably the air; and with equal certainty does the decomposition of vege. tables, and of the various animal products into which they are metamorphosed, finally restore this essential constituent to the atmosphere whence it has been originally abstracted. Nor are the mineral substances which plants extract from the ground, returned with less precision to the soil, save in so far as the necessities or usages of society are brought into direct antagonism with the laws of nature. On which side victory must lie, and what is the certain issue of a struggle between the demands of a natural law and the indifference or the ignorance of man-impoverished lands, diseased crops, and periodic visitations of disease, sufficiently proclaim. "The perpetual round of changes which the Creator has traced out, may, to a limited extent, be influenced by man, but it goes on without him." The case in which vegetation matures and decays upon the spot where it is grown, is self-explanatory; nor does it in any degree affect the questions of practical agriculture or social economy; and that in which the dead bodies of animals themselves become incorporated in the soil upon which they have lived, comes under the same category. It is in the excreta of the living animals, those portions of the food which they neither assimilate nor expire as carbonic acid, that we are to seek the great source of compensation for the mineral substances abstracted from the soil by all tribes of vegetation. The subject may be one which ignorance has too often dismissed with indifference or aversion, but it constitutes, nevertheless, a problem of momentous national and social importance. "Even now, Great Britain consumes nearly nine tenths of all the guano brought to Europe. In the actual position of English agriculture, America, by her guano beds, rules the prices of all the corn markets in Europe, and more especially of England; and should circumstances ever arise to prevent the importation of guano into England, a state of things would ensue of which the consequences might be incalculable. Bloody wars have sometimes sprung from causes of much less importance." (Liebig: Modern Agriculture.) Putting out of the question that part of the problem which relates to domestic animals and farm stock, the rejected mineral constituents of whose food generally go directly to fertilize the spot upon which they are fed, consider for a moment that portion which affects more directly the great masses of food-consumers collected in our towns and cities. The bread, meat, and other food, which forms the daily nutriment of London, are collected by a huge and complex system of supply from a vast acreage of ground in the adjacent counties, and represent a prodigious amount of mineral matter removed from their arable and pasture lands. The annual fluid and solid excreta of a million inhabitants of large cities, contain at least 10,300,000 lbs. of mineral substances, mostly the ash-constituents of bread and meat. "This enormous drain of these matters from the land to towns, has been going on for centuries, and is still going on, year after year, without any part of the mineral elements thus removed ever being restored to it." The presence of these substances being necessary to the growth of successive crops, their abstraction, if not compensated by other means, induces a gradual impoverishment of the whole area of cultivation, which is slowly, but with inevitable certainty, leading to a time when the fiels shall refuse to yield to the cultivator any profitable return at all. This compensation the more intelligent and richer farmer effects at great cost by the purchase of guano and other fertilizing matters, the supply of which is precarious and transient, while his ignorant or poorer neighbor leaves things to take their chance as best they may. "The law of compensation which makes the recurrence or permanency of |