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JOHN TYNDALL: THE ENERGY OF NATURE
From "Heat a Mode of Motion," New York, 1897, pp. 526–536.
This piece of exposition, as the analysis below proves, though of almost poetical fervor of imagination, is exact and logical in its thought and structure. I have printed it here, however, chiefly because it illustrates so admirably the way in which strong feeling can raise pure exposition to the level of literature. See the Introduction, p. 59. In its final results the scientific imagination is not very different from the poetic imagination; Mr. Kipling in "MacAndrews' Hymn" has made most stirring and inspiring poetry out of the product of mechanics. Perhaps the difference between the two forms of the imaginative power is that where the scientific man expresses his space and time destroying conceptions in symbols and abstractions which are intelligible only to his fellows, the poet puts his into concrete images which may be felt and understood by all men. This example, moreover, shows the way in which a larger view may illuminate and set glowing facts which to a smaller mind would be merely a mass of figures and formulæ. As I have insisted in the Introduction, no exposition can be thoroughly good which has not some of the warmth and color of a personal reaction to the facts.
The analysis is as follows:
The sun is the source of all the energy of nature.
I. The energy which is stored up by the separation of atoms can be released by combining them.
A. (1) In plants the solar rays decompose the carbonic acid and
(2) Burning reverses the process and sets free the heat, as in the consumption of coal.
B. Animals (1) take the separated atoms from plants, and
(3) The will creates nothing; it only directs.
(4) The contraction of a muscle makes heat and wastes oxygen.
II. (1) Just as water once raised performs work,
(2) so the energy of the sun once stored up is redistributed according to the form of the atoms and molecules.
III. Material combinations do not touch thought.
IV. These generalisations lead to inconceivably sublime conceptions of the universe.
SEVEN-AND-FORTY years ago, the following remarkable passage, bearing upon this subject, was written by Sir John Herschel.1 'The sun's rays are the ultimate source of almost every motion which takes place on the surface of the earth. By its heat are produced all winds, and those disturbances in the electric equilibrium of the atmosphere which gives rise to the phenomena of lightning, and probably also to terrestrial magnetism and the Aurora. By their vivifying action vegetables are enabled to draw support from inorganic matter, and become in their turn the support of animals and man, and the source of those great deposits of dynamical efficiency which are laid up for human use in our coal strata. By them the waters of the sea are made to circulate in vapour through the air, and irrigate the land, producing springs and rivers. By them are produced all disturbances of the chemical equilibrium of the elements of nature, which by a series of compositions and decompositions give rise to new products and originate 1 "Outlines of Astronomy," 1833.
a transfer of materials. Even the slow degradation of the solid constituents of the surface, in which its chief geological change consists, is almost entirely due, on the one hand, to the abrasion of wind or rain. and the alternation of heat and frost; on the other, to the continual beating of sea waves agitated by winds, the results of solar radiation. Tidal action (itself partly due to the sun's agency) exercises here a comparatively slight influence. The effect of oceanic currents (mainly originating in that influence), though slight in abrasion, is powerful in diffusing and transporting the matter abraded; and when we consider the immense transfer of matter so produced, the increase of pressure over large spaces in the bed of the ocean, and diminution over corresponding portions of the land, we are not at a loss to perceive how the elastic force of subterranean fires, thus repressed on the one hand and released on the other, may break forth in points where the resistance is barely adequate to their retention, and thus bring the phenomena of even volcanic activity under the general law of solar influence.'
This fine passage requires but the breadth of recent investigation to convert it into an exposition of the law of the conservation of energy, as applied to both the organic and inorganic world. Late discoveries have taught us that winds and rivers have their definite thermal values, and that, in order to produce their motion, an equivalent amount of solar heat has been consumed. While they exist as winds and rivers, the heat expended in producing them has
ceased to exist, being converted into mechanical motion; but when that motion is arrested, the heat which produced it is restored. A river, in descending from an elevation of 7,720 feet, generates an amount of heat competent to augment its own temperature 10° Fahr., and this amount of heat was abstracted from the sun, in order to lift the matter of the river to the elevation from which it falls. As long as the river continues on the heights, whether in the solid form as a glacier, or in the liquid form as a lake, the heat expended by the sun in lifting it has disappeared from the universe. It has been consumed in the act of lifting. But at the moment that the river starts upon its downward course, and encounters the resistance of its bed, the heat expended in its elevation begins to be restored. The mental eye, indeed, can follow the emission from its source; through the ether as vibratory motion; to the ocean, where it ceases to be vibration, and assumes the potential form, among the molecules of aqueous vapour; to the mountain-top, where the heat absorbed in vaporization is given out in condensation, while that expended by the sun in lifting the water to that elevation is still unrestored. This we find paid back to the last unit by the friction along the river's bed; at the bottom of the cascades where the plunge of the torrent is suddenly arrested; in the warmth of the machinery turned by the river; in the spark from the millstone; beneath the crusher of the miner; in the Alpine saw-mill; in the milk-churn of the châlet; in the supports of the cradle in which the mountaineer,
by water power, rocks his baby to sleep. All the forms of mechanical motion here indicated are simply the parcelling out of an amount of calorific motion, derived originally from the sun; and wherever the mechanical motion is destroyed, or diminished, it is the sun's heat which is restored.
ENERGIES OF PLANTS AND ANIMALS.
We have thus far dealt with the sensible motions and energies which the sun produces and confers; but there are other motions and energies, whose relations are not so obvious. Trees and vegetables grow upon the earth, and when burned they give rise to heat, from which immense quantities of mechanical energy are derived. What is the source of this energy? Let me try to put the answer into plain words. You see this iron rust, produced by the falling together of the atoms of iron and oxygen; you cannot see this transparent carbonic acid gas, which is formed by the union of carbon and oxygen. The atoms thus united resemble a weight resting on the earth; their mutual attraction is satisfied. But as I can wind up the weight, and prepare it for another fall; even so these atoms can be wound up, separated from each other, and thus enabled to repeat the process of combination.
In the building of plants, carbonic acid is the material from which the carbon of the plant is derived, while water is the substance from which it obtains its hydrogen. The solar rays wind up the weight. They sever the united atoms, setting the oxygen free, and allowing the carbon and the hydro