Popular Science Monthly/Volume 4/February 1874/The Dissipation of Energy
By H. F. WALLING.
THE dissipation of energy is a continuous process, quite familiar to mankind in its main features and results, since the days of the ancient philosophers. It was recognized by them that all mechanical motions, being dissipated by friction, gradually diminish, and must finally cease unless maintained by external power. In the language of modern science, the motion which thus disappears is converted from molar into molecular motion.
It may be added that molecular energy, existing mainly in the form called heat, tends to equalization of dynamic equilibrium, after the attainment of which it is powerless to produce molar or mechanical motion, a reconversion from the condition of equilibrium being impossible.
Accordingly, the power to produce mechanical motion, exerted by the heat of the sun, which is being lavished with such prodigious prodigality, can only last while the sun continues to be hotter than the other bodies in space. At present it is well understood that all terrestrial motive power is derived from this source with the single unimportant exception of that obtained from the tides, at the expense of the earth's energy of rotation. Among the more obvious processes of conversion of the sun's molecular into terrestrial molar motion, are the expansion and contraction of the atmosphere, the evaporation and condensation of water, and the less direct method by restoration of potential chemical energy accomplished in vegetation, whence are produced food and fuel.
But it is supposed that the sun will finally grow cold, and that the resistance of the ethereal medium, the evidence of whose existence is found in the demonstration of the undulatory theory of light, will cause satellites to fall into planets, planets into suns, and suns into one common centre, after which, unless by special interposition of divine power, darkness, silence, and death, will forever prevail.
This gloomy prediction is of course inconsistent with the theory of continuous evolution, which obviously excludes from cosmical economy catastrophes or extensive destructive effects.
A careful consideration, however, of the circumstances which will be likely to accompany the falling of a satellite into its planet may lead to the conclusion that this occurrence will not necessarily be catastrophic. The process must certainly be an exceedingly slow one, no progress in it having been detected throughout all the recorded observations of the moon's motion extending over thousands of years. The only practical evidence which has been adduced to prove the resistance of a medium, namely, a very slight diminution in the period of that nearly evanescent body, Encke's Comet, is very far from being definite and satisfactory. The mass of the moon being enormously greater, it is probable that many millions of years will pass before a diminution of her orbital period from this cause will be perceptible. The immense periods of time attributed to the past processes of geological evolution, and to the supposed metamorphoses of organic life, are therefore very brief when compared with those required for the returns of satellites to their parent orbs, admitting, as theoretical considerations seem to require, that such returns are ultimately inevitable.
The eccentricity being diminished by the resistance of a medium, the moon's orbit would eventually become, and afterward continue, circular, so that final contact would be unaccompanied by violent collision. But, before the time of actual contact, changes of form would be induced both in planet and satellite by mutual attractions, exemplified in the production of daily terrestrial tides. The investigations of Hopkins, Thomson, and recently of Barnard, in regard to tidal and precessional influences, indicate that, even at the present distance of the moon, they must cause elongations and contractions of the solid materials of the earth, which are quite appreciable. A considerable diminution of the distance between the earth and moon would give rise to changes in the form of the earth, and hence to bendings to and fro of its external shell even if the earth were solid throughout. This would be accompanied by earthquakes and kindred disturbances far exceeding in magnitude and destructiveness any thing of the kind now known to man. The frequency of these occurrences would be the same as that of the moon's meridian passage.
Resistances to this tidal action, however, would be developed, in consequence of which the molar motion of rotation would be converted into molecular motion, so long as the angular motion of rotation in either body was different from that of the moon's revolution, until the rotations became synchronous with the revolution, a condition already arrived at in the case of the moon. Synchronism once attained would be permanent, acceleration both of revolution and rotation occurring as the distance diminished, and both at the expense of the potential energy of gravity between the two bodies. Each body presenting the same face to the other, no meridian passage could take place, and hence no tidal action.
But there yet remains to be considered a continually increasing tendency to distortion of form consequent upon approach. This effect would be produced very gradually, being spread over such enormous durations of time. The curious and complicated foldings of the rocks in the Appalachian regions indicate that the solid materials of the earth are sufficiently plastic to allow it to take on any form toward which forces of sufficient magnitude direct it, provided the times be very greatly extended. Hence, considering the extreme slowness of the process, it may be reasonable to conclude that the forms ultimately developed would be identical with those which would be assumed by liquid masses having the same relative positions and velocities.
The determination of these forms is a problem for the mathematicians. In the absence of analysis, no reason is manifest for supposing that the forms of equilibrium would be materially different just before and just after contact. May it not be that the order of change would be a partial reversal of certain supposed processes of the nebular hypothesis? Thus the moon may be gradually elongated into a closed ring which will slowly contract upon the earth as the energy of angular velocity is gradually dissipated by the friction of the medium. In any event there seems to be no good reason to suppose that there will be such a sudden leap in the final osculation or embrace as would result in a catastrophe.
The same considerations apply to the gravitational relations between planets and suns. Other very important relations between these bodies, however, with which organic life is more especially concerned, require attention. One fundamental requisite to all known terrestrial organic life is the conversion, within living bodies, of molecular energy, either into molar motions, or into potential energy which may afterward be thus converted. All living animals and plants, therefore, depend for their existence upon the passage through their bodies, in the movement toward distribution and equalization, of heat, light, and other molecular forces originating in the sun.
The integrity of cosmical evolution in relation to organic life, accordingly, seems to require the maintenance of great central laboratories where molecular disturbances of sufficient intensity and quantity can be continually generated, and their effects distributed throughout the universe. Notwithstanding the enormous expenditure of heat by the sun, its temperature is supposed to have been maintained about the same as at present for a very long period of time in the past, and no reason is manifest why this fixed temperature will not continue for a very long time in the future. Doubtless, operations are going on in the sun which it would be impossible to imitate in terrestrial laboratories. May it not be that the conditions of materials and the circumstances of pressure, chemical affinity, etc., are such, that substances more elementary than our so-called chemical elements are uniting with an energy far exceeding that of any chemical combination we can effect, and so prodigious as to maintain, at comparatively small expenditure of material, the sun's temperature at that enormous degree which marks the dissociation point of the tremendously energetic combination? The duration of the combination or combustion would thus be prolonged to an enormously remote period. At last, when all the potential energy due to this particular reaction became exhausted by the combination of all the special materials required for it, new materials, whose dissociation point had a lower temperature, and which had consequently been prevented from combining previously, would commence upon a similar process of combustion. And so we may suppose combination to follow combination, until, finally, perhaps at a time when the planets, freighted with their living inhabitants, have begun to arrive at the sun's surface, long after the fires of the last combustion have expired, it has itself become a habitable globe, lighted and heated or served by other molecular forces from distant orbs where new conditions cause new chemical combinations, and conversions of newly-developed potential energies.
Finally, giving play to the imagination, may we not suppose further, that, in a universe extended throughout infinite space, processes of concentration similar to those supposed in the nebular hypothesis and supplemented by processes like those here indicated will go on forever, evolving worlds of continually-increasing magnificence, perhaps inhabited by living occupants of inconceivably transcendent and ever-expanding faculties?