In what does this differ from the problem of modern science? It is true that Comte would limit positive science to "the study of phenomena in their orders of co-existence, resemblance, and succession," an idea which the word "positive" by no means conveys. And Tyndall asserts that "the man of science, if he confine himself within his own limits, will give no answer to the question" as to the origin of things. At the same time he admits that "he can clearly show that the present state of things may be derivative."[136] The great masters of science, however, refuse to acknowledge any such arbitrary limitations. "The essence of science," says Sir William Thomson, "consists in inferring antecedent conditions, and anticipating future evolutions from phenomena which have actually come under observation."[137] If this be the essence of science, then we presume that it is competent to throw some light on the primitive condition of the universe, and give some prevision of its future destiny. Did not Comte himself teach that the solar system was once all nebula, and that it will yet collapse into an exhausted and extinguished sun?[138] Is it true, then, that physical science by its inductive inference of "antecedent conditions," does really furnish a solid confirmation of the à priori and native conviction of our race that the universe had a beginning? Then most assuredly even physical science is carrying us forward toward the ultimate unity of all truth—a unity which can be realized perfectly only by the constant mutual determination of à priori and empirical knowledge, a synthesis and equipoise of physical and metaphysical truths.
This is the most obvious tendency of modern science in its relation to the question under consideration. Nothing is more remarkable in the present aspect of physical research than what has been aptly called "the transcendental character of its results." As George Henry Lewes observes, "the fundamental ideas of modern science are as transcendental as any of the axioms of ancient philosophy."[139] Palætiological science in general has advanced by sure and steady steps, through careful observation and experiment, inductive inference, and the application of exact mathematical calculus to the recognition of the truth long ago announced by Paul: "The things which are seen are temporal, the things which are not seen are eternal." Dynamical Geology, Astronomical Palætiology, Cosmogony, Molecular Physics, Abstract Dynamics, have all landed in the same inevitable conclusion that "the existing order of things had a beginning." Sir William Thomson's doctrine of the "Dissipation of Energy" leads us, by sure steps of deductive reasoning, to the necessary future of the universe—necessary, that is, if physical laws remain unchanged—"so it enables us distinctly to say that the present order of things has not been evolved through infinite past time by the agency of laws now at work, but must have had a distinctive beginning, a state beyond which we are totally unable to penetrate—a state which must have been produced by other than the now acting causes."[140]
The science of Geology reduces all terrestrial phenomena to the great law of finite duration. If there be one scientific induction which may be fairly pronounced legitimate and irrefragable, it is this one—that the existing terrestrial economy had a beginning. "All organic existence, recent or extinct, vegetable or animal, had a beginning; there was a time when they were not. The geologist can indicate that time, if not by years, at least by periods, and show what were its relations to the periods that went before and that came after." He can carry us back to the time when man did not exist upon the earth, when no mammals existed; to the time when no birds, no reptiles, no fishes existed—when even Huxley's protoplasm had no being; "when all creation, from its centre to its circumference, was a creation of dead inorganic matter,"[141] and when there was not one spore or monad or atom of life throughout its dark domain. The form of the earth itself clearly reveals its history, and points us to that beginning. Its bulging equator and flattened poles, its pavement of congealed lava, which in some cases we name granite; nay, the oldest water-worn pavement composed of the detritus of the igneous rocks—all attest the emergence of our planet from a molten condition, and a temperature[142] in which no life could exist; so that even Tyndall admits "there are the strongest grounds for believing that during a certain period of its history the earth was not, nor was it fit to be, the theatre of life."[143]
The earth was once a molten mass heated to incandescence—a self-luminous globe. On this point there is scarcely any difference of opinion among scientific men. Furthermore, a large majority of modern scientists regard themselves as justified in the affirmation of a still anterior nebulous condition. If the nebular hypothesis is accepted, then we are required to contemplate a period when the earth did not exist, and when even the matter which now enters into its constitution was an undistinguished part of the nebula from which the whole solar system was evolved.
Many exact observations and mathematical computations as to the secular cooling of the earth give results which are in strict accordance with this theory of its primitive igneous condition. The observed facts clearly indicate that the earth is becoming, on the whole, cooler from age to age, and that the natural current of events is carrying it inevitably to a state of total refrigeration.[144] The fossil remains now found within the arctic circle indicate that at a period, not extremely remote, tropical vegetation flourished, and forms of animal life subsisted there which are now confined to the torrid zone. Mammoths lived in the now uninhabited polar regions, and tree-ferns and the tropical shell-fish found there a home.[145] The surface of the earth was then warmed by internal heat which since that period has waned; that heat has been gradually dissipated in the surrounding space, as a red-hot ball suspended even in the warm air of a room must, according to the well-known laws of radiation and absorption, necessarily part with its heat.
Many experiments carefully conducted in our time show that the temperature of the earth increases with the depth to which we penetrate: "In boring for the artesian well at Grenelle, which is 546 metres deep, it was observed that the temperature augmented at the rate of 1° Centigrade for every 30 metres. The same result was obtained by observations in the artesian well at Mondorf, in Luxemburg; this well is 671 metres in depth, and its waters 34° warm." As the result of many investigations in mines and borings, Sir William Thomson concludes that the average inference may be thus stated—there is on the whole about 1° Fahr. of elevation of temperature per 50 British feet of descent.[146] If this increase is uniform—and we have no reason to suppose the contrary—then at the depth of 50 miles there exists, says Helmholtz, a heat sufficient to fuse all our minerals.
The fact that the temperature of the earth increases with the depth necessarily involves a continual loss of heat from its interior by conduction outward into and through the upper crust, according to a well-known law of equilibrium of temperatures. "Hence, since the upper crust does not become hotter from year to year, there must be a secular loss of heat from the earth."[147] Thus it appears that from the surface of the earth and the ocean, from thermal springs, and from three hundred active volcanoes, the internal heat of the globe is incessantly radiated into space and is practically lost.
Now this average loss of heat may be at least approximately measured, and data are thereby furnished for determining the probable age of the earth, or, perhaps more correctly, its phase of life. If a man were to find a hot ball of iron suspended in a room, and if he were carefully to observe the distribution of heat in the ball, he would be able easily to determine whether the ball were becoming hotter or cooler. If he found that the inside were hotter than the outside, he would conclude that the ball was cooling, and had therefore been hotter than when he found it. So far common-sense would be his guide; but with the aid of mathematics, and some knowledge of the physical properties of iron and air, he could go much further, and be able to calculate how hot the ball must have been at any given moment, if it had not been interfered with. Thus he would be able to say, the ball must have been hung up less than, say, five hours ago, for at that time the heat of the metal would have been such that it would have been in a state of fusion, and hence not capable of hanging as a solid mass. Precisely analogous reasoning holds with regard to the earth: it is such a ball; it is hotter inside than outside. The distribution of the heat near its surface is approximately known—1° Fahr. of elevation in temperature for 50 British feet of descent.[148] The properties of the matter of which it is composed are approximately known. The temperature at which granite rocks are fusible has been found to be about 7000° Fahr. This must therefore have been the temperature of the earth in its primitive igneous condition. From these data, Sir William Thomson has, by rigid mathematical calculations, reached the conclusion that the consolidation of the earth's crust commenced 98,000,000 years ago.[149] The rates of increase of temperature inward in a great amount of average rock at various periods after the commencement of cooling, from the primitive heat of 7000° Fahr., are estimated by Sir William Thomson as follows:
"At 10,000 y'rs after commencement of cooling we should have 2° per ft.
At 40,000 " " " " 1° "
At 160,000 " " " " 1/2° "
At 4,000,000 " " " " 1/10° "
At 100,000,000 " " " " 1/50° "