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BEFORE the time of the Scotch geologist, James Hut- ton, some 6,000 years was believed to indicate the age of the earth, and, indeed, of the entire universe. The advent of the uniformitarian school of geologists marks a radical departure from the old estimates. The pendulum swings from one extreme to the other. Boundless distances of time were now drawn upon. So great an antiquity of the earth seemed to reveal itself to geologists, as to defy all attempts at measurement. In the further pursuit of Hutton's line of investigation, Playfair and Lyell were unable to discover among the records of the earth and in planetary motion either a beginning or an end of the present order of things. They found no indication of infancy or decaying old age. This convenient doctrine of infinite durability came to be rudely attacked by the physicists. Here, as in the history of the conservation of energy, the earliest investigator is Robert Mayer. To be sure, he did not attempt an estimate of the age of the solar system, but he discussed the preliminary question as to the source of solar heat. As soon as Mayer had convinced himself that energy can not be destroyed, and that the energy of the earth comes mainly from the sun, he began to study what Sir William Herschel had called “the great secret” of the maintenance of solar heat. In 1841, before the publication of his first paper, he asked questions relating to solar heat, in a letter to Baur, “Is it the glowing of the sun? Why does he not cool off? Is it a burning depending upon falling meteoric stones?” In 1846 he had a paper ready on this subject. Being reminded by a friend that no one can be a prophet in his own country, he sent the paper to the Academy of Sciences in Paris. A committee of the academy was directed to report on this paper, but it failed to do so and the paper was ignored. It could be published only at his own expense. It appeared in 1848 under the title, “Celestial Dynamics.” Mayer concludes that the sun can not be a glowing mass, sending out radiation without compensation; solar heat can not be due entirely to chemical changes; solar heat can not be due to solar rotation. He finally embraces the theory that solar heat is due to the energy of meteors falling into the sun. He did not overlook the fact that the resulting increase of mass of the sun would increase its attraction for the planets, and would shorten the sidereal year. He knew that observation does not disclose any variation in the length of the year. An easy explanation would be offered by Newton's corpuscular theory of light, according to which the sun sends out matter into space. But this theory was then known to be untenable. In this dilemma Mayer takes refuge in an idea which rests on a misconception of the undulatory theory of light, and he offers an explanation which is now easily recognized as invalid. From Mayer we pass to William Thomson, the late Lord Kelvin, who, six years later, took up the very same problem and arrived independently at almost identically the same conclusions. That solar heat may be due to falling meteors was first suggested in England by Waterston. Unlike Mayer, Thomson sees no objection to the increase in the sun's mass resulting from meteoric showers, for, “according to the form of the gravitation theory” which he proposed, “the added matter is drawn from a space where it acts on the planets with very nearly the same forces as when incorporated in the sun.” In an appendix to the paper, Thomson ventures an estimate of the age of the sun. This is the first attempt, made by a physicist, to compute the age of our great luminary and to prepare a mortuary estimate of it. He goes on the supposition that the solar energy of rotation is derived from the energy of falling meteors. He calculates that, allowing for the constant loss of solar energy by radiation, the sun could acquire its present energy of rotation in thirty-two thousand years. From an estimate of the limited amount of meteoric matter available near the sun, he concludes that “sunlight can not last as at present for three hundred thousand years.” This calculation of the year 1854 attracted no attention at the time. Later, the theory was abandoned by its author. Evidently the theory of solar heat was still in a very crude form. But important new ideas were brought into view in the same year, 1854, by Helmholtz, in a popular lecture at Kiinigsberg, delivered on the occasion of the Kant commemoration. Unlike Mayer and Thomson, he starts out with the nebular hypothesis of Kant and Laplace, and derives solar heat from nebular contraction. During the contraction of the nebula from which sun and planets were formed, and also during the contraction of the sun, now assumed to be in progress, the kinetic energy obtained thereby is converted into heat and compensates for the loss of solar heat by radiation. He concludes that if the sun contracts the ten-thousandth part of its radius enough heat iv generated to supply radiation for 2,100 years. His figures yield twenty-two million years as the probable age of the sun, on the assumption of uniform radiation and homogeneous density. Experimental data on the intensity of solar radiation, found later by Langley, reduced this age to eighteen million or less. Helmholtz's theory was a tremendous advance on that of falling meteors, assumed by Mayer and Thomson. No doubt meteors fall into the sun, as assumed by Mayer and Thomson, but the Mayer-Thomson theory made demands upon these meteors that bordered on extravagance. We are certain that a part of the solar heat is due to falling meteors, but its amount is as nothing, compared with the heat resulting from the gravitational energy of shrinkage. Until recently these were the only important sources considered. In the sixties fresh attacks were made on the problem of the age of the sun by William Thomson. In 1862 appeared in the Macmillan's Magazine an article, “On the Age of the Sun's Heat,” in which he favors a meteoric theory like that of Helmholtz, by which there is no difficulty in accounting for 20,000,000 years' heat radiated by the sun. He concludes that we may accept “as a lowest estimate for the sun's initial heat, 10,000,000 times a year's supply at present rate, but 50,000,000 or 100,000,000 as possible, in consequence of the sun,s greater density in his central parts.” “As for the future,inhabitants of the earth can not continue to enjoy the light and heat essential to their life for many million years longer, unless sources now unknown to us are prepared in the great store-house of creation.” More detailed studies of the same subject were made in 1887, in a lecture “On the Sun's Heat,” delivered before the Royal Institution of Great Britain. In this lecture Sir William Thomson refers to a very able paper, “On the Theoretical Temperature of the Sun,” by J. Homer Lane, of Washington, which establishes the apparently paradoxical statement that, within certain limitations, the more heat a gaseous body loses by radiation, the hotter it will become. This theorem. was discussed in connection with the solar heat by Benjamin Peirce, Simon Newcomb and Sir Robert Ball. Results similar to Lane's were reached in the years 1878-83 in a series of very exhaustive papers by A. Ritter. A rival to the Helmholtz-Thom- son theory of solar heat was advanced about 1882' by William Siemens, who imagined the rotating sun to hurl, by centrifugal action at his equator, enormous quantities of gas into space, which returned to him again at the poles. A refinement of the theory as presented by Helmholtz was introduced in 1899 by T. J. J. See, wherein he abandoned the Helmholtzian hypothesis of a sun of homogeneous density and, using Lane's law, investigated minutely the more complex case of central condensation. Thereby the probable solar age was extended from about 18 million to about 32 million years. Returning to the problem of the age of the earth, considered independently of the sun, we find William Thomson the great moving spirit. He approached the subject from more than one point of view. One argument for limitation of the earth's age was based on the consideration of underground heat. “The heat which we know by observation to be now conducted out of the earth yearly is so great that if this action had been going on with any apparent uniformity, the history of life on the earth could not exceed a few thousand million years.” Another consideration leading to similar conclusions was based on the shape and rigidity of the earth. With Sir William Thomson, the age of the earth continued to be a question studied with great predilection. His aim was not so much to determine the exact age as to fix an upper age limit. As the years passed by, investigation supplied much of the knowledge which was at first wanting regarding the thermal properties of rocks, and Sir William Thomson was able consequently greatly to reduce this upper limit. “The Physical Condition of the Earth” was the topic of Sir William. Thomson's presidential address in 1876, before Section A of the British Association. He took the gradual increase of temperature downward to be on an average 1 deg. C. for 30 meters of descent and gave reasons for his belief that for great depths the rate of increase does not diminish. He concludes that if at great depths the temperature does not exceed 4,000 deg. C., then the geological age of the earth does not exceed 90 million years. This argument involves some very uncertain factors. Sir William Thomson has shown quite conclusively that the earth's interior is solid, but at what temperature the substance of the earth would begin to melt under the high internal pressures was a matter of pure conjecture. About 1885 Carl Barus, of the United States Geological Survey, made a series of very important experimental researches on the physical properties of rocks at high temperatures, for the purpose of supplying trustworthy data for geological theory. Mr. Clarence King, in an article published in the American Journal of Science, used the data on specific heats, thermal conductivities and temperatures of fusion of rocks, which had been supplied by Barus, for a more accurate determination of the age of the earth. King concludes from. these experimental data on diabase, “that we have no warrant for extending the earth's age beyond 24,000,000 years.” A computation made by Lord Kelvin led to about the same figure. These results were embodied by him in his address of 1897 before the Victoria Institute. What was the attitude of geologists toward these researches? In England, geologists did not pretend to be able to find any flaw in the argument of Lord Kelvin, but they were in a position described in the well- known couplet, “A man convinced against his will Is of the same opinion still.” To the geologist, yonder snow-capped peaks symbolized eternity; to the physicist, the mountains were as transient as the clouds. A calm statement of the geologists' attitude was made before the British Association in 1892 by Sir Archibald Geikie. In one place hei expresses himself as follows: “Lord Kelvin is willing, I believe, to grant us some twenty millions of years, but Prof. Tait would have us content with less than ten millions...I frankly confess that the demands of the early geologists for an unlimited series of ages were extravagant... and that the physicist did good service in reducing them.That there must be some flaw in the physical argument I can, for mypart, hardly doubt, though I do not pretend to be able to say where it is to be found. Some assumption, it seems to me, has been made, or some consideration has been left out of sight, which will eventually be seen to vitiate the conclusions, and which when duly taken into account will allow time enough for any reasonable interpretation of the geological record.” Five years later an American geologist, Prof. T. C. Chamberlin, invaded the domain of physics and made a vigorous attack on Lord Kelvin's argument, challenging the correctness of some of his assumptions.' This criticism did not secure the attention it deserved, for scientific events soon took a different turn. Lord Kelvin's address of 1897 is permeated, as Prof. Ohamberlin puts it, “with an air of retrospective triumph and a tone of prophetic assurance.” “it is only by sheer force of reason,” says Kelvin, “that geologists have been compelled to think otherwise, and to see that there was a definite beginning and to look forward to a definite end of this world as an abode fitted for life.” Nor was this feeling of retrospective triumph confined to Lord Kelvin or to the students of the problem of the age of the sun and earth. At the close of the century physicists and chemists gloried in the triumphs of their predecessors, in such achievements as are indicated by the words “conservation of energy,” “conservation of mass,” and “atomic theory.” in physical research the nineteenth century was a golden age. it produced Faraday, Helmholtz, Mayer, Joule, Kelvin, Rayleigh, Rowland, and many other great men. With the close of the century timid souls doubtless feared that the golden age had come to a close, and they perhaps experienced strange emotions like those attributed to Adam in the Garden of Eden, on seeing the sun go down, not knowing that it would ever rise again. Others were perhaps haunted by another fear—a feeling that the great and fundamental truths of science were all revealed to the full sight of man, and it now remained only to work out the less important details. Some doubtless felt disheartened because of lack of opportunity, as did the Edinburgh anatomist, Dr. John Barclay, a century ago. Dr. Barclay looked upon the great anatomists of earlier periods as “reapers who, entering upon untrodden ground, cut down great store of corn from all sides of them.... Then come the gleaners who gather up ears enough from the bare ridges to make a few loaves of bread. Last of all come the geese, who still continue to pick up a few grains scattered here and there among the stubble, and waddle home in the evening, poor things, cackling with joy because of their success.” But the history of science shows that Dr. Barclay's reapers, gleaners, and geese do not belong to separate epochs. They are contemporaneous. The reaping, gleaning, and cackling go on as a rule in the same field, all at one time, in a grand comic medley of sounds. it is certain that anatomists had not so nearly exhausted their field one hundred years ago as Dr. Barclay believed that they had. We are told that, about 1878, the president of a certain chemical society informed his hearers in an annual address that the age of discovery in chemistry was closed, and that henceforth we had better devote ourselves to a thorough classification of chemical phenomena. But at that very time Crookes was experimenting in England on high vacua, and the year following he electrified the British Association by his brilliant experiments on “radiant matter.” Then came the Lenard rays and in 1895 the Roentgen rays, in 1896 the Becquerel rays and in 1899 radium, with its mysterious radiation. This was followed by the report that probably all matter is slightly radio-active. The study of these phenomena has shaken the old atomic theory, and calls for a re-examination of the principle of the conservation of energy and of matter. The earthquake in San Francisco did not shake buildings so violently as did these new facts shake the great edifice of physical science. The principle of the constancy of matter was called in question in an experiment of Kaufmann on particles shot off from radium. This experiment is hard to interpret, but i am not aware that J. J. Thomson, or Rutherford, or Soddy, or Boltwood, is denying the indestructibility of matter. One French experimentalist, however, Le Bon, has advanced the new theory of the destructibility of matter to explain the new phenomena. He advances his new theory as a demonstrated fact, and assumes to speak ex cathedra, when others observe extreme caution. Were he advancing the destructibility of matter merely as a working hypothesis, few could complain; but he puts it forward as a firmly rooted fact. The principle of the conservation of energy has quite withstood all attacks. To be sure, Le Bon claims to have overthrown it, too, but the validity of his argument is questionable. Even scientists sometimes play with logic. You have heard the story of the Assyriolo- gist who argued: “The Assyrians understood electric telegraphy, because we have found wire in Assyria.” “Oh,” replied the Egyptologist, “we have not found a scrap of wire in Egypt, so we know the Egyptians understood wireless telegraphy.' ' in the presidential address before the British Association in 1907, Prof. E. R. Lankester uttered the following weighty words: “The kind of conceptions to which these and like discoveries have led the modern physicist in regard to the character of that supposed unbreakable body—the chemical atom—the simple and unaffected friend of our youth—are truly astounding. But i would have you notice that they are not destructive of our previous conceptions, but rather elaborations and developments of the simpler views, introducing the notion of structure and mechanism, agitated and whirling with tremendous force, into what we formerly conceived of as homogeneous or simply built- up particles, the earlier conception being not so much a positive assertion of simplicity as a non-committal expectant formula awaiting the progress of knowledge and the revelations which are now in our hands.” This same address touches questions of cosmical physics. it says: “Radium has been proved to give out enough heat to melt rather more than its own weight of ice every hour; enough heat in one hour to raise its own weight of water from the freezing point to the boiling point. Even a small quantity of radium diffused through the earth will suffice to keep up its temperature against all loss by radiation! if the sun consists of a fraction of one per cent of radium, this will account for and make good the heat that is annually lost by it.” He continues to say: “This is a tremendous fact, upsetting the calculations of physicists as to the duration in past and future of the sun's heat and the temperature of the earth's surface. The physicists, notably Prof. Tait and Lord Kelvin,... have assumed that its material is self- cooling._ it has now, within these last five years, become evident that the earth's material is not self-cooling, but on the contrary self-heating. And away go the restrictions imposed by physicists on geological time. They now are willing to give us not merely a thousand million years, but as many more as we may want.” Some of the views relating to radium, expressed in the summer of 1906 at the York meeting of the British Association, appeared to Lord Kelvin open to objection. it seemed to him that some of the younger men were carried away by the strangeness of the new phenomena and were ready to adopt the most extravagant theories when there was no logical necessity for abandoning old views and, in their intoxication, were embracing new hypotheses without exercising due circumspection. After the meeting Lord Kelvin boldly opened a controversy in the London Times. Almost single-handed the old warrior fought with great intel- REMARKABLE SUCCESSIVE APPEARANCES OF THE SETTING SUN. lectual keenness against the transmutational and evolutionary doctrines, relating to the chemical elements, framed by the younger investigators to account for the properties of radium. Among his opponents were Sir Oliver Lodge, the Hon. Mr. Strutt, Mr. A. S. Eve and Mr. F. Soddy. it can not be said that Kelvin was victorious, but the controversy helped to define the points at issue. Among other things, Lord Kelvin said that there was no experimental foundation for the assertion that the heat of the sun was probably due to radium. He was still inclined to ascribe it to gravitation. Lord Kelvin also denied that it was proved that the heat of the earth is due to radium. it was possible, he claimed, that radium does not decompose under the conditions prevailing in the interior of the earth, and in that case it emits no heat. in considering the perturbations produced by radium in the progress of our ideas, it is well to remember that, thus far, we have been able to experiment with radium in only small amounts. Prof. Lankester remarks that the Curies never had enough of radium chloride to venture on any attempt to prepare pure metallic radium. “Altogether the Curies did not have more than some four or five grains of chloride of radium to experiment with, and the total amount prepared and now (1906) in the hands of scientific men in various parts of the world probably does not amount to more than 60 grains at most. When Prof. Curie lectured on radium four years ago at the Royal institution in London he made use of a small tube an inch long and of one- eighth inch bore, containing nearly the whole of his precious store, wrenched by such determined labor and consummate skill from tons of black shapeless pitchblende. On his return to Paris he was one day demonstrating in his lecture room with this precious tube the properties of radium when it slipped from his hands, broke, and scattered far and wide the most precious and magical powder ever dreamed of by alchemist or artist of romance. Every scrap of diist was immediately and carefully collected, dissolved, and re- crystallised, and the disaster averted with a loss of but a minute fraction of the invaluable product.” In a reinvestigation of the age of the earth it is extremely important to undertake extensive investigation of the amount of radium contained in the various rocks. Such researches have been begun by the Hon. R. J. Strutt. He has made determination of the amount of radium in rocks at the surface of the earth, and has found about 3 X 10-12 grains of radium as the average amount present in 1 cubic centimeter of soil. From the rate of increase of temperature below the earth's surface and the heat conductibility of rocks, Mr. Strutt concludes that radium is confined to a comparatively thin crust of the earth. While these reasons are not conclusive, they are weighty. Our incomplete knowledge of the properties and the distribution of radium and other radio-active substances makes it necessary to suspend judgment on the age of the earth. There is no necessity that the question be settled immediately. The same remark applies to the antiquity of the sun. Much depends upon the presence or absence of radium there. As yet this substance has not been found in the sun, but the presence of helium, combined with the fact that helium may be obtained from radium, renders the presence of radium in the sun quite probable. That radium affects the problem of the solar age was pointed out by Mr. G. H. Darwin in the following words: “Knowing, as we now do, that an atom of matter is capable of containing an enormous store of energy in itself, i think we have no right to assume that the sun is incapable of liberating atomic energy to a degree at least comparable with that which it would do if made of radium. Accordingly, i see no reason for doubting the possibility of augmenting the estimate of solar heat as derived from the theory of gravitation by some such factor as ten or twenty.” In conclusion, it is very evident that, however unpleasant it may be for the older men to revise their theories to meet the demands of new observations, we have in radio-activity the entrance into a region of new knowledge which will cast light upon many a dingy avenue of philosophy. Great are the trials and great the final triumphs of experimental science. in Norse mythology there is a wonderful tree called igdrasil, whose branches spread over the whole earth and reach up into the clouds. At the foot of the tree, away down at the deepest root, is a well from which the tree draws its sap. To us of the twentieth century that tree symbolizes science. The well which nourishes the tree is the fountain of eternal truth.—Popular Science Monthly.