IN his studies of the relative frequency of the different elements composing the crust of the earth, Mr. F. W. Clarke supposes that to a depth of ten miles below the level of the sea the composition of the ground is the same as is given by the examination of the surface strata and the depths which we have reached. The mean specific gravity of these strata is 2'5, or not quite half the density of the earth as a whole. Including the oceans. and the atmosphere, the exterior crust of the earth is composed half of oxygen and one-fourth of silicon, while the other fourth is represented by other elements—aluminum, 7 per cent.; iron, 510; calcium, 3'50; magnesium, 2'50; sodium, 2'20; and potassium, 2'20 per cent. Some of the elements of which the numerous compounds have long been very obvious to the human ken are, therefore, from the point of view of their quantity, of very little importance; thus, hydrogen stands for only 0'94 per cent. of the general composition of the crust of the earth, carbonic acid for 021 per' cent., phosphorus for 0 09 per cent., and nitrogen for 0'02 per cent. These elements, which are the constituents of immense seas and form the basis of life, therefore furnish only a minute fraction of the mass of the ten-mile-thick ring contemplated by Mr. Clarke. Since the soundings thus far made indicate that they do not exist or hardly exist at greater depths, we have a right to say that 80 far as regards quantity they may almost be neglected, in considering the mass of the whole globe. The content in chlorine does not exceed 0'15 per cent., yet the common salt alone held in the oceans is sufficient to cover all the continents and bury the highest mountains. We perceive from this showing how little the impression the outer surface of our globe gives us corresponds with its real nature as we judge of it from its mean density. There cannot be the least doubt that the internal parts of the globe are composed of different substances from those which appear in the external strata. But, while the elements of light specific weight or great volatility, which, like hydrogen and nitrogen, exist in large quantities around us, constitute only a very minute part of the constituents of our globe considered as a whole, we presume that the elements called rare only enter in an infinitesimal degree into the general composition of the earth; the more so, because, so far as we as yet know, these elements are not found at great depths. I, at least, do not - know that the heavier metals—gold, silver, lead, etc. —have ever been found in the materials extracted from deep soundings or volcanic ejections. After the mighty eruption of Krakatoa, for example, I sought in vain for these elements in the cinders cast out, which probably came from great depths. The supposed discovery of a new element in the ancient lavas of Vesuvius has been found to be erroneous. Elementary bodies seem to multiply as we approach the surface of the globe. Two hypotheses suggest themselves in explanation of the fact: that of displacements of cosmical matter and that of the new formation of elements on the surface. The displacements of cosmic materials are incessant; falls of meteorites furnish a particularly striking example of them, but it is probable that as to quantity the cosmical dusts are of more importance. Yet neither the meteorites found at various points nor the dust collected by Nordenskiold in the ice fields of the polar regions, the extra-terrestrial origin of which cannot be doubted, contain the rare elements of the earth. The hypothesis of an increase by accretions from without appears to lack foundation. The new formation of elementary bodies seems to be still less probable; at most it might be explained by the possibility, often indicated but never established, of a new reduction of bodies heretofore supposed to be simple. Spectrum analysis, it is true, reveals to us transformations which are gradually going on in the matter of the fixed stars, but they are only of known substances becoming converted into other substances equally known. Moreover, the conditions of temperature and aggregation of the fixed stars and those of the earth cannot be compared. It is evident that the increase of simple bodies in the outer strata of the earth is only apparent. It should be recognized, besides, that science has made greater progress, and this progress cannot be without influence on the discovery of new substances. The first electrolytic decompositions accomplished by Davy with an inferior voltaic pile made known at the beginning of this century the existence of metallic radicals in the salts and the earth of which there had not been before the slightest suspicion; while Moissan, by the employment of the powerful currents now available, has been able to disengage fluorine—hitherto almost unknown—from its combinations. Spectrum analysis has cast light on a whole series of elements of characteristic spectra. The presence of one of these elements, helium, had been demonstrated in the. sun before it was known that it likewise entered into the composition of our globe. The conclusions drawn by D. Mendeleef from the periodical law have also led to the discovery of several elements, the existence of which was indicated by theory before the chemTht had isolated them. I mention, first, scandium, discovered in 1879 by Nilson in exonite, gadolinite, and yttrotitanite. This metal, the oxide of which exists only in quantities of a few grammes, and which no person, perhaps, other than the author of the discovery has had in his hands, possesses considerable scientific importance, because its atomic weight of 44, as determined by Nilson, is precisely that indicated by Mendeleef for ekabor, an element the existence of which was predicated by the periodic law. In 1794, Gadolin had separated from the gadolinite of Ytterby an earth which he called the earth of Ytter, and which was afterward known under the forms of erbia, terbia, and yttria proper. These earths were found in a considerable number of rare minerals, but the oxides extracted from these minerals exhibited different natures and aspects, presenting themselves rather like mixtures in which the separation of the different constituents was attended by considerable difficulties, for the different elements gave no very distinct reaction. It was necessary to recur to spectrum analysis and to the determination of atomic weights, and to try to isolate them by repeated fractionings, under the action of sulphate of potassium or of ammonia, or else by the partial decomposition of the nitrates by heat. The bulk of these analyses, the results of which are not, however, entirely clear as yet on some points, have been performed within the last quarter of a century, and, besides securing more precise knowledge of scandium and yttrium, have revealed the existence of numerous other rare elements, the reduction of which does not seem impossible; among which we cite erbium, holmium, thulium, dysprosium, terbium, gadolinium, samarium, decipium, and ytterbium. Cerium, lanthanum, and didymium have been the object recently of very attentive researches having a practical end in view—the constitution of mantles for incandescent gaslights. Didymium has been long suspected of not being a simple substance; but Carl Auer von Welsbach, the inventor of this method of illumination, is entitled to the credit of having succeeded, in 1855, in separating didymium into its two elements of prresodidymium and neodidymium. The utilization of monazite afterward permitted the preparation of the salts of these remarkable metals in larger quantities, and the practical use of them. The existence of metacerium, announced by M. Brauner, does not yet appear to be fully established, nor that of russium, which M. Crushchow has found associated with thorium in some zircons and in monazite, and the atomic weight of which is calculated at 220. The jargonium of Sorby, the austrium of Linne- man, the norvegium of Dahll, the actinium of Phipson, the idumium of Websky. the masrium of Richmond and Off, and an unknown element which M. K. J. Bayer thought he had found in French bauxite, have returned to nothingness. We mention also merely as a matter of curiosity a kosmium and a neokosmium, dedving their names not from Cosmos, but from Kos- mann, who took out a patent for the preparation of their oxides. Gallium was discovered in August, 1875, by Lecoq de Boisbaudran in the blende of Pierrefitte, through two very distinct lines in the violet of the spectrum of that mineral, which, however, as afterward appeared, contained only a slight proportion of the new metal —not exceeding 0'000l per cent.—while in the richer blende of Bernbryer it amounted to 0 001 per cent. The preparation of gallium in any considerable quantities was attended with great difficulties on account of the want of a proper mineral to be practicably submitted to the extraction process, and none has as yet been found. Still, the study of the new metal was very interesting, in view of the theoretical speculations of Mendeleef. Scandium and germanium had not yet been discovered, and there was therefore nothing to justify or confirm the conclusions drawn from the law of periodicity. As early as 1869, Mendeleef had affirmed the existence of simple bodies still unknown, the atomic weights of which should be comprehended between 65 and 75; he had even gone so far as to describe in detail the properties of the three hypothetical elements—ekaboron, eka-aluminum, and ekasili- con. We can imagine the interest attached to the question whether the properties of gallium corresponded with the anticipations of the Russian chemist. At first the correspondence did not seem to exist; the determinations made on the small quantities of gallium that could be obtained gave the specific gravity the unexpected value of 4'7. But as many of the properties of the new metal—such as the precipitation of its solutions by carbonate of barium, its tendency to form basic salts, and its capacity of forming alums—denoted a relationship with aluminum, Mendeleef had no hesitation in declaring that the new element appeared to correspond with the one the existence of which he had indicated in 1874 as similar to aluminum, and which he had called eka-aluminum. A new determination, made with considerable quantities of gallium obtained by electrolysis, brought the value of the specific gravity up to 59, which corresponds exactly with the value calculated by Mendeleef for the hypothetical eka-aluminum. The specific heat (0 08) was afterward found to correspond with Mendeleef's estimate, and the justness of his previsions was established. It was, therefore, shown to be reasonable to deduce from the properties of known elements those of others still unknown, but the existence of which is anticipated. Mendeleef had not expected so quick a confirmation of his previsions; but his triumph was destined to be still more complete, for to gallium were afterward added scandium (ekaboron), discovered by M. L. F. Nilson in 1875, and germanium (ekasilicon), discovered by me in 1886. The discovery of germanium, predicted under the name of ekasilicon by Mendeleef, bears a resemblance to the discovery of the planet Neptune, the existence of which had been shown by the calculations of Adams and Leverrier. That discovery was not due to a concurrence of favorable circumstances or to a happy accident, but was the result of researches inspired by theoretic previsions, and the concordance between the predicted and the real properties was so great that Mendeleef regarded the discovery of germanium as an important verification of the periodic law. On only one point—that touching its formations in nature- did germanium completely fail expectations. The search for it would be more likely made as an oxide in the rare minerals of the north, along with titanium and zirconium, than as a sulphide accompanying similar compounds of arsenic and antimony in gangues of silver bearing minerals. This fact, with the comparative rarity of its mineral, argyrodite, has contributed no little to delaying the elucidation of its real character. For myself, I was at first inclined to regard it as elm- antimony, while Mendeleef, after my first incomplete communications, thought it was ekacadmium. At the same time, M. Von Richter expressed the conviction that germanium was nothing else than the long ex pected ekasilicon, a conclusion that was justified by the correspondence of atomic weights. The success of the bold speculations of Mendeleef permits the affirmation that the elaboration of the periodic system constitutes a great forward step for science. In the course of only fifteen years all the predictions of the Russian chemist have been confirmed. New elements have come to fill the vacant spaces in his table, and there is every reason to hope that a like fulflllment awaits the rest of the natural system. Yet the two elements last discovered, argon and helium, do not seem to present any relation with the periodic system. The physical properties of argon are very distinct; its characteristic. spectrum distinguishes it with great certainty from all other substances; but chemically the gas manifests an extraordinary indifference, and it has not so far been possible to make it enter into the usual compounds with other elements. This peculiarity, and the impossibility of introducing a simple body of the molecular weight of argon (39'88) into the periodic system, have given occasion to all sorts of hypotheses concerning the gas; and the question of its relations has not vet been answered. Another most interesting discovery is that of helium, which was made by Prof. Ramsay, in 1895, while examining the mineral cleveite for argon, when, besides the spectrum of argon, he observed another bright line not belonging to that spectrum, which Mr. Crookes recognized as identical w;th the line D3 which Prof. Lockyer had observed in 1868 in the spectrum of the solar chromosphere, and which he attributed to an element not yet known on the earth—helium. The same line was afterward found in the spectra of other fixed stars, and the conclusion was drawn that helium exists in large quantities outside of the earth. On our planet it seems, however, to be very rare, and may even be ranked among the rarest elements. Yet it has been almost discovered several times. Palmieri observed the line of helium in his researches on the lava of Vesuvius, but did not push the matter further; and Hillebrand in 1891 obtained, in the spectrum of the gas formed by uramte, lines which were presumptively, those of helium. Since its discovery, helium has been found in a considerable number of minerals, generally associated with uranium, yttrium, and thorium; in mineral waters and in extremely small quantities in atmospheric air. Next to hydrogen, it is the lightest of the gases, and from this peculiarity Stoney draws an explanation of the fact. that these two elements exist only in very small quantities in a free state on the earth, while they are diffused in enormous masses through the universe. The relatively small force of the earth's gravitation does not furnish an adequate counterpoise to the velocity of their molecules, and they escape from the atmosphere of the earth, unless they are restrained by chemical combination. They then collect around the great centers of attraction constituted by the stars, in the atmosphere of which they exist in large quantities. The study of the spectrum of helium is extremely important, because it gives interesting data concerning the nature of distant celestial bodies. It also, as the labors of Runge and Paschen have shown, suggests doubts concerning the elementary character of the new substance. Whatever it may be, if we have to suppose that helium is composed of two gases (Mr. Lockyer has proposed the name of asterium for the second), one of the two gases probably has a boiling point very near the absolute zero, and in any case below —264° C.; for the master in liquefaction of gases, M. K. Olszewsky, has not up to this time succeeded in provoking a change of state of helium, and he proposes to use this gas for filling gas thermometers for measuring extremely low temperatures. Helium has shown itself thus far as refractory as argon to all chemical combination, and so great an uncertainty reigns over the position to be attributed to it that I pass by the hypotheses that have been set forth with respect to the matter. It is not impossible that the discovery of these two new elements, argon and helium, may give occasion for a remodeling or a transformation of the periodical system—a remodeling by means of which some uncertainties and even contradictions now existing will undoubtedly be removed. Thus, for example, the atomic weight of tellurium, as recently determined by B. Brauner and Ludwig Standenmaler, does not enter at all into the periodical system; on the other haud, the existence in this substance of a foreign element, such as the austriacum suggested by B. Bra], lner, does not seem to be established. As to the much agitated question whether and to what extent the atomic weight of nickel differs from that of cobalt, I believe I have given a satisfactory answer, and have refuted the hypothesis of Gerhard Krl1ss and F. W. Schmid of the existence in one of the substances of a third element which has been called gnomium. The rapid glance which we have cast over the discovery of new elements during the last twenty-five years shows that researches have been pursued in this direction with great activity, and with the return of considerable results. Yet the speculations for which these researches have given occasion with respect to the possibility of an ultimate decomposition of apparently simple bodies, and reciprocally respecting the progressive development of the primitive substance and the formation of many of the present elements, may be considered very uncertain. I mention among these Mr. Lockyer's hypothesis of the dissociation of the elements within the solar atmosphere. Hypotheses of this kind must remain hypotheses so long as we do not succeed in splitting a substance unequivocally regarded as simple, or in transforming some element into another; yet they need not be considered wholly inadmissible. Something unexpected may happen at any time that will open to science new roads of investigation. — Translated for The Popular Science Monthly from the 'Revue Scientifique. The Boston and Albany road has sprayed 114 miles of its roadbed with oil this season, or from Boston to a point 15 miles beyond Springfield. It is intended to carry the work as far as Chester, or 126 miles from Boston, before the work is postponed. Eighty-two tanks of 6,000 gallons each were used. It is estimated that only one-third that amount will be required tosprinkle the same ground next year.