charged metal when immersed in water, there is a difficulty in taking its exact density by comparing its respective weights in air and water with one another. There is also a difficulty in determining the density by direct measurement of the charged palladium when in the form of wire; owing to the curious property of the wire, on being discharged, of not merely returning to its original volume, but of undergoing a considerable and permanent additional retraction. But in the case of certain alloys of platinum, silver, and gold with excess of palladium, while the absorptive power of the constituent palladium is still manifested, the excess of retraction on discharge of the wires does not occur; and the specific gravities deducible from the mere increase in length of wires of these alloys are found to accord approximatively with those deducible from the increase in length of the pure palladium wire, not above its original length, but above the length to which it retracts on discharge of its absorbed gas. It would thus appear that, simultaneously with its absorption of hydrogen, the pure palladium wire, unstably stretched by the process of drawing, suffers two opposite actions; that is to say, it undergoes a process of shortening by assuming a more stable condition of cohesion, and a process of lengthening by the addition to it of other matter-the lengthening due to the additional matter being the excess of the length of the charged above that of the discharged wire. In a particular experiment illustrative of this peculiarity, a new platinum wire took up a full charge of hydrogen electrolytically, namely, 956.3 volumes, and increased in length from 609.585 to 619.354 millimeters. With the expulsion of the hydrogen afterward, the wire was permanently shortened to 600.115 millimeters. The sum of the two changes taken together amounts to 19.239 millimeters, and represents the true increase in the length of the wire due to the addition of hydrogen. It corresponds to a linear expansion of 3.205 in 100, or to a cubical expansion of 9.827 in 100. The original volume of the wire being .126 cubic centimeter, the volume of the condensed hydrogen would accordingly be .01238 cubic centimeter. Then, as the charged wire, on being heated in vacuo, evolved 120.5 cubic centimeters of hydrogen gas, weighing .0108 gram, the density of the absorbed hydrogen would be—

Calculated from the mere increase in length of the charged wire above that of the wire originally, the density of the absorbed hydrogen would be 1.708. The following table gives the densities of condensed hydrogen in different experiments made with palladium wire, in which the excess of retraction on discharge was allowed for as above; and also the densities observed in experiments made with palladium alloys in which the contraction on discharge took place to the original lengths of the wires only:

Whether the absorption of hydrogen by palladium, alloyed or not with another metal, was large or small, the density of the occluded hydrogen was found to be substantially the same. That the excessive retraction of the palladium wire on the discharge of its absorbed hydrogeu is not a mere effect of heat was shown by the charged wire undergoing a similar retraction when discharged electrolytically instead of by ignition in vacuo; and also by the original wire not undergoing any sensible retraction as a result of annealing. That the retraction is merely in length was shown by the absence of any difference in specific gravity between the original and the discharged wire. Very curiously, the shortening of the wire, by successive chargings and dischargings of hydrogen, would seem to be interminable. Thus the following expansions of a particular wire, caused by variable charges of hydrogen, were followed, on expelling the hydrogen, by the contractions recorded in the other column :

The palladium wire, which originally measured 609.144 millimeters, thus suffered, by four successive chargings and dischargings of hydrogen, an ultimate contraction of 23.99 millimeters, or a reduction of its original length to the extent of nearly 4 per cent., each increment of contraction below the original length usually exceeding the previous increment of elongation above the original length of the wire. The alternate expansion and contraction of palladium by its occlusion and evolution of hydrogen is ingeniously shown by a contrivance of Mr. Roberts, in which a slip of palladium-foil, varnished on one side, is made to curl and uncurl itself, as it becomes alternately the negative and positive electrode of a battery, or is alternately charged and discharged of hydrogen on its free surface.

That hydrogen is the vapor of a highly volatile metal has frequently been maintained on chemical grounds; and from a consideration of the physical properties of his hydrogenized palladium, Mr. Graham was led

to regard it as a true alloy of palladium with hydrogen, or rather hydrogenium, in which the volatility of the latter metal was restrained by the fixity of the former, and of which the metallic aspect was equally due to both of its constituents. Although, indeed, the occlusion of upward of 900 times its volume of hydrogen was found to lower the tenacity and electric conductivity of palladium appreciably, still the hydrogenized palladium remained possessed of a most characteristically metallic tenacity and conductivity. Thus, the tenacity of the original wire being taken as 100, the tenacity of the fully charged wire was found to be 81.29; and the electric conductivity of the original wire being 8.10, that of the hydrogenized wire was found to be 5.99. In further support of the conclusion arrived at by Mr. Graham, as to the metallic condition of the hydrogen occluded in palladium, he adduced his singular discovery of its being possessed of magnetic properties, more decided than those of palladium itself, a metal which Mr. Faraday had shown to be "feebly but truly magnetic." Operating with an electromagnet of very moderate strength, Mr. Graham found that while an oblong fragment of electrolytically deposited palladium was deflected from the equatorial by 100 only, the same fragment of metal, charged with 604.6 times its volume of hydrogen, was deflected through 48°. Thus did Mr. Graham supplement the idea of hydrogen as an invisible incondensable gas, by the idea of hydrogen as an opaque, lustrous, white metal, having a specific gravity between 0.7 and 0.8, a well-marked tenacity and conductivity, and a very decided magnetism.

ON THE RELATION OF THE PHYSICAL SCIENCES TO SCIENCE IN GENERAL.

Delivered before the University of Heidelberg, by Dr. Herman Helmholtz.

[Translated for the Smithsonian Institution, by Prof. C. F. KROEH.]

Our university renews, on the annual return of this day, her grateful remembrances of Charles Frederic, the enlightened prince who, at a time when the whole established order of Europe was revolutionized, labored most zealously and efficiently to improve the well-being and facilitate the mental development of his people, and who clearly perceived that the revival of this university would be one of the principal means for the attainment of his benevolent object. Since it is my duty on this occasion to speak of our whole university as its representative, it is proper to review the connection between the sciences and their study in general, as far as may be possible, from the circumscribed point of view of an individual observer.

It would seem indeed, to-day, as if the mutual relations of all sciences, in virtue of which we unite them under the name of a universitas litterarum, had become looser than ever before. We see the scholars of our times absorbed in a study of details of such immense magnitude that even the most industrious cannot hope to master more than a small portion of modern science. The linguist of the last three centuries. found sufficient occupation in the study of Greek and Latin, and it was only for immediate practical purposes that a few modern languages were learned. Now, comparative philology has set for itself no less a task than to study all the languages of the human race, in order to deduce from them the laws of the formation of language itself, and its votaries have brought immense industry to bear upon this gigantic work. Even within classical philology they no longer confine themselves to the study of those writings which, by their artistic finish, the clearness of their thoughts, or the importance of their contents, have become the models of the poetry and prose of all times; the philologists are aware that every lost fragment of an ancient writer, every remark of a pedantic grammarian or of a Byzantine court-poet, every broken tomb-stone of a Roman official that is found in some remote corner of Hungary, Spain, or Africa, may contain some information or proof of importance in its proper place, and hence a large number of scholars are occupied in the gigantic task of collecting and cataloguing all remnants of classic antiquity so that they may be ready for use. Add to this the study of his torical sources, the examination of parchments and papers accumulated in states and towns, the collection of notes scattered through me

moirs, correspondences, and biographies, and the deciphering of the hieroglyphics and cuneiform inscriptions; add again to these the continually and rapidly augmenting catalogues of minerals, plants, and animals, living and antediluvian, and there will be unfolded before our eyes a mass of scientific material sufficient to make us giddy. In all these sciences, researches are pushed forward constantly in proportion to the improvement of our means of observation, and there is no perceptible limit. The zoölogist of the last century was generally satisfied with describing the teeth, fur, formation of the feet and other external characteristics of an animal. The anatomist, on the other hand, described the anatomy of man alone, as far as he could gain a knowledge of it by means of the knife, the saw, the chisel, or, perhaps, of injections into the tissues. The study of human anatomy was even then considered an extremely extensive and difficult branch of science. To-day we are no longer content with what is so-called descriptive human anatomy, which, although incorrectly, is considered as exhausted, but comparative anatomy, i. e., the anatomy of all animals, and miscroscopic anatomy, both sciences of unlimited scope, have been added and absorb the interest of the observer.

The four elements of antiquity and of mediæval alchemy have swelled to sixty-four* in our modern chemistry; the last three have been discovered according to a method originating in our university, which leads us to expect other similar discoveries. Not only, however, has the number of the elements increased extraordinarily, but the methods for producing complex compounds have been so greatly improved, that what is so-called organic chemistry, which comprises only the combinations of carbon with hydrogen, oxygen, nitrogen, and a few other elements, has already become a separate science.

"As many as the stars in heaven," used to be the natural expression for a number which exceeds all limits of our comprehension. Pliny considered it an undertaking bordering on arrogance when Hipparchus commenced to number the stars and determine their positions. Nevertheless, the catalogues of stars up to the seventeenth century, which were made out without the use of telescopes, contained only from 1,000 to 1,500 stars of the first to the third magnitude. At present they are engaged at the different observatories in extending these catalogues down to the tenth magnitude, which will make a sum total of more than 200,000 fixed stars in the whole heavens; and these are all to be noted down, measured, and their places determined. The immediate consequence of these observations has been the discovery of many new planets. Of these only six were known in 1781, while at present we know seventyfive. When we pass in review this gigantic activity in all branches of

*With Indium, recently discovered, sixty-five.

In the latter part of November, 1864, the eighty-second of the asteroids, Alcmene, was discovered. Add to this the eight large planets, and the whole number of planets known will amount to ninety. [1871, 120.]