It has long been suspected that hydrogen would ultimately prove to be a metal. Our readers will also recollect the announcement that during some recent experiments, a substance had been discovered, supposed to be the metallic base of hydrogen. Still more recent experiments by Thomas Graham, F.R.S., Master of the British Mint, throw additional light upon this most important subject It has often been maintained on chemical grounds that hydrogen gas is the vapor of a highly volatile metal. The idea forces itself upon the mind that palladium with its occluded hydrogen is simply an alloy of this volatile metal in which the volatility of the one element is restrained by its union with the other, and which owes its metallic aspect equally to both constituents. How far such a view is borne out by the properties of the compound substance in question will appear by the following examination of the properties of what, assuming its metallic character, would fairly be named hydro-genium The density of palladium, when charged with 800 or 900 times its'volume of hydrogen gas, is perceptibly lowered, but the change cannot be measured accurately by the ordinary method of immersion in water, owing to a continuous evolution of minute hydrogen bubbles which appear to be determined by contact with the liquid. However, the linear dimensions of the charged palladium are altered so considerably that the difference admits of easy measurement, and furnishes the required density by calculation. Palladium, in the form of wire, is readily charged with hydrogen by evolving that gas upon the surface of the metal in a galvanometer containing dilute sulphuric acid, as usual. The length of the wire before and after a charge is found by stretching it on both occasions by the same moderate weight, such as will not produce permanent distention, over the surface of a flat graduated measure. The measure was graduated to hundred-ths of an inch, and by means of a vernier, the divisions could be read to thousandths. The distance between two fine cross lines marked upon the surface of the wire near each of its extremities was observed. The wire had been drawn from welded palladium, and was hard and elastic. The diameter of the wire was 0*462 millimeter ; its specific gravity was 12'38, as determined with care. The wire was twisted into a loop at each end, and the mark made near each loop. The loops were varnished so as to limit absorption of gas by the wire to the measured lengtiu between the two marks. To straighten the wire, the low p was fixed, and the other connected with a string passing oier a pulley and loaded with 1*5 kilogrammes, a weight sufficiauit to straighten the wire without occasioning any undue strain. The wire was charged with hydrogen by making it the negative electrode of a small Bunsen's battery, consisting of two cells, each of half a liter in capacity. The positive electrode was a thick platinum wire placed side by side with the pal ladium wire, and extending the wliole length of the latter, within a tall jar filled with dilute sulphuric acid. The palladium wire had, in consequence, hydrogen carried to its surface lor a period of one and a half hours. A longer exposure was found not to add sensibly to the charge of hydrogen acquired by the wire. The wire was again measured and the increase in length noted. Finally, the wire being dried with a cloth, was divided at the marks, and the charged portion heated in a long narrow glass tube kept vacuous by a Sprengel aspirator. The whole occluded hydrogen was thus collected and measured; its volume is reduced by calculation to Bar. 760 m.m., and Therm. 0 C. The original length of the palladium wire exposed was 609-144 m. m. (23'982 inches), and its weight 1-6832 grm. The wire received a charge of hydrogen amounting to 936 times its volume, measuring 128 c.c, and therefore weighing 0101147 grm. When the gas was ultimately expelled, the loss as ascertained by direct weighing was 0-01164 grm. The charged wire measured 618"923 m.m., showing an increase in length of 9'779 m.m. (0'385 inch). The increase in linear dimensions is from 100 to 101'605 ; and in cubic capacity, assuming the expansion to be equal in all directions, from 100 to 104-908. Supposing the two metals united without any change of volume, the alloy may therefore be said to be composed of— By volume. Palladium.....................100 or 95'32 Hydrogenium................. 4-908 or 4-68 104-908 100 Tlie expansion which the palladium undergoes appears enormous if viewed as a change of bulk in the metal only, due to any conceivable physical force, amounting as it does to sixteen times the dilatation of palladium when heated from 0 to 100 C. The density of the charged wire is reduced by calculation from 12-3 to 11'79. Again, as 100 is to 4-91, so the volume of the palladium, 0-1358 c.c is to the volume of the hydrogenium 0-006714 c.c. Finally, dividing the weight of the hydrogenium, 0-01147 grm. by its volume in the alloy, 0-006714 c. c. we find Density of hydrogenium...........l'7O8 The density of hydrogenium, then, appears to approach that of magnesium, 1-743, by this first experiment. Further, the expulsion of hydrogen from tlie wire, however caused, is attended with an extraordinary contraction of the latter. On expelling the hydrogen by a moderate heat, the wire not only receded to its original length, but fell as much below that zero as it had previously ridden above it. The palladium wire first measuring 609-144 m.m., and which increased 9-77 m.m., was ultimately reduced to 599-444 m.m., and contracted 9'7 m.m. The wire is permanently shortened. The density of the palladium did not increase, but fell slight-; ly at the same time, namely, from 12-38 to 12-12 ; proving i that this contraction of the wire is in length only. The result | is the converse of extension by wire-drawing. The retraction of the wire is possibly due to an effect of wire-drawing in leaving the particles of metal in a state of unequal tension, a tension which is excessive in the direction of the length of the wire. The metallic particles would seem to become mobile, and to right themselves in proportion as the hydrogen escapes; and the wire contracts in length, expanding, as appears by its final density, in other directions at the same time. ' A wire so charged with hydrogen, if rubbed with the powder of magnesia (to make the flame luminous), burns like a waxed thread when ignited in the flame of a lamp. Numerous other experiments were also performed, with remarkable unanimity of result; the specific density of hydrogenium being found by calculation from several successive experiments to be, respectively, 1-708, '1-898, 1-977, 1-917, 1-927, 1-930, 2'055, the variations resulting from different volumes being used in the alloy, the highest densities being obtained when small quantities were used. In these experiments the hydrogen was expelled by exposing the palladium placed within a glass tube to a moderate heat short of redness, and exhausting by means of a Sprengel tube; but the gasVas also withdrawn in another way, namely, by making the wire the positive electrode, and thereby evolv-| ing oxygen upon its surface. In such circumstances, a slight film of oxide of palladium is formed on the wire, biJ it appears not to interfere with the extraction and oxidation of the hydrogen. The wire measured— Difl'crcncc. Before charge....:.........448-25 m.m. With hydrogen............449-90 " -f 6'65 m.m. After discharge............437-81 " —5-94 " The retraction of the wire, therefore, dees not require the concurrence of a high temperature. This experiment further proved that a large charge of hydrogen may be removed in a complete manner by exposure to the positive pole—for four hours in this case; for the wire in its ultimate state gave no hydrogen on being heated in varuo. 131 Experiments were also made to determine the conducting power of the palladium and hydrogenium wire, and its magnetic properties, the details of which may be hereafter referred to. The record of these experiments, as communicated to the Royal Society, January 14, by Mr, Graham, forms one of the most important contributions to science that has been recently made, and will immediately arrest the attention of the entire scientific world.