tion, however, was observed on charging the retracted wire more than once; and the expansion continued to be in the usual proportion to the hydrogen absorbed. The final density of the wire was 12.18.

The wire retracted by heat is found to be altered in another way, which appears to indicate a molecular change. The metal gradually loses much of its power to take up hydrogen. The last wire, after it had already been operated upon six times, was again charged with hydrogen for two hours, and was found to occlude only 320 volumes of gas, and in a repetition of the experiment 330.5 volumes. The absorbent power of the palladium had therefore been reduced to about one-third of its maximum. The condition of the retracted wire appeared, however, to be improved by raising its temperature to full redness by sending through it an electrical current from a battery. The absorption rose thereafter to 425 volumes of hydrogen, and in a second experiment to 422.5 volumes.

The wire becomes fissured longitudinally, acquires a thready structure, and is much disintegrated on repeatedly losing hydrogen, particularly when the hydrogen has been extracted by electrolysis in an acid fluid. The palladium in the last case is dissolved by the acid to some extent. The metal appeared, however, to recover its full power to absorb hydrogen, now condensing upwards of 900 volumes of gas.

The effect upon its length of simply annealing the palladium wire by exposure in a porcelain tube to a full red heat, was observed. The wire measured 556.075 millims. before, and 555.875 millims. after heating; or a minute retraction of 0.2 millim. was indicated. In a second annealing experiment, with an equal length of new wire, no sensible change whatever of length could be discovered. There is no reason, then, to ascribe the retraction after hydrogen, in any degree, to the heat applied when the gas is expelled. Palladium wire is very slightly affected in physical properties by such annealing, retaining much of its first hardness and elasticity.

2. Tenacity. A new palladium wire, similar to the last, of which 100 millims. weighed 0.1987 grm., was broken, in experiments made on two different portions of it, by a load of 10 and of 10-17 kilogrammes. Two other portions of the same wire, fully charged with hydrogen, were broken by 8-18 and by 8-27 kilogrammes. Hence we have

Tenacity of palladium wire

Tenacity of palladium and hydrogen

100

81.29

The tenacity of the palladium is reduced by the addition of hydrogen, but not to any great extent. It is a question whether

the degree of tenacity that still remains is reconcilable with any other view than that the second element present possesses of itself a degree of tenacity such as is only found in metals.

3. Electrical Conductivity.-Mr. Becker, who is familiar with the practice of testing the capacity of wires for conducting electricity, submitted a palladium wire, before and after charging with hydrogen, to trial, in comparison with a wire of German silver of equal diameter and length, at 10°.5. The conductingpower of the several wires was found as follows, being referred to pure copper as 100:

A reduced conducting-power is generally observed in alloys; and the charged palladium wire falls 25 per cent. But the conducting-power remains still considerable, and the result may be construed to favour the metallic character of the second constituent of the wire. Dr. Matthiessen confirms these results.

4. Magnetism.-It is given by Faraday as the result of all his experiments, that palladium is "feebly but truly magnetic;" and this element he placed at the head of what are now called the paramagnetic metals. But the feeble magnetism of palladium did not extend to its salts. In repeating such experiments, a horseshoe electromagnet of soft iron, about 15 centims. (6 inches) in height, was made use of. It was capable of supporting 60 kilogs. when excited by four large Bunsen cells. This is an induced magnet of very moderate power. The instrument was placed with its poles directed upwards; and each of these was provided with a small square block of soft iron terminating laterally in a point, like a small anvil. The palladium under examination was suspended between these points in a stirrup of paper attached to three fibres of cocoon silk, 3 decimetres in length, and the whole was covered by a bellglass. A filament of glass was attached to the paper, and moved as an index on a circle of paper on the glass shade divided into degrees. The metal, which was an oblong fragment of electro-deposited palladium, about 8 millims. in length and 3 millims. in width, being at rest in an equatorial position (that is, with its ends averted from the poles of the electromagnet), the magnet was then charged by connecting it with the electrical battery. The palladium was deflected slightly from the equatorial line by 10° only, the magnetism acting against the torsion of the silk suspending thread. The same palladium charged with 604-6 volumes of hydrogen was deflected by the electromagnet through 48°, when it set

itself at rest. The gas being afterwards extracted, and the palladium again placed equatorially between the poles, it was not deflected in the least perceptible degree. The addition of hydrogen adds manifestly, therefore, to the small natural magnetism of the palladium. To have some terms of comparison, the same little mass of electro-deposited palladium was steeped in a solution of nickel, of specific gravity 1.082, which is known to be magnetic. The deflection under the magnet was now 35°, or less than with hydrogen. The same palladium being afterwards washed and impregnated with a solution of protosulphate of iron of specific gravity 1·048, of which the metallic mass held 2.3 per cent. of its weight, the palladium gave a deflection of 50°, nearly the same as with hydrogen. With a stronger solution of the same salt, of specific gravity 1.17, the deflection was 90°, and the palladium pointed axially.

or

Palladium in the form of wire or foil gave no deflection when placed in the same apparatus, of which the moderate sensitiveness was rather an advantage in present circumstances; but when afterwards charged with hydrogen, the palladium uniformly gave a sensible deflection of about 20°. A previous washing of the wire or foil with hydrochloric acid, to remove any possible traces of iron, did not modify this result. Palladium reduced from the cyanide and also precipitated by hypophosphorous acid, when placed in a small glass tube, was found to be not sensibly magnetic by our test; but it always acquired a sensible magnetism when charged with hydrogen.

It appears to follow that hydrogenium is magnetic, a property which is confined to metals and their compounds. This magnetism is not perceptible in hydrogen gas, which was placed both by Faraday and by M. E. Becquerel at the bottom of the list of diamagnetic substances. This gas is allowed to be upon the turning-point between the paramagnetic and diamagnetic classes. But magnetism is so liable to extinction under the influence of heat, that the magnetism of a metal may very possibly disappear entirely when it is fused or vaporized, as appears with hydrogen in the form of gas. As palladium stands high in the series of the paramagnetic metals, hydrogenium must be allowed to rise out of that class, and to take place in the strictly magnetic group, with iron, nickel, cobalt, chromium, and man

ganese.

Palladium with Hydrogen at a high Temperature.-The ready permeability of heated palladium by hydrogen gas would imply the retention of the latter element by the metal even at a bright red heat. The hydrogenium must in fact travel through the palladium by cementation, a molecular process which requires time. The first attempts to arrest hydrogen in its passage Phil. Mag. S. 4. Vol. 37. No. 247. Feb. 1869.

K

through the red-hot metal were made by transmitting hydrogen gas through a metal tube of palladium with a vacuum outside, rapidly followed by a stream of carbonic acid, in which the metal was allowed to cool. When the metal was afterwards examined in the usual way, no hydrogen could be found in it. The short period of exposure to the carbonic acid seems to have been sufficient to dissipate the gas. But on heating palladium foil red-hot in a flame of hydrogen gas, and suddenly cooling the metal in water, a small portion of hydrogen was found locked up in the metal. A volume of metal amounting to 0.062 cubic centim., gave 0.080 cubic centim. of hydrogen; or, the gas, measured cold, was 1.306 time the bulk of the metal. This measure of gas would amount to three or four times the volume of the metal at a red heat. Platinum treated in the same way appeared also to yield hydrogen, although the quantity was too small to be much relied upon, amounting only to 0.06 volume of the metal. The permeation of these metals by hydrogen appears therefore to depend on absorption, and not to require the assumption of anything like porosity in their stucture.

The highest velocity of permeation observed was in the experiment where four litres of hydrogen (3992 cubic centims.) per minute passed through a plate of palladium 1 millim. in thickness, and calculated for a square metre in surface, at a bright red heat, a little short of the melting-point of gold. This is a travelling movement of hydrogen through the substance of the metal with the velocity of 4 millims. per minute.

The Chemical Properties of hydrogenium also distinguish it from ordinary hydrogen. The palladium alloy precipitates mercury and calomel from a solution of the chloride of mercury without any disengagement of hydrogen; that is, hydrogenium decomposes chloride of mercury, while hydrogen does not. This explains why M. Stanislaus Meunier failed to discover the occluded hydrogen of meteoric iron by dissolving the latter in a solution of chloride of mercury; for the hydrogen would be consumed, like the iron itself, in precipitating mercury. Hydrogen (associated with palladium) unites with chlorine and iodine in the dark, reduces a persalt of iron to the state of protosalt, converts red prussiate of potash into yellow prussiate, and has considerable deoxidizing powers. It appears to be the active form of hydrogen, as ozone is of oxygen.

The general conclusions which appear to flow from this inquiry are that in palladium fully charged with hydrogen (as in the portion of palladium wire now submitted to the Royal Society) there exists a compound of palladium and hydrogen in a proportion which may approach to equal equivalents*; that Proceedings of the Royal Society, 1868, vol. xvi. p. 425. [Phil. Mag. July 1868. p. 66.]

both substances are solid, metallic, and of a white aspect; that the alloy contains about 20 volumes of palladium united with 1 volume of hydrogenium, and that the density of the latter is about 2, a little higher than magnesium, to which hydrogenium may be supposed to bear some analogy; that hydrogenium has a certain amount of tenacity, and possesses the electrical conductivity of a metal; and, finally, that hydrogenium takes its place among magnetic metals. The latter fact may have its bearing upon the appearance of hydrogenium in meteoric iron, in association with certain other magnetic elements.

I cannot close this paper without taking the opportunity to return my best thanks to Mr. W. C. Roberts for his valuable cooperation throughout the investigation.

XVIII. On some Phenomena of Binocular Vision. By JOSEPH LECONTE, Professor of Chemistry and Geology in the University of South Carolina *.

TWO

I. Adjustments of the Eye.

every

volun

WO kinds of ocular adjustment take place in tary act of sight, viz. (1) a proper convergence of the optic axes so that they shall meet on the object of sight, and (2) an adjustment of each eye so that the diverging pencil of rays which enters the pupil shall be brought to perfect focus, and therefore produce a perfect image on the retina. The first or binocular adjustment is necessary for single vision; the second or focal adjustment is necessary for distinct vision. The first is distinctly sensible for all distances within 100 yards, and perhaps for much greater distances; the second is scarcely, if at all, sensible for distances beyond two yards.

To the two adjustments mentioned above may be added a third, viz. contraction of the pupil. The design of the contraction of the pupil is probably to increase the clearness of definition of the retinal image by cutting off the most divergent rays from very near objects, and thus to decrease the spherical aberration which is not entirely corrected in the eye by the form of the lens. The pupil, however, also contracts involuntarily under the stimulus of strong light, without regard to distance. This must be carefully distinguished from the adjustive contraction, which is (to some extent at least) voluntary.

These three adjustments of the eye, viz. binocular or axial adjustment, focal adjustment, and contraction of the pupil, are associated in every voluntary act of sight. They are accomplished by one act of volition. They are so intimately associated that they cannot be voluntarily separated. It is usually * From Silliman's American Journal for January 1869.