W2 where w, and we are the weights of the substance, S, and S, the specific gravities, and S the mean specific gravity found by experiment. This formula assumes that no condensation takes place in the palladium. TABLE I.-Specific Gravity of Bar Palladium at different times during Saturation with Hydrogen. Weight of original Palladium 31.802. When the calculated values of the specific gravity of the hydrogen in the eight determinations of Table I. are examined, it will be observed no material alteration can be said to take place in the specific gravity during the course of the saturation, regard being paid to the difficulty of getting accurate weighings immediately after removal from the electrolytic cell. All the above determinations were made as quickly as possible after the palladium was taken from the pole of the battery. The mean of the results gives a specific gravity of 0-620 for the hydrogen. The atomic volume of this condition of hydrogen is therefore 1.6; and the alteration of volume is equivalent to saying that, in the formation of this condition, 7 litres of gas are condensed into 1 cubic centimetre. The maximum charge of hydrogen the palladium would take was given to No. 7 experiment; and this very nearly accords with the composition Pd H2. In treating masses of palladium the above proportion was never exceeded. As all the above experiments were made with bar palladium increasing to saturation, it was necessary to find similar values for a mass decreasing from saturation, the hydrogen being expelled by heating. For this purpose a piece of plate palladium was employed. When the plate was fully charged for the first time, and the specific gravity of the hydrogen found as in the previous experiment, the result was a density of 0.621: the plate in this case contained 0.21 grm. of hydrogen. When a portion of the hydrogen was expelled by heat, leaving 0·127 grm. in the plate, the density of the hydrogen was 0.633; and where only 00495 grm. was left, the value was 0.615. The mean specific gravity of the three different alloys of plate was 0.623, nearly the same as previously found for the bar. After palladium has been treated with hydrogen and heated, the metal becomes porous, and often blisters, thus rendering specific-gravity determinations very uncertain. The curious discovery of Graham, that the strain on the particles of palladium produced by wire-drawing induces such a curious shrinking in the length after the hydrogen has been expelled, suggests the importance of investigating how a saturated piece of bar would resist the action of sustained pressure or tension. If the formation of the alloy is attended with an increase of volume in excess of the sum of the volumes of the constituents regarded as in the solid state, then partial decomposition ought to occur under great pressure. Experiments on this subject are left over for the present. After palladium has been used in the above experiments, and the last trace of hydrogen is removed by heating, the specific gravity is found to have diminished from 12.0359 to 11.9546. If this final value is taken in calculating the mean density, then the average result of the three hydrides of plate is 0.707 for the specific gravity of the condensed hydrogen. The mean of the first and second series of experiments is thus 0-664. A piece of palladium, weighing 31 grms., was fused in a lime crucible with the oxyhydrogen-flame; and the specific gravity was now found reduced to 10.5549. When this sphere of palladium was hydrogenized for a very long time (twenty-four hours) only 0.0684 grm. of hydrogen was absorbed, and by further treatment nothing was added. The specific gravity of the hydrogen in this case was 0.655, nearly identical with the mean of the valnes found from the plate experiments. Specific-Heat Observations. The apparatus devised for this purpose is represented in the figure (p. 338); the calorimeter used in the experiments had the form A. It held conveniently 100 grammes of water, and was inserted in the middle of a stout brass envelope, thoroughly exhausted of air, the whole being placed in the middle of a large cylindrical tin vessel (E), having an outer annular compartment. The tin vessel was filled with water; and a constant current from the town supply was kept circulating by means of a siphon through the outer chamber. To the thermometer a ring of thin sheet india rubber was attached as a stirrer, and, when it was removed before the immersion of the palladium, was placed in the little tin tube represented at B. Immediately before an observation the whole apparatus was moved below the steamPhil. Mag. S. 4. Vol. 47. No. 313. May 1874 bath (C), and the palladium dropped in. This arrangement of the calorimeter is very convenient in a small chemical labora tory, where uniformity of temperature cannot be easily commanded. The constancy of the radiation in the calorimeter makes the correction for cooling very exact. The same arrangement of the calorimeter is employed for registering very small amounts of heat by placing in A bisulphide of carbon or chloroform, packing the middle and outer compartments of the tin vessel full of pounded ice, and covering the exposed surface with sawdust. The thermometer is now provided with a thin sheet-copper stirrer, instead of the india-rubber one formerly used. The rate of cooling is in this case determined once for all for a range of 5° above zero, and plotted in a curve. From this curve the correction for the radiation is determined for all subsequent experiments. Two series of experiments were made. In the first series bar, and in the second plate palladium was used, three different hydrides of each. The experimental results are given in Tables II. and III. In the case of the bar the specific heat of the occluded hydrogen increased as the charge diminished, the extreme values being 3.79 and 5:05. Similarly with the plate the values are greater, and have a wider range, viz. from 3.93 to 5.88. These results are calculated in each case for the heat given to the * In this paper the term hydride is not used in its strict chemical sense, but as a convenient abbreviation. calorimeter by the hydride in excess of that of the original palladium. But if a comparison is made between the different hydrides in both series of experiments, then the specific heat is found to exhibit no such regular increase as in the former series. In the bar the extreme values are 3.21 and 3.77, and the mean of the three results is 3:47. The plate, on the other hand, gives a range of from 2-7 to 3.94, the mean value being 3:31. The amount of variation in the plate is very great as compared with the bar, and is clearly due to some secondary action taking place. The increase of specific heat for small charges of hydrogen, when comparison is made with pure palladium, is clearly due to some regular increase of the specific heat of the palladium, or of the hydrogen, or of both individually or conjointly. The mere fact that we have only a very small weight of hydrogen relatively to the amount of palladium does not explain the anomaly, because a small amount of hydrogen by the second mode of comparison does not yield such high results. The observational errors, although much greater when we are dealing with small quantities, cannot be expected to fall always in the same direction; and thus we are forced to admit there is some regular sequential change taking place in the relations of the hydrogen and palladium. The palladium, after use, is not found to have increased in specific heat, but rather the reverse; so that the explanation can only rest on some altered condition of the bodies when united. So far as my experiments have led me, I am inclined to regard this alteration as a kind of molecular dissociation that increases with diminished charge of hydrogen. I am led to this conclusion from observing that the rate at which palladium loses bydrogen at constant temperature is dependent on the amount present increasing with diminished charge. In conducting these specific-heat experiments, a greater variation in the results has always been observed when small charges were under experiment, especially in the first two or three determinations. This increase of dissociation may be explained in part from the formation of regular cracks or channels in the mass of palladium, together with the effect of pressure resulting from the contraction of the external layers of palladium from the loss of hydrogen. It is, however, premature to discuss the cause of this increase of dissociation until a more extended series of experiments is made, and the behaviour of the alloy to pressure is investigated. |