VI. Additional observations on hydrogenium

From the elongation of a palladium wire, caused by the occlusion of hydrogen, the density of hydrogenium was inferred to be a little under 2. But it is now to be remarked that another number of half that amount may be deduced with equal probability from the same experimental data. This double result is a consequence of the singular permanent shortening of the palladium wire observed after the expulsion of hydrogen. In a particular observation formerly described, for instance, a wire of 609·14 millims. increased in length to 618·92 millims. when charged with hydrogen, and fell to 599·44 millims. when the hydrogen was extracted. The elongation was 9·78 millims., and the absolute shortening or retraction 9·7 millims, making the extreme difference in length 19·48 millims. The elongation and retraction would appear, indeed, to be equal in amount. Now it is by no means impossible that the volume added to the wire by the hydrogenium is represented by the elongation and retraction taken together, and not by the elongation alone, as hitherto assumed. It is only necessary to suppose that the retraction of the palladium molecules takes place the moment the hydrogen is first absorbed, instead of being deferred till the latter is expelled; for the righting of the particles of the palladium wire (which are in a state of excessive tension in the direction of the length of the wire) may as well take place in the act of the absorption of the hydrogen as in the expulsion of that element. It may indeed appear most probable in the abstract that the mobility of the palladium particle is determined by the first entrance of the hydrogen. The hydrogenium will then be assumed to occupy double the space previously allotted to it, and the density of the metal will be reduced to one half of the former estimate. In the experiment referred to the volume of hydrogenium in the alloy will rise from 4·68 per cent, to 9·36 per cent., and the density of hydrogenium will fall from 1·708 to 0·854, according to the new calculation. In a series of four observations upon the same wire, previously recorded, the whole retractions rather exceeded the whole elongations, the first amounting to 23·99 millims., and the last to 21·38 millims. Their united amount would justify a still greater reduction in the density of hydrogenium, namely to 0·8051. The first experiment, however, in hydrogenating any palladium wire appears to be the most uniform in its results. The expulsion of the hydrogen afterwards by heat always injures the structure of the wire more or less, and probably affects the regularity of the expansion afterwards in different directions. The equality of the expansion and the retraction in a first experiment appears also to be a matter of certainty. This is a curious molecular fact of which we are unable as yet to see the full import. In illustration, another experiment upon a pure palladium wire may be detailed. This wire, which was new, took up a full charge of hydrogen, namely 956·3 volumes, and increased in length from 609·585 to 619·354 millims.