Art. XXXVIII. — Researches on the Absorptivs Properties of Platinum.
[Read before the Wellington Philosophical Society, October 22, 1870.]
The researches embodied in this paper were collateral with those given in Art. XXXVII. in point of time, and of a kindred nature, but I preferred to state the results of them separately, as the subject itself is foreign to gold mining interests, to which the other appears intimately connected.
Shortly stated, these results are as follows:—
When a piece of freshly-cleaned platinum is placed in the vapour of sulphuretted hydrogen, or in solution of sulphide of ammonium, at common temperature for a few minutes, then well washed in distilled water, it will be found to have acquired such a condition upon its surfaces that metallic contact cannot be established between it and mercur; whereas, before treatment with these sulphur compounds, amalgamation rapidly proceeded over the whole surface of such piece on the application of mercury.
When the sulphuretted hydrogen was thoroughly dessicated, before administration to the platinum, amalgamation was neither prevented nor retarded; at least, I could not observe any such effect.
Platinum rendered thus non-amalgamable, becomes again readily amalgam able at a temperature of 400° to 600° F., also by a short contact with any of the following substances at common temperatures,—chromic acid, nitric acid, nascent hydrogen, or chlorine.
Sulphuric and hydrochloric acids had not this effect, neither had cyanide of potassium, not even when boiled with it.
Upon the surfaces of platinum thus treated with either of these sulphides, sulphur was readily detected by digesting them in a boiling solution of cyanide of potassium, and applying the nitro-prusside test.
It further appeared that platinum is also brought into a non-amalgamable condition by a short contact with either aqueous solution of potash or ammonia; even distilled water had the same effect if allowed contact for an hour or two; this may be owing, however, to traces of ammonia present in it.
In these cases, however, the application of hydrochloric or sulphuric acid to the platinum, rendered it readily amalgamable.
Clean platinum has been found to amalgamate readily, after twenty-four hours contact, with dry air.
In none of these cases did the metal appear to sustain any visible change upon its surfaces.
The results thus stated, tend to show,—
1st. That platinum, like gold, is capable of absorbing sulphur at common temperature, from either a solid or gaseous compound of it.
2nd. That this absorption is chemical.
3rd. That this metal is superficially oxydized in alkaline solutions.
These results, therefore, to a certain extent appear to impugn the correctness of the opinion that gaseous absorption by platinum is, in every case, simply mechanical.