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Art. LXV.—On the Nature of the Precipitate formed by certain Mercuric Salts in presence of Essential Oils.
[Read before the Wellington Philosophical Society, 21st February, 1880.]
In a paper read before you last year* I showed that mercuric-chloride, when added to a mixed solution of mercuro-iodide of potassium and any essential oil, determined a white precipitate therein in the place of that red coloured one which would form were the oil omitted; but while I showed how these precipitates could be distinguished from those produced by such mercurial salts in the presence of the alkaloids and the albumenoids, I did not inform you as to the precise nature of these precipitates, being then ignorant of it myself. Since then, however, I have, in order to settle this point, acquired a wider knowledge of the whole matter, and so am enabled to inform you that other substances, not exactly essential oils but partaking somewhat of their nature, also behave like such oils in respect to the mercurial compounds named; and by this wider knowledge of the subject I am also enabled now to describe the nature of these precipitates to you.
These substances are camphor, carbolic acid, kerosene, gasoline, picrotoxia, and guaiacum resin.
In regard to kerosene and gasoline it is only a very small portion of these substances which is ever concerned in the production of any colourless mercurial precipitate; and so I consider this part to be a portion of the oil which has been oxydized by the air to an acid hydrocarbon, or to something on the way to this.
And now, as to the nature of the precipitates:—
Camphor.—As I have got that obtained in presence of camphor,—none of the camphor is present. I merely decanted the liquid portion off, and allowed the residue to dry in the air at a low temperature. The pale yellow mass thus resulting turned to a bright scarlet colour when pressed, as the iodide of mercury does, but kept its yellow colour for months; I may say, indeed, it would do this permanently, if left alone. As this yellow mass does not contain camphor, it is certainly the pure mercurial iodide; and the reason, therefore, that it does not (as it is termed) spontaneously redden, is that its particles are so detached among themselves that, except for some outside agency, they cannot get sufficiently within each other's influence to favour the molecular action necessary to produce this change of colour.
Guaiacum Resin.—This resin, in acetic acid diluted with water, furnishes a solution which, like the other oils and resins named here, does not afford a precipitate with mercuro-iodide of potassium, but when afterwards treated
[Footnote] * Trans. N.Z. Inst., Vol. XI., p. 470.
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with mercuro-chloride, a pale one forms, of some permanency as to colour. This compound, however, differs from the others which I have described in being of a soft plastic nature. It passes through filter-paper as readily as the analogous compound with digitaline. It reddens under pressure—a singular circumstance, considering that a resinous matter is certainly present throughout it, and as it seems, combined therewith. That a purely mechanical force, as pressure, can not only modify the chemical (or, at least, molecular arrangement) of a substance like mercuro-iodide,—but actually eliminate a substance (and one which is, besides, non-volatile at the temperature used) from its combination with this salt (mercuro-iodide), in order to pass such mercuro-salt to its more fixed or final form,—would be a phenomenon of so unexpected a nature, that one hesitates to accept this view of the case, and feels inclined, for the present, to consider the colorific change in question to be favoured by a decomposition of the precipitate prior to its being dried for the pressing. Urea behaves with these mercurial salts much the same as this resin.
Carbolic Acid.—The compound formed by these mercurial salts in presence of this acid, appears in the form of opalescent oily globules, which neither subside nor are retained by paper-filters; however, by the addition thereto of a little common salt, these agglomerate and finally precipitate in the form of a light yellow solid, which when dried preserves its colour even when strongly pressed by a hard substance; it also sublimes without undergoing any notable chemical change, and the sublimate is also unaffected by pressure. From these results it appears that the pale precipitate in question contains carbolic acid, and is a compound possessed of considerable stability.
Picrotoxia.—Picrotoxia, by the process described, forms a compound with mercuro-iodide having considerable stability; its colour is not affected by pressure; it is not chemically affected except at a comparatively high temperature, in which case it chars and evolves gaseous matters.
I need not detail any further results, as the cases I have selected for this are typical of all I have investigated. I will only refer back to the precipitates formed in presence of the more volatile substances cited, for the purpose of stating my belief that as the less volatile oils certainly combine with the mercuro-iodide, the more volatile ones in the first instance are also in combination with it, but a combination so unstable that it is difficult to isolate them for examination.
I should state that these mercuro-compounds are, as a rule, soluble in alcohol, ether, mercuro-iodide of potassium, or mercuric-chloride.
These results show that in examining for albumenoids or alkaloids by the mercuro-iodide and chloride test, it is necessary to remove all resinous
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or oily matter which may be present in the solution to which such test is applied. This can generally be accomplished by shaking such solution with a little pure hard fat, at its fusing-point, and removing the crust which forms on cooling.
In conclusion, I would desire to point out to you that by some of the facts above stated may be explained that difference which has arisen between Reynoso and Schiff respecting the character of the precipitate produced by adding water to alcohol in which has long been dissolved mercuro-iodide; the former has stated it to be red; the latter, to be yellow—a discrepancy which is readily susceptible of explanation upon the very likely assumption that in the case where such precipitate is yellow, an oil was present in the alcohol used.