This section is from the book "An Experimental History Of The Materia Medica", by William Lewis.
Hibernicus Lapis & Tegula hibemica & Ardefia hibemica. Hardesia Pharm. Paris. Irish slate: a kind of slate or very soft stone, found in different parts of Ireland} in the mass of a bluish black colour and staining the hands; when powdered, pale or whitish at first, and in keeping growing black; in the fire yielding sulphureous fumes, and acquiring a pale red colour with an additional hardness.
This mineral appears from Dr. Rutty's experiments in his synopsis of mineral waters, to be a matrix of serrugineous vitriol; which it discovered by its taste, and by the black colour which infusions of it struck with galls. The specimens 1 examined, procured from our shops, had no vitriolic taste, and infusions of them in water suffered no visiblc change from galls: but that they contained materials from which vitriol is producible, appeared from the sulphureous fumes they emitted in the fire, and from their giving out, when burnt, a calx of iron to aqua regis; the iron discovered itself by the tincture linking a blue colour with the lixivium described under the article ferrum: their burning hard shews their earthy matter to be of the argillaceous kind. It may therefore be prefumed, that this fossil consists of argillaceous earth impregnated, like the pyritae, with sulphur and iron; and that, like the pyritse also, it is capable of becoming vitriolic by long exposure to the air and moisture. It has been sometimes taken by the common people, powdered and mixed with spruce beer, against inward bruises; but its medicinal use in any intention is not much to be commended, on account of the variability of its qualities.