4. _Indelible Writing_.—As the art of man can unmake whatever the art of man can make, we have no right to expect an _indelible ink_: however, a sort of approximation to it may be made as follows:—Let a saturated solution of indigo and madder in boiling water be made, in such proportions as give a purple tint; add to it from one sixth to one eighth of its weight of sulphuric acid, according to the thickness and strength of the paper to be used: this makes an ink which flows pretty freely from the pen, and when writing, which has been executed with it, is exposed to a considerable, but gradual, heat from the fire, it becomes completely black, the letters being burnt in and charred by the action of the sulphuric acid. _If the acid has not been used in sufficient quantity to destroy the texture of the paper, and reduce it to the state of tinder, the colour may be discharged by the oxymuriatic and oxalic acids, and their compounds, though not without great difficulty_. When the full proportion of acid has been employed, a little crumpling and rubbing of the paper reduces the carbonaceous matter of the letters to powder; but by putting a black ground behind them, they may be preserved, and thus a species of _indelible writing_ is procured, (for the letters are, in a manner, stamped out of the paper,) which might be useful for some purposes, perhaps for the signature of bank-notes. 5. _Peculiar Crystals of Quartz_.—Mr. W. Phillips has met with some remarkable crystals of quartz, which occurred imbedded in the [p224] limestone of the Black-rock, near Cork. They are from the fourth to the half of an inch in length, and about half their length in width: they are smooth, externally, for the most part, and sometimes considerably bright; they are of the colour termed smoky, or brown quartz, externally, and may easily be separated from the limestone, leaving a cavity of their exact form. On trying to cleave them, they yielded parallel to one or other of the planes of the pyramid, like common quartz, but at such fractures appeared to consist of alternate and concentric prisms of smoky transparent quartz, and of gray opaque, and somewhat granular limestone. On applying muriatic acid to the surface, effervescence occurred along the gray parts, proving the presence of limestone, but soon ceased: after an action continued for some weeks, the gray parts became cellular, and so soft, as to admit of being scraped by a knife. Mr. Phillips says, it seems reasonable to conclude that such part of the gray substance as does not yield to the action of the acid is siliceous or quartzose; and that the prime difference between it and the smoky quartz surrounding it consists in the different circumstances of crystalline aggregation under which they are deposited. The crystals, with the somewhat analogous case of the Fontainebleau sandstone, may serve to assist in the illustration of some points relative to the laws of affinity, as operating in the