Dodecaëdral Carbonate of Lime

Nature, ever instructing us with useful lessons, at first sight often puzzles us. I had long since seen Quartzose mixtures cast, as it were, over tabular Sulphate of Barytes, forming a very accurate mould of them, small or large. Such are commonly called Hacked Quartz, from the notion that they resembled a substance partly chopped or hacked with a hatchet or sharp instrument; and their formation was long left unaccounted for.

It is not uncommon to find Quartz moulds, if I may so call them, of the Cubic Fluor^*, on which it often forms coats; and I have a very curious specimen of white semi-opaque Quartz having formed over the Metastatic Carbonate of Lime, from Polgooth in Cornwall, with which I was favoured by Phil. Rashleigh, Esq.: but I had not seen Carbonate of Lime forming the mould of Sulphate of Barytes till very lately, I received it from Alstone Moor in June, 1809. The specimen represented in the figure is crystallized in the equiaxe form; and I had another specimen sent at the same time, which was cast over larger Sulphate of Barytes, crystallized in dodecaëdrons with rhomboidal faces—see tab. 128—which is very rare: but a still rarer form is when truncated at the edges, as they are in the specimen spoken of—see the right hand outline.

It is a curious lesson in the study of Chemistry to search for the menstruum in which the Sulphate of Barytes may he dissolved without hurting the Carbonate of Lime; for it is too much undercut, as the workmen would say, to deliver the cast whole; and the facets of the Barytes are very sharp in the mould, and indicate some rare ones that appear to have been extremely smooth with bright and polished surfaces. Whether the Ochre, or other pulverulent matter which is about the specimen, helped it to separate, I dare not say; but I scarcely think that would have been sufficient of itself.