Sulphate of Lime, or Gypsum

I have two reasons for making my readers acquainted with this variety of Gypsum,—its perculiar situation, and its form. It is from a Lead mine in Westmoreland, is accompanied by Galæna, and is formed in a hollow of Limestone, which is very ochraceous, has a little Pyrites, and is lined with small bubbled Calamine.

Whether the Sulphate of Lime be of late formation from the Sulphur in the Pyrites and the ochraceous Lime, or has old as Galajna, may be a geological question; but if not known at present, a hint of tills kind may cause observation, and, with other subjects, if newly formed, teach us the utility of exposing some parts of a mine to the air, as mentioned in the description of tab. 23: and at other times endeavour to prevent decomposition or loss, by excluding the air: thus exerting judiciously our chemical and other faculties (allowed us for these pursuits,) to use the many agents winch Providence has prepared tor us.

I believe finer examples of this are in the hands of Mr. Walker, who kindly promises to lend me any thing. I, however, sometimes think that moderate examples should be employed in preference for instruction, because they help us to overcome difficulties that we ought to be accustomed to, as all substances cannot be equally favourable. Its modification belongs to the prominule mackle of Haüy, tab. 34, fig. 99; which latter he had not seen in its simple form, fig. 102; I believe it is therefore the greater curiosity. I have placed the nucleus within the geometrical outline, to make it the more perfectly understood.

The columns vary in the number of their sides, from six to a much greater number, formed by many striæ. The base and apex are nearly flat. The right hand figure is that of a small one selected from the group, and is extremely clear and neat. The larger ones are chiefly many-sided columns, being multiplied by striae on the narrower face on the right hand, which is the same as the broad faces in tab. 405, and is one of the primitive faces, P of Haüy.