Sulphate of Barytes

When any thing unusual comes under our investigation, it is a pleasant task to account for it if we can. Subjects of the present nature may not be uncommon, therefore it is the more convenient to comprehend them. The specimen here represented is particularly interesting and instructive, as the forms are strikingly curious, and show the tendency of the molecules to form according to the laws of Crystallography, notwithstanding certain interruptions. In this instance the gravity of a quantity of Sulphate of Barytes in solution has seemingly caused it to settle in contact with a loose powder, consequently having more air in its interstices, which, as the Barytes subsided, has risen into the substance, and, in part, interrupted the mass, while at the same time it gives them particular forms and a curious appearance ; some of the powder filling them up more or less towards their bases, which are sometimes quadrangular, but mostly hcxangular—see the right hand figure—where it appears that the greatest part of the hollows and the larger sides of their bases are diagonal to the primitive fracture. The variety in the forms of the hollows are innumerable; and there are some hair-like appearances on the tops of some of them, which are bending or otherwise modified: in the point of one is a little black speck like soot. The specimen has broken rather conchoidaily, but not far from the primitive fracture. The somewhat zigzag line is interruptedly six-sided, depending on the particular interruption of the primitive molecules, and is chiefly filled with the same powder as the others; whence its opacity. It is not much tinlike in the angles to the lottom figure of tab. 72, and is remarkable for its contrary and prostrate appearance; and the combination of the two might lead the imagination very far, of any one who had not attended to Crystallography. I mentioned in the description of tab. 71 the water or liquid in the little hollows of this substance. I add an outline or sketch of them at the bottom of this plate, as hitherto they appear to be a great curiosity. The hollows are rather irregular, a little angular, and have sometimes more or less tendency to the form of crystallization. The air bubbles of course upwards, in whatever position they are held; but in the long one it only moves the space marked with dots.

I have specimens of solid crystals of Sulphate of Barytes covered with, and passing into one another; and so it happens with different substances; but these I thought sufficiently remarkable to illustrate the present phænomenon, which may be very useful. Both these specimens serve to show the double refraction completely through the columnar sides. Thus the bubbles and hollows are seen double as in the bottom specimen—see the magnified figure. And where we can look into the upper specimen, the pyramids and odd forms may be seen double.

These specimens are from near Alstone Moor, Cumberland, and are in the possession of—Walker, Esq. I have specimens with Copper Pyrites in spicules, hair-formed, &c., in them.