that some modifications of the primitive crystal of this substance are principally the production of particular districts, as I am led to suspect will be the case, might not an investigation of the nature and peculiarities of the veins, and of the country through which they pass, tend to throw some light on the circumstances, or laws, by which the several modifications are produced: may not these circumstances be supposed in some degree to depend on the purity of the substance itself, or to be affected by the various proportions of other substances entering into combination with it? The Bohemian oxyd has not hitherto been observed to assume so great a diversity of crystalline forms as the Cornish, which by the analysis of Klaproth already noticed, appears to be by far the most pure.
The crystals of this substance from Bohemia are generally much larger than those from Cornwall, but Pryce mentions one he had seen that weighed upwards of two ounces. Very large crystals, mostly of the macle, I believe, were found in Seal-hole and Trevonance mines in St. Agness; in the former they were lying loose in the vein, and were conveyed without first breaking or purifying them, immediately from the mine to the smelting-house. Some have also lately been brought from a mine in the neighbourhood of the Tamar, and others from near the Land's End; but instances of this kind are by no means common. The crystals of this substance are generally in part imbedded in the matrix; they are not commonly so disposed as to shew both paramids, and are sometimes confusedly grouped, but this appearance of confusion principally arises from a circumstance which will be hereafter explained in speaking of the macle, to which the oxyd of tin is so liable. The crystals are rarely disposed in radii, but I have one specimen on which they are so disposed. Radiated schorl has often been mistaken for tin, to which it frequently bears considerable.