An introduction to the study of minerals : with a guide to the mineral gallery / by L. Fletcher.

  • British Museum (Natural History). Department of Mineralogy.
Date:
1914
Catalogue details

Licence: In copyright

Credit: An introduction to the study of minerals : with a guide to the mineral gallery / by L. Fletcher. Source: Wellcome Collection.

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    “ Crystal ” is bounded by plane surfaces. Later meaning of the term Crystal. Steno’s discovery against the vulgar error) Madagascar and other countries in the Torrid zone abound with Crystal.” It was observed by the ancients that this mineral, wherever found, has a characteristic shape (Fig. 1). It is naturally bounded by flat surfaces (or planes) arranged in a definite way —six of them generally forming a column (or prism), at each end of which are arranged six other planes so inclined that they form a pyramid. The relative sizes and the shapes of these planes vary in different specimens; the prism may be com- paratively short, or even be entirely absent. 15. This peculiarity of being naturally bounded by flat sur- Specimens, faces, and not by the curved ones which are so characteristic both of Plants and Animals, was afterwards found to belong not only to “ Crystal” but to other minerals both transparent and opaque; so that by an extension of its meaning the term Crystal was eventually used to signify, not the particular kind of mineral still known as Rock-crystal, but any mineral naturally limited by plane faces. 16. It was not till 1669 that any important addition to the knowledge of the properties of minerals was made. In that year Nicolaus Steno,* a Danish physician, announced that, amid all the variations in the sizes and shapes of the faces of the mineral termed Rock-crystal, there was something constant besides the number and the grouping of the faces. Cutting- each of a series of specimens in a direction at right angles to the edges of the prism, he found that the edges of the six- * De solido intra solidum naturaliter contento dissertationis prodromus. Florentise, 1.669 : English translation, London, 1671. Figs. 2 and 3 are repro- ductions of figures given by Steno.