in mineralogy, a name given to those earths which break easily into rhomboidal, cubical, or laminated fragments with polished surfaces. As the term spar is thus applied to stones of different kinds, without any regard to the ingredients of which they are composed, some additional term must be used to express the constituent parts as well as the figure; for instance, calcareous spar, gypseous spar, &c. The spars found in Britain and Ireland are of four different species; opaque, refracting, diaphanous, and stalactitical. 1. The opaque spar is rhomboidal, hexangular, and triangular, of various colours, and is found in mines in Wales, Derbyshire, &c. and at Ovens near Cork. 2. The refracting spar is rhomboidal, shows objects seen through it double, and sometimes 8, 12, or 16 images at once. It is frequent in the lead mines of Derbyshire, Yorkshire, &c. 3. Diaphanous spar is rhomboidal, triangular, hexangular, pyramidal or columnar; and is found in mines, quarries, and caverns, in many different places. 4. Stalactitical spar, icicle or drop-stone, is formed by the running or dropping of water, containing a large proportion of calcareous earth. It is opaque, generally laminated, but from accidental circumstances assumes various forms. It occurs at Knaresborough in Yorkshire, and at Ovens near Cork.

A new species of spar has lately been found in the East Indies, which, from its extreme hardness, approaching to that of a diamond, is called adamantine spar. It was discovered by Dr Black of Edinburgh to be a distinct species. Happening one day to visit a lapidary, it was shown to him among other specimens as a stone that was used in the East Indies for polishing gems, and grinding other hard substances. Dr Black immediately singled out a specimen which he sent to Mr Greville, who requested M. Klaproth to analyze it.

There are two varieties of this spar; one of them comes from China, and crystallizes in hexagonal prisms without pyramids, the length of the sides varying from six to twelve lines; their breadth being about nine, of a grey colour with different shades. Though the entire pieces are opaque, the thin laminae are transparent, and when broken, its surface appears slightly striated. Its crystals are covered with a very fine and strongly adhering crust, composed of scales of silvery mica, mixed with particles of red feldspar. Sometimes the surface has martial pyrites or yellow sulphuret of iron adhering to it. Its hardness is so great, that it not only cuts glass as easily as the diamond, but even scratches rock-crystal and other very hard stones. Its specific gravity is to that of water as 3710 to 1000. Sometimes it contains crystallized grains of magnetic oxyd of iron, which may be separated from the stone when pulverized by means of the loadstone.

The other kind found in Hindostan is of a whiter colour, and of a more laminated texture than the former; the grains of iron contained in it are likewise of a smaller size than those of the former; they are not diffused through its substance, but only adhere to its surface.

This spar is exceedingly difficult to analyze. To do so, M. Klaproth was obliged to melt it no less than 12 times with 15 parts of soda or mineral alkali, in a silver crucible; the heat being each time continued for five hours as strong as the crucible could bear. After each fusion the mass was softened by boiling distilled water, filtering and precipitating by acids the small quantity of earth which the alkali had dissolved; and lastly, that portion which had not been decomposed was digested at different times with concentrated and boiling acids. By this tedious process he at length found, that the spar consisted of alumine and another kind of earth, in the proportion of 2 to 1, the nature of which is not understood. It is not siliceous earth, as it does not combine with fixed alkalis in a melting heat; and for want of opportunities to make a sufficient number of experiments, our author was unable to determine whether it be a fifth simple earth, or a composition of two or more earths which he was not able to separate.

From a letter of M. Morveau to Mr Crell, it appears that this stone is also found in France. A small bit of this was tried by him in presence of Mr Wedgwood, and he found that its specific gravity was superior to the spar of China, being no less than 4.1803, and the true adamantine spar of China gave 3.8222.