Page:Proceedings of the Royal Society of London Vol 1.djvu/103

Rh tion of specimens of this stone, and of various other species which were long thought analogous, but are.now found to constitute with it a genus of some extent. This substance, we are now told, pre- sents itself to our senses, especially to our sight, under two very dis- similar appearances. The ﬁrst is the original corundum imported ﬁnder that name and also under that of adamantine spar. At times it is brought in irregular fragments, but often in crystals, which are generally of a pretty large size. Those of the other appearance are, in fact, the gems hitherto known by the names of Sapphire, Ruby, &c. Another distinction is next made, according to the texture or ﬁneness of the grain of these stones; the former being called imper- fect, and the latter, or the gems, perfect Corundum.

The author now enters into a minute account of the appearances and the principal properties of this substance. And ﬁrst he treats of its colour. This, he tells us, is at best but an uncertain character in stones, but yet it may at times afford some secondary marks of di- stinction. The common or imperfect corundum varies, in this respect, according to the country where it is found. That from the Camatic is of a grayish White, sometimes approaching to a pale green, and sometimes, though rarely, of a red or blue colour : that from China and Ava is generally of a dull green or brown colour; and that from the coast of Malabar appears of a reddish brown.

The perfect corundum which is found in Pegu and in Ceylon is either red, blue, or yellow. The former is the Oriental ruby; the blue is the sapphire; and the yellow the topaz.- From a duly pro- portioned mixture of the blue and the red is produced the purple colour, which constitutes the Oriental amethyst. The union of the blue and yellow forms the green colour, which is proper to the Ori- ental emerald; and a larger proportion of yellow produces the colour proper to the chrysolite.

2. As to transparency, the crystals of corundum from the Carnatic, having usually rough surfaces, are of course very imperfectly diapha- nous; but when broken, their fragments have generally a certain de- gree of semi-transparency : most of these fragments, when held up to the light, show a number of ﬁssures in their substance, which in a great measure prevent the transmission of light. These ﬁssures arise from a want of cohesion between all the parts of the crystalline la- minae. The red and blue corundum of the Carnatic has a greater degree of transparency than those of any other colour ; the blue in particular having generally the preference in this respect.

3. In hardness the corundum comes next to the diamond; but this quality, with regard to intensity, differs greatly, according to the co- lour an“ degrees of purity of the specimens. The corundum of the Carnatit When it is neither blue nor red, is less hard then any other sort; wnilst the imperfect blue species exceeds all the other varieties of this kind in hardness,—such is its density that it will emit pretty bright sparks when struck with steel.

4. This substance, like quartz, becomes phosphorescent by collision. The crystals of a red colour, whether of the perfect or im-