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PRECIOUS STONES.

given off to the extent of 20 to 24 per cent. With moderate heating no loss of weight, but merely change of colour, occurs, the bulk of the stone remaining unaltered, and consequently its specific gravity suffering no increase or diminution. The sherry-coloured, the brown, and the other tinted topazes, which are susceptible of being "pinked" by heat, exhibit a very curious phenomenon during the operation. When a suitable stone is packed in magnesia or other inert material, and heated in a crucible, the specimen, if removed before it is cold and laid upon a white surface, shows scarcely any trace of colour; but after a little time, when the stone has acquired the temperature of the air, the desired pink hue makes its appearance. If the temperature reached has not been sufficiently high, a salmon tint, or a hue like that of a drop of blood mingled with much water, is obtained instead of a rose-petal pink. What the cause of these changes of colour is remains doubtful: it may be a change in the molecular or physical condition of some minute trace of a coloured constituent in the topaz, or it may be an actual chemical change. Anyhow the colour of topaz is a very unstable one, for light, or at least the solar rays, soon exerts a bleaching effect on many pale-coloured specimens; so that the fine suite of wine-hued Russian crystals, collected by Colonel de Kokscharow, and now in the British (Natural History) Museum, is kept shrouded from the light of day.

Topaz occurs in several Scotch and Irish, and in some English, localities—St. Michael's Mount in Cornwall, may be named amongst the latter. Villa Rica, Minas Novas in Minas Geraes in Brazil, Flinders Island, and many places in the United States, as well as several Siberian localities, furnish splendid specimens of colourless and coloured topaz. The white topazes from Flinders Island are less brilliant than those from Brazil. Good topazes come from Pegu and Ceylon, and they have been found in Australasia. A magnificent deep blue topaz was found in Ceylon in 1899: when cut, it weighed no less than 355 carats. The topazios of the