Quantities of Foreign Matter on Crystallization. 397 



always covered. In this state they remain for two or three 

 weeks, when they are transferred to a vessel containing com- 

 mercial oil of vitriol, and gently heated. After some hours 

 the stones are washed, dried on the stove, and transferred to 

 the lapidary. After being cut they are steeped in oil for a 

 day ; this causes small crevices to disappear, and gives a 

 better lustre to the stones after polishing with oil and bran. 

 The action of the sulphuric acid is to fill up the porous bands 

 with carbon so as to produce greyish-brown or black stripes, 

 while the non-absorbent bands become opaque white. Onyxes 

 are prepared from Brazilian pebbles. A fine citron-yellow, 

 uniform or in stripes, or in cloudy patches, is given to chal- 

 cedony by first drying the stones on a stove, and then 

 immersing them for two or three weeks in hydrochloric acid. 

 This contains a little iron, which is probably the source of the 

 colour. A blue colour is produced by steeping the chalcedony 

 in a solution of protosulphate of iron, and then in one of 

 prussiate of potash. Other colours are similarly produced 

 by forming precipitates within the stone. Even cochineal 

 will impart its colour to agate. By the action of heat and 

 various chemicals a great variety of ornamental pebbles are 

 manufactured, some of which fetch a high price, such as the 

 milky-white chalcedonies, with dendritic brown and black 

 figures. Striped stones are converted into sardonyxes, and 

 stones of one colour contract true carnelian tints. In some 

 cases the pebbles are dried on a very hot stove during some 

 weeks, then wetted with sulphuric acid, and raised to a red 

 heat in a luted crucible ; they are then allowed to cool slowly, 

 and the effect of the heat has been to deprive the hydrated 

 oxide of iron of water, when the oxide combining with the 

 silica develops the carnelian colour. 



It would be a useful but difficult inquiry to determine the 

 influence exerted by small quantities of apparently foreign 

 or accidental impurities in modifying the structure of siliceous 

 and other minerals. These presumably accidental ingredients 

 bear the odd and apparently inappropriate name of Mineral- 

 izers. Their effect in producing a molecular change in 

 structure seems to be undoubted, but the necessity for a 

 searching inquiry seems to be evident, on account of the con- 

 tradictory statements that are occasionally met with. For 

 example, the green colour of the emerald is said to be due to 

 chromium oxide, but Lewy* attributes it to the presence of a 

 minute proportion of an organic hydrocarbon, which is dissi- 

 pated by a strong heat, which would have no effect on chromium 



* Ann. de Chemie. 1858, t, liii. 



