COMMON GALENA OR LEAD-GLANCE. 367 with its oxygen, flies off in the form of carbonic acid gas, while the lead is reduced to the metallic state, and sinks to the bottom of the furnace. Almost all the va rieties of galena or lead-glance contain a greater or less Portion of silver. The silver, after the reduction of the lead, may be separated by the process of cupellation; but in the greater number of instances, the quantity of silver is so inconsiderable, as not to repay the expence of labour; and hence the lead of commerce almost always contains a minute portion of silver. Galena is also used for glazing pottery. Observations. 1. Some dark-coloured varieties of galena might, with a superficial observer, be confounded with blende, or sul- Phuret of zinc; but the lustre of the zinc-ore is destroyed by scratching the surface with a knife, which is not the case with the galena or lead-glance. If both are breath ed upon, the galena recovers its lustre in a moment; the blende very slowly. Galena is distinguished from gra phite by its colour, greater specific gravity, and by the comparatively faint trace it leaves on paper. The same frarks will serve to distinguish it from mohjbdena, which Possesses, besides a foliated fracture, a considerable de- 8 r ee of flexibility. 2. Galena, which occurs in beds, is said to contain les* silver than that found in veins. Second