MINERALOGY. 79 
crystallized nearly like arragonite. The crystals occupy either the hollow 
of the druse (pl. 36, fig. '7), or present themselves in superficially radiating 
needles (jig. 20). 
4. Carbonate of Lead. 
The white lead of the arts is a carbonate of lead. This substance is 
found native, though rarely, in acicular, fibrous, distorted prisms, of the 
trimetric system (pl. 36, figs. 47 and 48) ; fig. 21 represents a druse of this 
white lead ore. 
5. Beracite. 
This is a biborate of magnesia found at Lineburg and Segeberg, in very 
fine crystals, associated with anhydrite (pl. 35, fig. 12). The most usual 
forms of the Liineburg boracite are the rhombic dodecahedron (pl. 35, 
jig. 48), and their transitions into octahedrons (fig. 42) and rhombic dode- 
cahedrons, with cubic and octahedral surfaces (jig. 30). The cube is also 
found with angles truncated or replaced (figs. 38, 39), but the opposite solid 
angles differ in their modifications. The crystals are imbedded in anhydrite. 
6. Sulphate of Lame 
This compound of sulphuric acid and lime constitutes, at times, whole beds 
of rocks, specially in connexion with rock salt. Without the water of 
crystallization, sulphate of lime is known as anhydrite or karstenite. In this 
form it occurs in amorphous masses (pl. 36, fig. 31), traversed or not by 
crystallized anhydrite, and is of a greyish-white, bluish, or blackish color. 
Combined with water of crystallization sulphate of lime forms gypsum or 
plaster of Paris. When gypsum is ground into an impalpable powder, and 
strongly heated in an iron vessel over a fire, its water of crystallization will 
be given off in the form of bubbles, rising through the powder, presenting a 
strong resemblance to ebullition. This, indeed, is technically termed 
“boiling.” This anhydrous powder, when mixed with water, will, by 
taking up a portion of the latter, soon harden, taking the shape of any form 
or mould into which the mixture may be poured. For this reason it has 
extensive applications in the arts. The unheated powder, if mixed up by 
means of a solution of carbonate of potassa, possesses the same property of 
fixing or hardening. 
7. Sulphate of Baryta. 
The crystallized forms of this substance have the rectangular octahedron 
for their base. It received the name of barytes, or heavy spar, from its 
great specific gravity, by which it may readily be distinguished from all 
other minerals, excepting ores. The most common among the manifold 
crystallizations of barytes are shown in figs. 1, 2, 4, 6, 8, 9, 10, 13. It is 
generally of a white, or bluish-white color, and very abundant in some 
localities. It is used, when ground up, to adulterate white lead. 
8. Vitriols. 
By the term vitriols miners understand the sulphates which are formed by 
the exposure of sulphurets to the weather, and become dissolved in the 
waters of the mines. Iron, or green vitriol ( fig. 27), with its most common 
crystallized form (fig. 45), is sulphate of iron: it is of a green color, 
becoming ferruginous on exposure to the air from the formation of an 
509 
