SCHISTOSE, OR SLATE-ROCKS. 337 



bonate and muriate of barytes, appear to produce symptoms simi- 

 lar to those occasioned by arsenic, but less violent. In such 

 cases, some alkaline, or earthy sulphate, such as that of mag- 

 nesia, or of soda, has been found to neutralize the poison, by form- 

 ing an insoluble salt which exerts no action on the animal 

 system . 



SCHISTOSE, Or SLATE-ROCKS. 



1. Aluminous Schistus, or Alum-Slate. — Pl. XLV. fig. 3. 

 — This rock, which affords the greater part of the alum of commerce, 

 is of a greyish, blueish, or iron-black colour, and often contains 

 marine shells, and other similar organic remains. Its lustre is 

 glistening or glimmering, or semi-metallic, and it is sometimes 

 iridescent on the surface. The structure is slaty, and it splits, 

 by exposure to the air, into thin plates. It is generally soft, and 

 yields easily to the knife. It consists of clay-slate impregnated 

 with a large portion of carbonaceous or bituminous matter, and 

 sulphate of iron. Alum-slate forms a bed several hundred feet in 

 thickness, at Whitby, in Yorkshire, which, according to Mr. 

 Bakewell, extends over a great part of the Cleveland hills, and is 

 intersected by a vertical dyke of basalt. A full account of the 

 processes for the manufacture of alum was published by Mr. 

 Winter, in the 26th volume of Nicholson's Journal. One hundred 

 and thirty tons of this schistus produce, on an average, one ton of 

 alum. 



Alum, prepared according to the process adopted at Whitby, is a sul- 

 phate of alumina and potass. It is in large irregular semi-transparent 

 colourless masses, having a vitreous fracture, and crystalizes from its 

 solutions in regular octahedrons. It has a sweetish, acidulous, styp- 

 tic taste, and from its excess of acid it reddens the vegetable blues. 

 It is soluble in about 14 parts of water at 60°, but insoluble in 

 alcohol. When exposed to a moderate heat, its water of crys- 

 talization is expelled, and a white, friable, opaque, spongy mass 

 remains, named burnt alum [alumen ustum) ; it is the ulumen 

 exsiccatum of the pharmacopoeias. The variety called Roche or 

 Rock alum {alumen rupeum), is brought from the Levant ; it is 



