CHAMBERS'S INFORMATION FOR THE PEOPLE. 



,is found in Siberia, near Lake Baikal, in China, 

 Persia, and India. Lapis lazuli is highly esteemed 

 for costly vases, for inlaying, and for mosaics. It 

 .is also used to some extent in jewellery. When 

 powdered and washed, it furnishes ultramarine 

 blue, the most beautiful, durable, and costly of all 

 colours. Its price is five guineas per ounce. One 

 of the triumphs of modern chemistry has been the 

 production of an almost perfect imitation of this 

 splendid colour, which, under the name of French 

 or artificial ultramarine, is now consumed in vast 



quantities as a pigment and colouring material. 



Basalt Greenstone Felstone. These sub- 

 stances, and many similar ones, are often classed 

 under the general name of trap-rocks ; that is, all 

 those rocks that are not distinctly granitic or of 

 recent volcanic origin. The nomenclature of igne- 



ous rocks, however, is at present in a veiy unsettled 

 state. Greenstone and basalt are now rather 

 applied to groups of rocks than to individual 

 kinds. The various felspars, hornblende, augite, 

 and quartz are the minerals of which these igneous 

 rocks are chiefly formed. Chemically, they are 

 either magnesium or aluminium silicates, in both 

 cases mixed with smaller quantities of other sili- 

 cates. Industrially, they are extensively used for 

 causewaying, macadamising, and, where other 

 stone is scarce, in building houses. 



Volcanic Products. The products of recent 

 volcanoes are not only interesting geologically, 

 but the chief ones, as lava, obsidian, and pumice, 

 are also useful. Lava, when compact enough, is 



employed for building, and in Iceland, such things 

 as hand-mills for grinding corn and coffee are 

 made of it. In Naples, lava of a peculiar texture 

 from Vesuvius is largely made into cameos and 

 other ornaments. Obsidian is a volcanic glass 

 often quite undistinguishable from ordinary black 

 bottle-glass ; but sometimes it is prettily figured, 

 and such varieties are made into boxes and orna- 

 ments. The natives of some countries make 

 rude hatchets and knives of common obsidian. 

 Pumice or pumice-stone is a light, porous, hard, 

 and brittle substance. It is really the same 

 material as obsidian or trachyte, ejected from 

 volcanoes in an extremely vesicular instead of 

 a compact state. Large quantities are exported 

 from the Lipari Islands and other localities 

 to different countries, as a polishing material 

 for metals, marble, ivory, hard wood, and 

 for smoothing parchment and some kinds of 

 leather. Pozzuolana is a loosely coherent volcanic 

 sand much used in the preparation of hydraulic 

 mortar. 



Polishing Slate, or Tripoli, &<;. Tripoli is an 

 earthy silica or a sandy variety of quartz mixed 

 with clay, found in various countries. It usually 

 consists of the cases or skeletons of Diatomaceae 

 and Infusoria, and each individual being invisible 

 to the naked eye, the effect of the substance, 

 when used for polishing, is only to produce fine 

 invisible scratches. Tripoli is used to polish 

 metals, marble, optical glass, and gems. Rotten- 

 stone is a soft, earthy stone, supposed to be 

 derived from the decomposition of a silicious 

 limestone or shale. It is found in Derbyshire 

 and South Wales, and near New York, but 

 hardly anywhere else. When reduced to powder, 

 it is employed for polishing the softer metals and 

 alloys, as silver, brass, Britannia metal, and also 

 . glass. 



396 



SALINE SUBSTANCES. 



We have grouped under this head, along with 

 common salt, such substances as alum, nitre, 

 natron, and borax, which in chemical language 

 are called salts. They all occur native, but some 

 of them are prepared on a large scale artificially 

 from mineral substances which yield them. Those 

 we describe are of great and indispensable service 

 in various manufactures, and in the case of com- 

 mon salt for culinary purposes as welL 



Rock-salt Common Salt. Common salt is a 

 compound of the metal sodium with the gas 

 chlorine, hence it is also called the chloride of 

 sodium its old name being muriate of soda. 

 The supply of this substance is inexhaustible, as 

 sea-water contains it in the proportion of nearly 

 four ounces to the gallon. It also exists in the 

 water of salt lakes, and in many countries it is 

 obtained from brine springs, and in the solid form 

 as rock-salt. Sometimes rock-salt is found so 

 pure and white as to require nothing but grinding, 

 but more frequently it is mixed with clay, bitumen, 

 and other foreign matters, from which it requires 

 to be purified. It occurs transparent and colour- 

 less ; also white or some shade of yellow, red, 

 blue, or purple. Very impure kinds are often 

 dirty gray. Rock-salt is found crystallised in 

 cubes, the finer specimens of which are objects of 

 great beauty. 



Formerly, a great deal of salt was obtained by 

 evaporating sea-water in shallow iron pans, but in 

 this country the process is now almost entirely 

 abandoned, although still practised in some parts 

 of Europe. 



Rock-salt and salt springs the great source 

 of common salt commercially occur in many 

 countries, and in almost all geological formations. 

 In England, beds of rock-salt are worked in 

 Cheshire, and brine springs both there and in 

 Worcestershire. In 1863, a bed of rock-salt one 

 hundred feet thick was discovered at a depth of 

 thirteen hundred feet at Middlesborough-on-Tees. 

 On the continent of Europe there are vast de- 

 posits of rock-salt : in Spain, where, in the valley 

 of Cardona, it forms hills from three hundred to 

 four hundred feet high ; in Poland at Wieliczka, 

 near Cracow, where the mines are perhaps the 

 most ancient and celebrated in the world ; in 

 several of the Austrian provinces, Prussian Saxony, 

 Switzerland, France, and in Southern Russia. It 

 occurs in Asiatic Russia, Persia, China, and India, 

 where, in the Lahore district, it forms large hills. 

 Northern Africa, as well as both North and South 

 America, also contains abundant supplies of salt, 

 either as rock-salt or in salt lakes and brine 

 springs. 



In the Cheshire salt-mines, the beds of rock- 

 salt occur in the New Red Sandstone associated 

 with marl and gypsum, and vary in thickness 

 from six inches to nearly forty feet They are 

 worked at a depth of from fifty to one hundred 

 and eighty yards below the surface, where the salt 

 is mined much in the same way as coal, shale, or 

 any other mineral occurring in strata. The shafts 

 are usually square and lined with timber, and are 

 employed for the usual purpose of raising the 

 produce of the mine, the ascent or descent of the 

 miners, ventilation, &c. Gunpowder is used to 

 blast the rock, the ordinary mining tools being 

 employed for boring and removing the loosened 



