CHAMBERS'S INFORMATION FOR THE PEOPLE. 



countries. In the middle ages, it was, and in 

 Prussia is still, called black amber. Jet was 

 valued by the Romans chiefly for its medicinal 

 qualities, although they also made ornaments of it. 

 Its lightness, toughness, and the beautiful polish 

 which it takes, render it peculiarly suitable for cut- 

 ting into bracelets, brooches, ear-rings, and other 

 objects which are largely worn as mourning jewel- 

 lery. The jet district of Yorkshire is around Wnitby, 

 and there and at Scarborough, the centres of the 

 trade, the annual value of the articles made of it 

 is upwards of .125,000. Good imitations of jet 

 are now made from hardened india-rubber called 

 ' vulcanite,' but they are apt to turn gray. Those 

 made of black glass are easily known by their 

 vitreous glare. Perhaps the best are obtained 

 from a material called bois durci, a peculiar 

 composition of charred sawdust and other in- 

 gredients. 



Amber. Amber is a fossil resin found in 

 several countries, but in marketable quantities 

 only on the coast of the Baltic between Konigs- 

 berg and Memel. It is said to claim the highest 

 antiquity among minerals used for personal orna- 

 ment, and to be the only one known to the early 

 Greeks. It was carved into elegant forms by the 

 Etruscans, and maintained its high value among 

 the Romans long after they obtained it in large 

 quantities from the Baltic. Amber ornaments are 

 less esteemed now than of old, probably on account 

 of their softness and brittleness ; yet the finer 

 pieces of this substance are still prized for beads, 

 brooches, necklaces, and like articles, and also for 

 the mouth-pieces of pipes. It is found both trans- 

 parent and opaque, from a pale primrose to a deep 

 golden orange the transparent kind, which is the 

 more common, having a high refractive power. 

 Amber is largely used in the manufactuie of var- 

 nishes, but the greater portion of it is sent to 

 China for burning as incense. The largest piece 

 known weighs eighteen pounds, and is in the 

 Royal Museum at Berlin. Some specimens con- 

 tain extinct species of insects, and even small 

 fish, which must have been entombed when it 

 exuded as a soft gum from trees. 



CALCAREOUS SUBSTANCES. 



The familiar substance limestone, when pure, 

 consists of the elements of calcium oxide and 

 carbonic acid, and is called calcium carbonate, 

 or carbonate of lime. Calcium occurs largely in 

 nature combined with other substances. Thus, 

 besides limestone, marble, and chalk, which are 

 identical in chemical composition, we have gypsum 

 or alabaster, formed of calcium oxide and sulphuric 

 acid ; apatite, formed of calcium oxide and phos- 

 phoric acid ; fluorspar, formed of calcium and fluoric 

 acid ; and so on. These and other compounds of 

 calcium are called calcareous bodies. Limestone 

 of various kinds forms by far the most abundant 

 of these, and is chiefly of organic origin. Chalk, 

 for example, usually consists of more than ninety 

 per cent, of the cases of Foraminifera and com- 

 minuted shells ; coral rock, of the calcareous 

 skeletons of compound animals allied to the sea- 

 anemones ; and common limestone, of the shells 

 of molluscous animals. In some situations, the 

 same bed of roc"k can be seen passing gradually 

 from a dark limestone, full of fossil shells, to a 

 white crystalline marble, with all trace of fossils 



390 



obliterated by metamorphic action. Some lime- 

 stones, however, have been formed by the pre- 

 cipitation of carbonate of lime from calcareous- 

 waters. 



Common Limestone. Few substances are of 

 greater economic importance than common 

 limestone, and it is fortunately abundant in 

 one form or other through all the geological 

 formations, from the oldest to the newest. 

 Most countries, accordingly, have extensive and 

 widely distributed deposits of this material. 

 Limestones vary much in their texture, being 

 soft and earthy, like some chalks ; concretion- 

 ary, as in oolitic roe-stones ; hard and compact, 

 or crystalline, as they pass into the state of 

 marble. Besides these and other kinds of simple 

 limestones, there are also composite varieties, as 

 magnesian, silicious, argillaceous, arenaceous, 

 bituminous, conglomerate, and hydraulic lime- 

 stones. Limestone is usually obtained by work- 

 ing it in open quarries, but sometimes it is worked 

 downwards by a rude kind of mining. When it 

 is wanted for such purposes as the preparing of 

 mortar, cements, or plaster, it requires to be 

 heated in a kiln, so as to convert it into what is 

 called lime- shell, quicklime, or caustic lime. la 

 this process, the carbonic acid is driven off by 

 the heat, and what remains is oxide of calcium or 

 lime. The kiln is a stone building, square, round, 

 or oval in shape, in which layers of limestone and 

 coal are laid alternately, so that the latter may 

 effectually calcine the limestone. If limestone is 

 underburned, it retains some carbonic acid; if 

 overburned, it combines with silicic acid ; and in. 

 neither case will it slake well. Quicklime is 

 slaked or converted into hydrate of lime by 

 sprinkling it freely with water, and is then mixed 

 with sand when made into mortar. Hydrate of 

 lime is soluble in water to some extent, but, curi- 

 ously enough, more so in cold than in hot water. 

 Lime-water thus obtained is used for several 

 chemical and pharmaceutical purposes. 



The uses of lime are so numerous, that we can 

 only mention a few of its principal applications. 

 Some of these have already been referred to, but 

 it is also an indispensable ingredient in the manu- 

 facture of bleaching-powder, soda, soap, candles,, 

 glass, nitre ; in the preparation of leather; as a. 

 flux in metallurgy ; in the purification of coal-gas ; 

 in agriculture, as a reclaimer of peaty soils, and 

 in liberating alkali from such as consist of clay.. 

 Some limestones, especially those from the oolitic 

 formation, furnish valuable building-stones. Like 

 many sandstones, these cut with equal freedom in: 

 any direction all building-stones with this prop- 

 erty being called free-stones. Of these oolitic 

 limestones, the best known are Portland stone,, 

 used in the construction of St Paul's Cathedral 

 and other London buildings ; Bath stone, also 

 employed in the metropolis ; and the beautiful 

 French stone from Caen. 



Marble. The term marble is frequently applied 

 to serpentine, alabaster, porphyry, and other 

 stones which take a polish, and are used in a. 

 similar way to marble for decorative purposes. 

 True marble, however, is merely a limestone suffi- 

 ciently hard and compact to take on a fine polish. 

 British marbles are usually obtained from the 

 older or palaeozoic rocks, and chiefly from the- 

 Carboniferous and Devonian strata ; but they also- 

 here and there occur in the newer formations^ 



