AIR-BRICK 37 



The green stones are largely used as artificial chrysoprase, and the colour is more 

 intense and said to be more durable than that of the natural chrysoprase. A yellow 

 tint is obtained by prolonged digestion in warm hydrochloric acid, the acid acting on 

 the oxide of iron normally present in the stone, and thus forming a yellow perchloride. 

 One of the commonest and at the same time most easily developed colours is the red 

 tint of the carnelian. It was long ago observed that yellow and grey chalcedony 

 might be caused to assume a bright red colour by mere exposure to sunshine, the heat 

 being sufficient to expel the water more or less completely from the hydrated peroxide 

 of iron normally present in the stone. In India it has long been the practice to 

 convert yellowish chalcedony into red carnelian by solar heat. The Idar workers 

 generally expose the agates to the heat of an oven, gradually raised until all hygro- 

 scopic water is expelled. The stones are then moistened with sulphuric acid, and 

 raised to a red heat, whereby an anhydrous peroxide of iron, of fine red colour, is 

 developed. Or a pernitrate of iron may be formed by throwing a handful of old nails 

 into half-a-pint of aquafortis mixed with a pint of water, and the stones having been 

 soaked in this solution are heated so as to decompose the absorbed salt, and cause a 

 precipitate of ferric oxide in the pores of the agate. Several organic colours have 

 been introduced by the agate-worker, but they are generally fugitive : mauve and 

 magenta agates, and other equally unnatural stones, are common in the market. The 

 ingenuity of the Idar workmen also enables them to imitate successfully the dendritic 

 markings in mocha-stones. 



Another important branch of industry connected with the agate-trade is that of 

 mounting the finished objects. Though dignified by the name of Goldschmiede, the 

 mounters usually work in gilt tomback ; some of the better kinds of agate-ware are 

 now, however, set in silver. Drilling, engraving, cameo-cutting, and etching with 

 hydrofluoric acid, are also extensively practised in Idar ; and, in addition to agates, 

 largo quantities of Oriental blood-stones and other semi-precious stones, and even 

 pastes, are largely cut and mounted in this district. 



In 1867 there were in Birkenfeld and the neighbourhood 724 grindstones working 

 in 153 mills. The number of grinders and polishers in Birkenfeld was 1,129, but in 

 addition to this number about 300 workmen dwelt beyond the limits of the Princi- 

 pality. There were also about 258 persons engaged in drilling and boring the agates, 

 and about 700 so-called goldsmiths. Including those employed in the sale of the 

 stones, in making paper cases, and otherwise connected with the trade, it may be 

 said that upwards of 3,000 persons depend for their support on the agate -industry 

 in the neighbourhood of Idar and Oberstein. 



[For further information on the subject of this Article the reader may consult the 

 following works: Billing's 'The Science of Gems,' 1867, p. 48 et seq. ; Kluge, 

 ' Edelsteinkunde,' 1860, p. 401; Lange, 'Die Halbedelsteine aus der Familie der 

 Quarze und die Geschichte der Achatindustrie,' 1868 ; Noggerath, ' Die Kunst, Onyxe, 

 Carneole, Chalcedone u. andere verwandte Steinarten zu farben,' Karsten's ' Archiv. 

 xxii.' 1848, p. 262, and 'Edin. New Phil. Journ.', xlviii. 1850, p. 166; Noggerath, 

 ' Ueber die Achatmandeln in den Melaphyren,' Haidinger's ' Naturwissentschaftliche 

 Abhandlungen,' iii. 1850, p. 93; Kenngott, 'Ueber die Achatmandeln in den 

 Melaphyren, namentlich iiber die von Thesis in Tirol,' Op. cit. iv. 1851, p. 72 ; 

 Haidinger's 'Berichte,' vi. 1850, p. 62 ; Bischof, 'Lehrbuch d. Chem. u. Phys. Geologie,' 

 ed. 2, ii. p. 853 ; iii. p. 457, 464, 623 et seq. ; Ileusch, 'Ueber den Agat' ; Poggendorff's 

 'Annalen,' cxxiii. 1864, p. 94]. F. W. E. 



AGATE. An instrument used by gold-wire drawers, so called from the agate 

 fixed in the middle of it. 



AXCR MET AL. An alloy patented in 1860 by Johann Aich, and recommended 

 for use in ship-building and sheathing. In composition it approaches close to Keir's 

 metal. The following are found to be the best proportions : copper, 60 Ibs. ; zinc, 

 38 Ibs. 2 ozs. ; and iron, 1 Ib. 8 ozs. The proportion of zinc may, however, be 

 increased to 40 per cent., and that of iron may vary between 0'6 and 3 per cent. 



AXXEIVXTE. A native sulphide of bismuth, lead, and copper, crystallising in 

 needle-shaped crystals belonging to the prismatic or orthorhombic system. It is 

 found at Bersow in the Urals, and is believed to occur in Georgia and North Carolina 

 in the JtTnited States. 



AIR. The gaseous envelope which surrounds this earth is emphatically so called ; 

 it consists of the gases nitrogen and oxygen. 



About 79 measures of nitrogen, or azote, and 21 of oxygen, with jjgth ^ carbonic 

 acid, constitute the air we breathe. The term air is applied to any permanently 

 gaseous body. And we express different conditions of the air, as good air, bad air, 

 foul air, &c. See ATMOSPHERE. 



AIR-BRICK. A brick of the ordinary size and kind, but perforated so as to 

 admit the passage of air through the openings when built into the walls. 



