40 ALABASTER 



by means of a piston, powerfully condensed. Here is a reserved force, which, upon 

 its being relieved from pressure, is at once exerted. When air has been condensed 

 to about Jjth of its bulk, it exerts a force which is still very inferior to that of gun- 

 powdor. In many other respects the air-gun is but an imperfect weapon, consequently 

 it is rarely employed. 



AXU-HOZiES. The cavities in a metal casting produced by the escape of air 

 through the liquid metal. 



AIK-PTTlttP. A machine by which the air can be exhausted from any vessel con- 

 taining it It is employed in scientific investigations for exhibiting many very 

 interesting phenomena in connection with the pressure of air, and its presence 

 or absence ; and it is connected with, and forms an important part of, the improved 

 modern steam-engine. Similar machines are also used for condensing atmospheric 

 air ; these have been employed on a large scale in some civil engineering purposes. 

 For a description of the Sprengel pump, see ASPIRATOR. 



AIR-SHAFT. In Mining, a shaft devoted to the purpose of maintaining the 

 circulation of air in a colliery or metalliferous mine. (See MINING, VENTILATION.) 



AIRO-HITOROGEN- BLOWPIPE. A blowpipe in which air is used in the 

 place of oxygen, to combine with and give intensity of heat to a hydrogen flame for 

 the purposes of soldering. See AUTOGENOUS SOLDERING. 



AJUTAGE. A tube through which water is discharged as in a fountain. 



ALABASTER. Gypsum (Albatre, Fr.; Alabaster, Ger.), a sulphate of lime. 

 (See ALABASTER, ORIENTAL.) When massive, it is called indifferently alabaster or 

 gypsum ; and when in distinct and separate crystals, it is termed selenite. Massive 

 alabaster occurs in Britain in the new red or keuper marl : in Glamorganshire, on the 

 Bristol Channel ; in Leicestershire, at Syston ; at Tutbury and near Burton-on-Trent, 

 in Staffordshire ; at Chellaston, in Derbyshire ; near Droitwich it is associated in the 

 marl with rock salt, in strata respectively 40 and 75 feet in thickness; and at 

 Northwich and elsewhere the red marl is intersected with frequent veins of gypsum. 

 At Tutbury it is quarried in the open air, and at Chellaston in caverns, where it is 

 blasted by gunpowder : at both places it is burned in kilns, and otherwise prepared 

 for the market. It lies in irregular beds in the marl, that at Chellaston being about 

 30 feet thick. There is, however, reason to suppose that it was not originally 

 deposited along with the marl as sulphate of lime, but rather that calcareous strata, 

 by the access of sulphuric acid and water, have been converted into sulphate of lime 

 a circumstance quite consistent with the bulging of the beds of marl with which the 

 gypsum is associated, the lime, as a sulphate, occupying more space than it did in its 

 original state as a carbonate. At Tutbury, and elsewhere, though it lies on a given 

 general horizon, yet it can scarcely be said to be truly bedded, but ramifies 

 among the beds and joints of the marl in numerous films, veins, and layers of fibrous 

 gypsum. A thick bed of white crystalline alabaster, or gypsum, has been discovered 

 during the Sub-Wealden Exploration in Sussex. It occurs in beds which are probably 

 of Purbeck age. 



A snow-white alabaster occurs at Volterra, in Tuscany, much used in works of art 

 in Florence and Leghorn. In the Paris basin it occurs as a granular crystalline rock, 

 in the lower Tertiary rocks, known to geologists as the upper part of the Middle 

 Eocene freshwater strata. It is associated with beds of white and green marls ; but 

 in the Thuringerwald there is a great mass of sulphate of lime in the Permian strata. 

 It has been sunk through to a depth of 70 foot, and is believed to be metamorphosed 

 magnesian limestone or Zechstein. In the United States this calcareous salt occurs 

 in numerous lenticular masses in marly and sand strata of that part of the Upper 

 Silurian strata known as the Onondaga salt group. It is excavated for agricultural 

 purposes. For mineralogical character, &c., see GYPSUM. A. C. B. 



The fineness of the grain of alabaster, the uniformity of its texture, the beauty of its 

 polished surface, and its semi-transparency, are the qualities which render it valuable 

 to the sculptor and to the manufacturer of ornamental articles. 



The alabaster is worked with the same tools as marble ; and as it is many degrees 

 softer, it is so much the more easily cut ; but it is more difficult to polish, from its 

 little solidity. After it has been fashioned into the desired form, and smoothed down 

 with pumice-stone, it is polish od with a pap-like mixture of chalk, soap, and milk ; 

 and, last of all, finished by friction with flannel. It is apt to acquire a yellowish 

 tinge. 



Besides the harder kinds, employed for the sculpture of large figures, there is a 

 softer alabaster, pure white and semi-transparent, from which small ornamental 

 objects are made, such as boxes, vases, lamps, stands of timepieces, &c. This branch 

 of business is much prosecuted in Florence, Leghorn, Milan, &c., and employs a great 

 many turning-lathes. Of all the alabasters, the Florentine merits the preference, 

 on account of ite beauty and uniformity. Other sorts of gypsum, such as that 



