266 ATMOSPHERE 



Atmospheric Air, in roasting. 

 Nitre, by conversion into nrsenato of potash. 



COLLECTING AGENTS, used for collecting or dissolving gold and silver in an assay. 

 Lead, granulated. 



Litharge, or galena, which yields lead when heated with carbon or iron respectively. 

 Mercury. 



REAGENTS, &c., EMPLOYED IN THE WET WAY. 



SOLVENTS, or agents used in dissolving various substances. Nitric Acid. Hydro- 

 chloric Acid. Sulphuric Acid. 



REDUCING AGENTS, or substances used to reduce a metallic compound in solution to 

 H lower state of combination. Zinc, granulated. That in the form of bean shot, 

 obtained by pouring the metal into hot water, is preferable. 

 Sulphite of Soda. Protochloride of Tin. 



METALLIC PRBCIPITANTS. Zinc, in form of sheet. Iron. Wrought-iron, in form of 

 thin clean sheet, or round or flat bar. Protochloride of Tin. 



SPECIAL REAGENTS, as permanganate of potash, sulphide of sodium, iodide of potas- 

 sium, &c., will bo noticed when necessary. 



For the methods of assaying the various metals, see COPPER, LEAD, GOLD, &c. K.S. 

 ASTER! A. (Starstone). The name first used by Pliny to denote those 

 98 varieties of sapphire which display diverging rays of light. 



ASTRAGAXi. An ornamental moulding, generally used to conceal a 

 junction in either wood or stone. 



ASTRAGAXi PX.AXTES. Planes fitted with cutters for forming 

 astragal mouldings. They are commonly known as moulding-planes. 



ASTRAGAXi TOOL, for turning. By using a tool shaped as in fig. 98, 

 the process of forming a moulding or ring is greatly facilitated, as one 

 member of the moulding is completed at one sweep, and we are enabled to 

 repeat it any number of times with exact uniformity. 



ASTRAXiXTE. A glass resembling AVENTUHINE, but containing 

 crystals of a compound of copper, which by reflected light exhibit a 

 dichroic iridescence of dark rod and greenish-blue. It is said to be mado 

 with 80 parts of flint, 120 parts of oxide of lead, 72 parts of carbonate of 

 soda, and 18 parts of anhydrous borax. Tt, this are added 24 parts of scalo 

 oxide of copper, and 1 part of scalo oxide of iron. These are melted in 

 a Hessian crucible, at the heat of an ordinary air-furnace, and left to cool 

 slowly without being moved. 



ATACAIKXTE. A native oxychlorido of copper, originally found in 

 the desert of Atacama in Peru. It forms small rhombic crystals, varying in colour 

 from leek to emerald green. Splendid examples of this mineral have been found at 

 Wallaroo, on Yorko Peninsula, and at Hurra Hurra, in South Australia. 

 ATHERXNA. See SARDINE. 



ATIYIOIYXETER. (it.rft.bs, vapour ; ptTpov, a measure.') An instrument to mcasxire 

 the quantity of water evaporated in a given time under ordinary atmospheric 

 conditions. 



ATMOSPHERE. The gaseous envelope surrounding this globe. The term is 

 also applied to any gaseous body enveloping any mass of matter. 



The extent of the earth's atmosphere has not been determined with any great 

 degree of accuracy. The height of the air above the surface of the earth should vary 

 with the increase of the attractive force at the poles, and its diminution at the 

 equator, and the variations of temperature also affect the mass, but, owing to the 

 influence of centrifugal force, the spheroidal mass of air has a diameter shorter at the 

 poles than at the equator. The volume which a given quantity of air occupies 

 is directly dependent upon the pressure to which it is subjected and upon the 

 temperature so that the density diminishes as the distance from the earth's surface 

 increases. 



All the influences having been carefully examined and allowed for, calculations 

 have been made which appear to show that the atmosphere reaches, in a state of 

 density, which can bo measured, to the height of about 45 miles. It may be assumed 

 that in a state of continually increasing tenuity, it extends to a considerably greater 

 distance from the earth's surface. 



The composition of the air has been determined under a considerable number of 

 conditions, the mean result being to show that it is a chemical mixture of about 20 

 volumes of oxygen and 80 volumes of nitrogen (see Watts's ' Dictionary of Chemistry ' 

 for a full account of tho chemical examinations of the air.) Carbonic acid exists in 



