Page:The American Cyclopædia (1879) Volume I.djvu/805

 AESENIO 769 even s^Vinr P ar * f arsenic will not escape de- tection by this test. Nitrate of silver gives with it a yellow precipitate. It should be borne in mind, in attempting to determine the presence or absence of arsenious acid in any mixture in which organic substances, partic- ularly those which are not volatile, are present, that some of these substances often produce very similar reactions, and, on the other hand, that they prevent or modify those which ar- senious acid should produce in mixtures where no organic substances are present. "Marsh's apparatus " has been long known as affording an easy means of detecting the presence of arsenious acid. The process depends on the property possessed by arsenic of forming a gas with hydrogen, and depositing itself in the me- tallic state upon the surface of a cold plate held over the flame of the burning gas. Hy- drogen is prepared in the usual way, with granulated zinc and diluted sulphuric acid, in a glass flask provided with a tube of glass drawn out to a small orifice at its outer end ; or a mere tube itself may be used, bent in the form of the letter U, one end drawn out, the other left open for introducing the materials, and closed with the thumb when in use. The hydrogen evolved should first be tested by burning it against a porcelain plate to prove that it is free from arsenic, and then the sus- pected liquid is to be introduced into the appara- tus. If it contain any traces of arsenious acid, it will be shown by the bluish- white color of the flame, by the fumes of the acid, and brown shining spots of arsenic of metallic appearance will be deposited upon the porcelain plates. By heating the glass tube with a spirit lamp, metallic arsenic will be deposited in the cplder part of it, forming a beautiful incrustation. The tube may be cut off at this point, the arse- nic be converted into arsenious acid by heat, dissolved in hot water, and tested by the am- monio-sulphate of copper and nitrate of silver. This apparatus has been modified by Dr. Ure, so that the gas may be made at will to pass through the solutions by which the arsenic is precipitated, or to deposit the metallic incrus- tation in the tube, or the spots upon the plate. In its most simple form, however, it is a very useful contrivance for detecting arsenic. Anti- mony combined with hydrogen produces a spot that may be confounded with that of arsenic ; but a solution of hypochlorite of soda instantly dissolves arsenical spots, and has no effect upon those of antimony. The arsenical spots also are volatilized at a temperature of 500 applied by a bath of olive oil, while the antimonial are unchanged. The proper solvent for organic matters supposed to contain arsenic is. a mix- ture of 3 parts of hydrochloric and 1 part of nitric acid r and the quantity of this should be equal in weight to the organic substance, which before being dissolved should be cut into small pieces and dried at a gentle heat. The mix- ture being distilled, the arsenic, if present, comes over in the form of the volatile terchlo- 50 VOL. i. 50 ride, which is then to be converted into the tersulphuret by sulphuretted hydrogen. Ar- senious acid is manufactured on a large scale at Altenburg and Keichenstein, in Silesia, from the ore called arsenical iron. In many other places it is obtained as a secondary product in the treatment of cobalt ores, and of other me- tallic ores with which arsenic is associated. The process consists in roasting the ore in large muffles, 10 ft. long and 6 ft. wide, in charges of 9 or 10 cwt. each, and collecting the vapors as a sublimate upon the walls of a succession of chambers, arranged in a tower through which they pass, and from which the uncondensable gases escape by a chimney. The muffles are placed inclining upward from their mouth, and are left open for the passage of heated air to aid in subliming the arsenic and converting it all into arsenious acid. A charge is worked off in about 12 hours, and is imme- diately followed by another. Charcoal is the fuel used, and as very little more heat is required than what is evolved by the chemical changes, the quantity consumed is very small. The purest arsenic is found in the flues and cham- bers nearest the furnace ; in the upper cham- bers it is intermixed with the condensed sulphu- rous vapors. To purify it for market, it is all sublimed again. It is placed in cast-iron or porcelain pots, which hold 3 cwt. each, and these are set vertically in a furnace. They open above into sheet-iron drums, which serve as condensers, and which are connected by a funnel with the condensing chamber. The fire must be carefully regulated to maintain the proper temperature for the acid to sublime in the form of a glassy cake. If the heat is too high, metallic arsenic is apt to be sublimed and mixed with the acid, appearing in dark spots. This must be picked out, or the w-hole sublimed over. The preparation of arsenious acid is a most dangerous occupation. The workmen employed generally die before the age of 40 ; indeed, their mean term of life is stated to be only from 30 to 35 years. Dumas states that they are compelled to avoid alcoholic drinks, and live principally upon leguminous vegeta- bles, with plenty of butter, taking very little meat, and that very fat ; and to each man two small glasses of olive oil are administered daily. In removing the acid from the chambers the workmen are completely enveloped in a dress and helmet of leather, the latter furnished with glass eyes. The passage for the air is protected with a wet sponge, by which it is filtered as it passes to the mouth and nostrils. Arsenious acid is also found native, crystallized in octahe- drons and capillary forms, at Andreasberg, in the Hartz, and at mines in Hungary and Bohe- mia. Combined with iron and sulphur, it forms the common ore of arsenic, called arsenical iron, or mispickel, which is of frequent occur- rence in veins of iron pyrites, and of copper, lead, silver, zinc, cobalt, nickel, and tin ores. This ore is found in many localities in Connec- ticut and New Hampshire particularly,, but is