A R S

A R S

( <54 )

consists of sulphuret of iron, retaining sometraces of arsenic. The component parts ofthis ore, as deduced from a separate analysisof the sublimate and of the residue, are43.418 arsenic34.938 iron20.134: sulphur

98.490

Iron is well known to have a strongeraffinity for sulphur than arsenic has, and as inthe present instance the proportion of iron andsulphur is just sufficient to form the sub-sul-phuret, it appears probable that mispickel isa combination of iron pyrites and of metallicarsenic.

Arsenical P//rifes. c

The primitive form of this substance is astrait rhomboidal prism, the angles of whichare alternately TIP-18' and 68°-42'.

It is composed, according to Lampadius, of58.9 iron42.1 arsenic

101.0

§ 2. Chemical Properties.

It is often desirable to detect minute quan-tities of arsenic mixed with other substances.Some of these means have been already men-tioned under this article, (Ch. Diet. 1. p. 105 ,)but another should be noticed on account ofthe great extent of its operation. It comistsin first dissolving the arsenic in water, addinga small quantity of sub-carbonat of potash,and then applying nitrat of silver. The pro-cess is thus described by Mr. Hume who firstsuggested it. d Put in a glass flask two orthree grains of the powder suspected to bearsenic, add eight ounces of distilled waterand gradually bring it to boil over a lamp orclear fire. Then add a grain or two of sub-carbonat of potash or soda, and shake themixture. When clear, pour some of the solu-tion into a glass, and dip just under the surfaceof the liquid a stick of lunar caustic, and abeautiful yellow precipitate will appear, whichC3.n hardly be confounded with any othersubstance.

Dr. Mareet e has given a variation of thisprocess which has the advantage of being per-formed in a few seconds. Pour some of the

clear fluid suspected to contain arsenic intoany glass vessel, stir into it a glass rod dippedin pure ammonia, and immediately after ano-ther glass rod wetted with nitrat of silver.The same orange yellow precipitate will im-mediately appear. Care should be taken toadd only a very small quantity of each of thereagents, particularly of the ammonia. In eithermode it is obvious that any muriatic acid pre-sent will give a precipitate with nitrat of silver,and this metallic salt will also be decomposedby the carbonated potash or soda where thismethod is used, so that it is not the mereappearance of any precipitate, but of an orange-fellow precipitate that will determine the pre-sence of arsenic. Where'the arsenic is in thestate of arsenic acid the colour of the precipitatewill be brick-red.

This precipitate is a compound of oxyd ofsilver and arsenic, either partially or entirelyoxydated, and when heated per sc, especiallyon charcoal, the arsenic sublimes and leaves aglobule of pure silver.

Orpiment and Realgar. The followingdistinctive characters of these substances aregiven by VI. Thenard. f

Both of them melt and sublime unaltered inclose vessels and without giving out any sul-phureous acid. Both are acted on by thesulphuric nitric and oxymuriatic acids. Sul-phuric acid acts much more easily on orpimentthan on realgar. In both cases sulphureousadd and arsenious acid are formed, but moreof the former acid and less of the latter areproduced with orpiment than with realgar.Nitric acid is easily decomposed even withoutheat on both these substances, and sulphur andarsenious acid are formed. The fixed alkalies dis-solve both without heat; when potash is used,hydro-sulphuret of potash and arsenite of potashare formed, for on adding lime-water to thesolution a copious white precipitate appears,which, whi*i treated with carbonat of potash,gives a liquid that yields abundance of arseniousacid on saturation with muriatic acid and eva-poration.

Sulphur fused with realgar converts it intoorpiment, and arsenic added to orpiment changesit to realgar. Three parts of sulphur and fourof arsenic form orpiment: and one part ofsulphur with three of arsenic form realgar.'The latter is the most fusible of the two.Orpiment made artificially by sublimation is

Hauy. Table Comp. 05, 272. A Phil. Mag. vol. 33. « Medico-Chirurp-ical Transact, vol. 2, p. 15, and vol. 3. 3 IS.

*■ An. chim. No. 177. p. 284.