380

ELEMENTS OF MATERIA MEDICA.

little, it will produce a precipitate with phosphate of soda similar incolour to that produced with arsenious acid. The only certain way ofknowing when the proper quantity has been employed is to test it.Arsenious acid, but not phosphate of soda, ought to occasion a precipitatewith it. The impediments to the operation of this test, are, free acids (ashydrochloric nitric, acetic, citric, or tartaric), chlorides, and organic mat-ters. The acids may be readily neutralized by an alkali. If commonsalt, or other metallic chloride be present, ammoniaco-nitrate of silverthrows down a white precipitate (chloride of silver), even though a con-siderable quantity of arsenic be present. To obviate this, add a fewdrops of nitric acid, then an excess of a solution of nitrate of silver.Filter to get rid of the precipitated chloride of silver, and apply theammoniaco-nitrate of silver. The presence of much organic matter im-pedes the action of this test. Ammoniaco-nitrate of silver, when pro-perly prepared, does not occasion a yellow precipitate with any substancesave arsenious acid; and hence is not subject to any fallacy of that kind.If, however, it be not properly prepared, it may occasion a yellow preci-pitate (subphosphate of silver) with phosphate of soda. There is anoptical fallacy, against which the student should be put on his guard:if ammoniaco-nitrate of silver be added to certain yellow liquids con-taining common salt, a white precipitate (chloride of silver) is produced,which, seen through a yellow medium, might, by a careless observer, bemistaken for a yellow precipitate.

4. Hydrosulphuric Acid {Sulphuretted Hydrogen ).—If this gas be passedthrough a solution of arsenious acid, a yellow precipitate of sesquisul-phuret of arsenicum (orpiment) is produced, while the oxygen of thearsenious acid, and the hydrogen of the hydrosulphuric acid, unite toform water. In order, however, for this effect to be produced, it is neces-sary that the liquid be slightly acidified by some acid (as the hydro-chloric). If the liquid be already acid, we must neutralize it by cau-tiously adding an alkali, and then acidify by hydrochloric acid.

In applying this test we may place the suspected liquid in a test-tube,or conical wine or ale-glass (fig. 60); the gas being developed in a com-mon Florence flask (or two-necked bottle, as recom-mended by Dr. Christison): the mouth of the flask isclosed by a cork, perforated by a tube curved twice atright angles. The ingredients for developing the gasare a metallic sulphuret (as of iron or of antimony) andsulphuric or hydrochloric acid. I prefer the sulphuretof iron with sulphuric acid diluted with water. Theseare to be introduced into the flask previous to theadaptation of the cork. After the gas has passedthrough the arsenical liquid for a few minutes, portionsof the yellow sesquisulphuret of arsenicum (orpiment)begin to fall down. The separation of the precipitateis promoted by ebullition, and the exposure of thesolution for a few hours to the air. The essential cha-racters of the precipitate are, its yellow colour, its

Fig. 60 .

Mode of passingHydrosulphuricAcid through an.arsenical solu-tion.

rapid solution in liquor ammonia:, forming a colourless and very limpidliquid, and its yielding metallic arsenicum when dried and heated withblack flax, or a mixture of ignited carbonate of soda and charcoal.When the quantity of sesquisulphuret is small, some difficulty may be