378
ELEMENTS OF MATERIA MEDICA.
tube, into octahedral crystals of arsenious acid, which may be dissolvedin distilled water, and tested by the liquid re-agents presently to bementioned; and its yielding arsenic acid by dissolving it in nitro-hydrocliloric acid, and carefully evaporating the solution to dryness.The arsenic acid is known by the red precipitate (arseniate of silver)produced on the addition of nitrate of silver: but if the evaporation hasnot been carried on sufficiently far, some hydrochloric acid or chlorinewill be left, which will form a white precipitate (chloride of silver) withnitrate of silver. The arseniate of silver may be reduced, if necessary, bymixing it with charcoal and boracic acid, and heating it in a glass tube.
In some cases the metallic crust is imperfectly formed, or is masked bysome decomposed organic matter. Whenever any cloubt respecting itsnature is entertained, proceed as follows:—Cut off with a file the portionof the tube which contains the suspected crust, roughly powder it, intro-duce it into another glass tube, and apply heat.
The fallacies to which this test is liable are principally two—a char-coal crust may, by an inexperienced experimenter, be mistaken for thearsenical crust; and I have seen students confound a stratum of globulesof mercury (obtained by reducing calomel) with the arsenical crust.Careful examination, especially by a magnifying glass, will, however,easily enable the experimenter to distinguish them: the inner surface ofthe charcoal crust is brown, powdery, and dull, whereas that of the arse-nical crust has a crystalline texture, grey colour, and shiny appearance:the sublimate obtained by reducing calomel or mercurial compounds hasall the brilliancy of arsenicum, but by a glass is found to consist ofminute globules which may be made to coalesce by the point of a knife.Lastly, the arsenical may be distinguished from all other crusts byoxidating it, as before directed, and converting it into arsenious or arsenicacid which can be readily recognised by the tests already mentioned :—a proceeding which ought never to be omitted.
As a deoxidizing agent I have directed freshly ignited charcoal to beemployed to convert arsenious acid into arsenicum. If carbonate of sodaor of potash be mixed with the charcoal, a part only of the arsenicum isdisengaged, an arseniuret of sodium or of potassium being formed: hencewhen the quantity of acid to be reduced is small, charcoal only shouldbe employed. “ Where the quantity of material, however, is considera-ble, it is preferable to employ the black flux, or still better, as not beingdeliquescent, a mixture of charcoal and carbonate of soda, deprived ofwater of crystallization by heat.” (Christison ’s Treatise on Poisons, 3d ed.237.) If the substance to be reduced be an arsenite (as of silver, copper,or lime), or an arseniate (as of silver), a mixture of charcoal and boracicacid should be used. For the reduction of the arsenical sulphurets (asthe precipitate obtained by passing hydrosulphuric acid gas through asolution of arsenious acid) a mixture of 2 parts of ignited carbonate ofsoda and 1 of charcoal should be employed. The alkali is here essentialin order to combine with the sulphur. Black flux (prepared by defla-grating 1 part of nitre and c l\ of bitartrate of potash) is objectionable onaccount of its deliquescent property. Various other deoxidizing agentshave been recommended, as formate of soda by Goebel (Griffin’s Chem.Reereat . 8th ed. 140), oxalate of lime by Du Menil ( Handb. d. Reag. n.Zerlegungslehre, ii. 268), and oxalate of soda by Dr. M‘Grcgor {LondonMed, Gaz. xxii. 613.) I find that binoxalate of potash answers very