MORDANTS.

547

Chloride or Muriate of Copper.—This compound is best made by thedouble decomposition of a mixture of chloride of calcium and sulphate ofcopper in hot water, the whole, after the solution of the ingredients and afterthorough stirring, being left to settle, and the clear liquid drawn off for use.It is much used on the Continent, but in English dye- and print-works it isgenerally replaced by the sulphate, or the nitrate used along with sal-ammoniac.

Nitrate of Copper.—This salt is sold both in the form of crystals and insolution. It may be easily prepared either by dissolving copper clippings andturnings in nitric acid to decided saturation, or else by a process of doubledecomposition. For this purpose a saturated solution of sulphate of copperis mixed with a solution of the nitrate of lime, or, better, the nitrate of lead,in equivalent quantities. The solution thus obtained is perfedtly neutral, andmay be concentrated to any strength required. The nitrate of copper,whether in the crystal or in solution, is of a deeper and less green shade thanthe sulphate. It is less generally met with in the solid state, as it is verydeliquescent, and attracts moisture from the atmosphere. In solution it isgenerally sold at from 8o° to 100° Tw. Those who judge of its qualitymerely by means of the hydrometer often find themselves mistaken, as it isnot unfrequently contaminated with zinc and other bodies. The presence ofsuch foreign matters will be easily understood if we remember that nitrate ofcopper is now generally made, not by the dyers or printers themselves, norby manufacturing chemists whose speciality it is to prepare tindtoria!requisites, but by workers in metals. These parties use nitric acid to a con-siderable extent in cleansing articles of copper and its alloys, &c. Theresultant liquids, mixed promiscuously and concentrated, are sold at a verycheap rate. Such samples of nitrate of copper, beside containing foreignmetals, may be too acid, and may then prove very injurious. The comparativeacidity of different samples may be judged by noting the relative amounts ofcaustic soda solution of uniform strength which are required to produce apermanent precipitate, i.c., one which will not disappear on agitation. If anexcess of ammonia is added to the nitrate of copper, the precipitate formedat first should entirely re-dissolve. If anything remains undissolved, iron orlead is probably present. If a portion of the nitrate of copper is mixed witha little pure hydrochloric acid, and a current of sulphuretted hydrogen passedthrough it until it is completely saturated, the precipitate produced shouldcontain the whole of the fixed matter present. If it, therefore, be filtered off,the clear filtrate, on evaporation to dryness and raising to very low redness,should leave nothing. Any permanent residue is probably due to the presenceof zinc, a frequent, though for the most part an unintentional, impurity.

Sulphate of Copper, known also as Blue Stone, Blue Vitriol, and RomanVitriol, is now generally prepared on the large scale direct from the ores. Itforms large, hard, deep blue crystals, soluble in 3 parts of cold and in | partof boiling water. In a dry atmosphere the crystals are apt to effloresce, losingpart of the 36 per cent of water normally present. A solution of blue vitriolsaturated at 62° F. stands at 36° Tw. If the crystals are exposed for somelittle time to a temperature slightly exceeding that of boiling water, the waterof crystallisation is expelled, and there remains the anhydrous sulphate in theform of a white powder. In contact with minute traces of water this powderresumes its blue colour, and may, therefore, serve to detedt the presence of

2 n 2