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silky, dry, and smooth to the touch. They should dissolve in ten times theirweight of distilled water, without turbidity, and if a few drops of pure hydro-chloric acid be added no turbidity or precipitate should appear on the furtheraddition of a solution of chloride of barium. If a white cloud or deposit isproduced, sulphate of zinc, sulphate of magnesia, or some other sulphate, ispresent as an adulterant.

Chloride of zinc is also sometimes present. In such cases the samplegrows damp on exposure to the atmosphere, more rapidly than do genuinecrystals. To ascertain its absence a sample of the crystals should be dissolvedin water, with the addition of a few drops of hydrochloric acid. A current ofsulphuretted hydrogen is next passed through the liquid until it is fully satu-rated. The precipitate produced by this means is filtered off, and the filtrateevaporated down to dryness. If the sample was genuine no fixed residue willremain after exposure to a red heat. Should a fixed residue be found, then, ifno sulphuric acid can be detected'in the crystals, the presence of chloride ofzinc is highly probable.

If it is required to find the amount of metallic tin present in a sample oftin crystals, or in any solution in which this metal is present as a proto-salt,the following volumetric method can be recommended :—•

A standard solution of tin is first prepared, by dissolving, in pure hydro-chloric acid, exadtly 500 grains of pure tin. The solution is then made upwith distilled water to the exadt volume of 20 ounces.

A solution of iodine is next prepared in the following manner:—127 grs. ofpure iodine and 180 grs. of pure iodide of potassium are weighed out and dis-solved in 10,000 grain-measures of water, without the aid of heat. Thissolution is preserved in small, well-stoppered bottles. The iodine solution isnext standardised. For this purpose such a portion of the standard tin liquidis measured off as may contain exadtly 2 or 4 grs. of metallic tin. It is putin a beaker, and mixed with bicarbonate of soda in excess, along with doubletartrate of potash and soda sufficient to keep the liquid bright. A littlestarch-liquid is then added, and the iodised solution is allowed to drop in froma burette until a permanent, though faint, blue tint appears in the glass. Thenumber of degrees of the burette consumed indicate what measure of theiodine solution represents 1 gr. of tin. A known quantity—say 5 grs.—of thetin-crystals under examination must next be weighed out and dissolved inwater, with the addition of a very little hydrochloric acid. Bicarbonate ofsoda and the double tartrate are then added as before. The starch-liquidfollows, and the iodine solution is then dropped in, as above, till a faint bluetint makes its appearance. The amount of tin present in the crystals canthen be easily calculated.

Double Muriate of Tin is often the subjedt of a misunderstanding. Somewriters conceive that it is a bichloride or per-salt of tin. This is an error.After examining hundreds of samples from different makers, and as used indifferent establishments, we can affirm that it is a proto-salt of tin, differingfrom single muriate merely by being stronger and having a higher specificgravity, just as is the case with “ double oil of vitriol ” as compared with“ single.” Double muriate of tin ranges in specific gravity from 70° to120° Tw., and contains.from 2i to 5 ounces of metallic tin to the pound ofliquid. It is made in the same manner as tin-crystals, with the only difference