PUTTY POWDER is the pulverised oxide of tin, or generally of tin and lead mixed in various proportions, - the process of manufacture is alike in all cases. The metal is oxidized in an iron muffle, or a rectangular box, close on all sides, except a square hole in the front side. The retort is surrounded by fire and kept at the red heat, so that its contents are partially ignited, and they are continually stirred to expose fresh portions to the heated air; the process is complete when the fluid metal entirely disappears, and the upper part of the oxide then produced, sparkles somewhat like particles of incandescent charcoal. The oxide is then removed with ladles and spread over the bottom of large iron cooling pans and allowed to cool. The lumps of oxide, which are as hard as marble, are then selected from the mass and ground dry under the runner, the putty powder is afterwards carefully sifted through lawn.

As a criterion of quality it may be said that the whitest putty powder is the purest provided it be heavy, some Of the common kinds are brown and yellow, whilst others from the intentional admixture of a little ivory black are known as grey putty. The pure white putty, and which is used by marble workers, opticians, and some others, is the smoothest and most cutting; it should consist of the oxide of tin alone, but to lessen the difficulty of manufacture, a very little lead, (the linings of tea chests,) or else an alloy called shruff (prepared in ingots by the pewterers) is added to assist the oxidation.

The putty powder of commerce of good fair quality, is made of about equal parts of tin and lead, or tin and shruff; the common dark coloured kinds are prepared of lead only, but these are much harsher to the touch and altogether inferior.

Perhaps the most extensive use of putty powder is in glass and marble works, but the best kind serves admirably as plate powder, and for the general purposes of polishing. 2. - Putty Powder for Fine Optical Purposes is prepared by Mr. A. Ross by the following method, which is the result of many experiments. Metallic tin is dissolved in nitro-muriatic acid, and precipitated from the filtered solution by liquid ammonia, both fluids being largely diluted with water. The per-oxide of tin is then washed in abundance of water, collected on a cloth filter, and squeezed as dry as possible in a piece of new clean linen; the mass is now subjected to pressure in a screw press, or between lever boards, to make it as dry as possible. When the lump thus produced has been broken in pieces and dried in the air, it is finely levigated while dry, on a plate of glass with an iron spatula, and afterwards exposed in a crucible to a low white heat.

Before the per-oxide has been heated, or whilst it is in the levigated hydrous state, the putty powder possesses but little cutting quality, as under the microscope the particles then appear to have no determined form, or to be amorphous, and on being wetted to resume the gelatinous condition of the hydrous precipitate, so as to be useless for polishing; whereas when the powder is heated, to render it anhydrous, most of the particles take their natural form, that of lamellar crystals, and act with far more energy, (yet without scratching,) than any of the ordinary polishing powders. The whole mass requires to be washed or elutriated in the usual manner after having been heated, in order to separate the coarser particles.

Mr. Ross usually adds a little crocus to the putty powder by way of colouring matter, as it is then easier to learn the quantity of powder that remains on the polishing tool; and it may be added that this is the polishing powder employed by Mr. Ross in making his recently improved achromatic object glasses for astronomical purposes.