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290 a slag and coke rake (89); and a stoking iron, with its extremity bent, for the purpose of breaking the clinkers off the bars from beneath upwards.

Preparation of spongy platinum.—The platinum used for this preparation should be pure, and may be the refuse pieces resulting from such plate and foil as has been in use for trays in former experiments. This, after being taken out of the pickle (93), and condemned as useless for other purposes in the glass house, should be trimmed from all alloyed parts, if any such are adhering to it, and then digested in a Florence flask, with a mixture of five measures of strong hydrochloric acid, one measure of strong nitric acid, and three measures of water. But little heat should be applied at first until the action diminishes. According to Dr. Wollaston, one ounce of platinum will be dissolved by about four ounce measures of such acid, and it is advantageous to have a considerable excess of platinum present. The solution obtained is to be precipitated bya strong solution of muriate of ammonia; a bright yellow pulverulent substance will fall, and a mother-liquid having moreor less colour remain. The precipitate being allowed to subside, the liquor is to be poured off, and the former then washed with two or three portions of water. The washing liquors and the mother-water may afterwards be concentrated together; but it is better not to prepare spongy platinum for this particular use from these fluids, but only from the precipitate which falls on adding the muriate of ammonia.

The yellow precipitate, when washed, is to be dried on a filter, or in a basin, and then decomposed by the application of a dull red heat. This may be done in a clean white earthenware crucible. The heat should be continued until vapours cease to arise; but this will be found along operation, in consequence of the low temperature which is to be applied, and the exceedingly bad conducting power of platinum for heat when in this spongy state. The reduction may also be performed by putting the precipitate upon a piece of platinum foil in a layer about ⅛th of an inch in thickness, and covering it with another piece of foil; a spirit lamp will then suffice to reduce the metal, but the foil and powder must be turned occasionally, that both sides may be exposed to the flame. The platinum will appear as a dull grey spongy metallic mass. It should be broken up, mingled, and