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a week of such treatment it will stand the atmosphere of any town. Percussion. Shots of known weight are dropped on to the surface of the ware from different heights. Enamels on sheet iron that are to be used for automobile tags are liable to percussion from flying stones, and this test should be especially applied to them and to advertisement signs made of enamelled metal. Surface tests of the glaze are made by rubbing lamp- black on to it, or by soaking the body in a coloured liquid, such as red ink. Holes or crevices are immediately detected by the retention of the dirt, or the stain of the liquid creeping up from under the glaze. Soluble salts, which are a measure of the liability to efflorescence, are found by crushing a weighed piece of the ware, boiling in distilled water, filtering, evaporating to dryness, and weighing the residue. If it be not desirable to crush a piece, a minimum figure can be obtained by allowing distilled water to percolate through the whole brick, or whatever ware it is, placed in a basin. Change of temperature, apart from freezing action, is not sufficiently marked to produce much influence on building ware. But enamels on iron, especially in cooking vessels, must be prepared for extremes of temperature. To test their reliability, they are heated to dull redness and plunged CERAMIC CHEMISTRY. 81 into cold water. They should withstand such treatment