পাতা:পদার্থবিদ্যাসার.djvu/১৮১

| 70 Tut. The harder kinds of chalk are used as building-stones. It is also applied to make mortar: nearly all the houses in London are cemented with chalk mortar. In agriculture it is used like lime. By artists it is employed for making moulds to cast metals in, and by carpenters for marking. Chalk is one of the most useful absorbents employed in medicine. Pup. For what is marble employed 2 Tut. Marbles are exceedingly numerous ; but they are applied principally to ornamental architecture, and to the making of statues, floors, &c. Some marble when burnt, forms a manure equal to that of chalk. Pup. I think you said the second class of minerals consisted of Raits? pray how many orders does it contain : Tut. The second class consits of salts, and here are two kinds of them, Earthy and Mineral. Pup. Tell me soule of the principal salts included under the first order, Tut. Alum, common salt, medicinal salts, and nitre. Pup. To what purpose is aluin applied ' Tut. By dyers it is used to fix the colours of cloth; by tanners to give consistency to their skins; by chandlers to make the candles look glassy; by paper-makers to render the paper less liable to catch fire; by printers to remove any greasiness that would prevent the ink from sticking. Alum is also frequently used to purify water. Pup. Is salt productive of any further benefit to us beside that of adding a relish to our food?