Issue 24 and Volume 48.

GYPSUM A FIRE-RESISTING MATERIAL In a pamphlet issued by the National Fire Protection Association of Chicago, H. G. Perring discusses the use of gypsum as a fireproof substance made up in blocks, plaster board and plaster or metal lath. Regarding the fire-resisting qualities of gypsum it is shown that it is a sulphate of lime, chemically a hydrous calcium sulphate, the formula of which is expresed as Ca SO4+2 H2O. Interpreted this formula may be expressed as 32.5 per cent, by weight of calcium and 46.6 per cent, sulphuric acid chemically compounded and holding 20.9 per cent, of waater of crystallization. Plaster of paris is made by calcining the gypsum rock to drive off part of the water of crystallization. This material has the valuable quality of taking up water and, by a crystallizing process, becoming again converted into gypsum. Gypsum is a soft mineral, ranking number 2 in a…

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