Alum is an article of indispensable importance to dyers, not only on account of its cleansing and opening the pores of the substances to be dyed, and thus rendering them fit to receive the colouring particles, but also from its more essential property of fixing the colours in such manner that they cannot afterwards be washed out. By tanners it is in great request for giving firmness to the skins after they have been rendered flaccid in the lime─pits. It is employed in the manufacture of paper, and by engravers, and other artists. In the making of candles, alum is added to the tallow, to render it glossy, and to give it greater firmness and consistence; and, mixed with cream, it aids the separation of butter. It has a tendency to retard ignition. Paper soaked in alum water does not easily take fire, and is thereby better fitted for the preservation of gunpowder. Such paper is likewise used in the whitening of silver, and the silvering of brass. It has been recommended that ladies’ muslin dresses should be dipped in a solution of this substance, for the purpose of rendering them less liable to catch fire. A solution of alum also retards the putrefaction of animal substances, and affords useful, as well as economical, means of preserving natural productions that are imported from foreign countries. Alum is frequently mixed with paste, to prevent its losing its tenacity by the absorption of moisture. It is asserted that bakers occasionally use it as an ingredient in bread, and that its presence may be discovered by thrusting a heated knife into a loaf before it is cold: if free from alum, scarcely any alteration will be visible on the blade, but if the contrary, the surface, when cool, will appear slightly covered with an incrustation of alum. A very important purpose to which alum may be applied is in the purifying and sweetening of water that has become fetid and unfit for use; from five to ten grains of burned alum, and double or treble that quantity of pounded charcoal, will correct the fetor of a gallon of water. Printers’ cushions, and the blocks used for the printing of calicos, are rubbed with burned alum to remove any greasiness, which otherwise would prevent the ink or colour from sticking. This substance is also occasionally employed by surgeons to stop the bleeding of small vessels, to corrode fungous or proud flesh, and for other purposes in medicine. 198. MAGNESIA FAMILY. This is a family of minerals which comprehends all the combinations of magnesia with acids. When freed from extraneous matters, magnesia is a powdery substance of limpid white colour. 199. _EPSOM SALTS, or SULPHAT OF MAGNESIA, consist of magnesia in conjunction with sulphuric acid (24)._