~Boiler scale.~ When water which contains certain salts in solution is evaporated in steam boilers, a hard insoluble material called _scale_ deposits in the boiler. The formation of this scale may be due to several distinct causes. 1. _To the deposit of calcium sulphate._ This salt, while sparingly soluble in cold water, is almost completely insoluble in superheated water. Consequently it is precipitated when water containing it is heated in a boiler. 2. _To decomposition of acid carbonates._ As we have seen, calcium and magnesium acid carbonates are decomposed on heating, forming insoluble normal carbonates: Ca(HCO_{3})_{2} = CaCO_{3} + H_{2}O + CO_{2}. 3. _To hydrolysis of magnesium salts._ Magnesium chloride, and to some extent magnesium sulphate, undergo hydrolysis when superheated in solution, and the magnesium hydroxide, being sparingly soluble, precipitates: MgCl_{2} + 2H_{2}O <--> Mg(OH)_{2} + 2HCl. This scale adheres tightly to the boiler in compact layers and, being a non-conductor of heat, causes much waste of fuel. It is very difficult to remove, owing to its hardness and resistance to reagents. Thick scale sometimes cracks, and the water coming in contact with the overheated iron occasions an explosion. Moreover, the acids set free in the hydrolysis of the magnesium salts attack the iron tubes and rapidly corrode them. These causes combine to make the formation of scale a matter which occasions much trouble in cases where hard water is used in steam boilers. Water containing such salts should be softened, therefore, before being used in boilers. ~Magnesium sulphate~ (_Epsom salt_) (MgSO_{4}.7H_{2}O). Like the chloride, magnesium sulphate is found rather commonly in springs and in salt deposits. A very large deposit of the almost pure salt has been found in Wyoming. Its name was given to it because of its abundant occurrence in the waters of the Epsom springs in England. Magnesium sulphate has many uses in the industries. It is used to a small extent in the preparation of sodium and potassium sulphates, as a coating for cotton cloth, in the dye industry, in tanning, and in the manufacture of paints and laundry soaps. To some extent it is used in medicine. ~Magnesium silicates.~ Many silicates containing magnesium are known and some of them are important substances. Serpentine, asbestos, talc, and meerschaum are examples of such substances. ZINC ~Occurrence.~ Zinc never occurs free in nature. Its compounds have been found in many different countries, but it is not a constituent of common rocks and minerals, and its occurrence is rather local and confined to definite deposits or pockets. It occurs chiefly in the following ores: Sphalerite (zinc blende) ZnS. Zincite ZnO. Smithsonite ZnCO_{3}. Willemite Zn_{2}SiO_{4}. Franklinite ZnO.Fe_{2}O_{3}.