LIMESTONES. 



311 



variations in physical properties are of economic interest chiefly in 

 their bearing upon two points: the percentage of water carried by 

 the limestone as quarried, and the ease with which the rock may be 

 crushed and pulverized. To some extent the two properties counter- 

 balance each other, for the softer the limestone the more absorbent 

 it is likely to be. These purely economic features will be discussed in 

 more detail in later chapters. 



Effect of heating on limestone. On heating a non-magnesian lime- 

 stone to or above 750 F., its carbon dioxide will be driven off, leav- 

 ing quicklime (calcium oxide, CaO). If a magnesian limestone be simi- 

 larly treated, the product would be a mixture of calcium oxide and 

 magnesium oxide (MgO). The rapidity and perfection of this decom- 

 position can be increased by passing steam or air through the burning 

 mass. In practice this is accomplished either by the direct injection 

 of air or steam, or more simply by thoroughly wetting the limestone 

 before putting it into the kiln. 



FIG. 65. Working thick limestone-bed. 



If, however, the limestone contains an appreciable amount of silica, 

 alumina, and iron, the effects of heat will not be of so simple a char- 

 acter. At temperatures of 800 C. and upwards these clayey impuri- 

 ties will combine with the lime oxide, giving silicates, aluminates, and 



