Lime and Calcium Compounds 137 



thoroughly heated, it gives up the carbonic acid, which 

 goes off as a gas, leaving calcium oxide, commonly known 

 as lime, and often termed "burned lime," "quick-lime," 

 "stone-lime" or "lump-lime." In fact, 100 pounds of 

 pure limestone when properly burned gives up 44 pounds 

 of carbonic acid, leaving 56 pounds of calcium oxide. 

 This material calcium oxide has become known as 

 actual lime through the practice of farmers, manufac- 

 turers and chemists who are accustomed to using it as a 

 basis of comparison in estimating the quantities of lime 

 in the different forms. It was the custom of farmers 

 located in limestone regions to maintain kilns and do the 

 burning themselves, but the increased cost of wood and 

 high cost of coal for burning and scarcity of labor have 

 made it practically impossible for the farmer to compete 

 with the manufacturer and the home practice of burning 

 has been abandoned. 



The resultant material after burning must be slaked 

 before it is applied to the soil. This is done by adding 

 water, which is absorbed and the lime falls to a fine powder 

 ready for distribution. When chemically pure, burned 

 lime contains 100 per cent of actual lime (calcium oxide), 

 but the commercial product seldom contains more than 

 92 per cent and varies from 78 to 96 per cent of actual 

 lime, depending upon the amount of impurities in the 

 limestone used for burning. 



Ground limestone. 



By the use of powerful grinding machinery used in the 

 manufacture of cement, ground or pulverized limestone 

 has been put upon the market. The rock limestone is 

 simply ground, bagged and sold to the farmer. Chem- 

 ically pure ground limestone contains 56 per cent of actual 



