AVAILABILITY OF LIME 249 



It is in fact the only fertilizer which contains practically 

 100 per cent, of the valuable constituent. 



(c) Calcium Hydroxide, Slaked Lime, Hydrated Lime, 

 Ca(OH) 2 .- When water is added to a lump of calcium oxide 

 it swells, gives off heat, and finally crumbles to a fine, dry 

 powder. The process is called "slaking," and the product 

 slaked lime. The reaction is expressed thus: 



CaO+H 2 0=Ca(OH)>. 



The volume of the calcium oxide is increased two or three 

 times, and the weight is increased one-third. To put it in 

 another way, 100 pounds of slaked lime contain about 75 

 pounds of calcium oxide. A solution of calcium hydroxide 

 in water is called lime water. A thin paste of the hydroxide 

 and water is called milk of lime. 



(d) Air-slaked Lime — When burnt lime is allowed to 

 remain exposed to the air it first takes on water and then 

 carbon dioxide until it finally becomes calcium carbonate, 

 thus : 



CaO + H 2 - Ca(OH) 2 . 

 Ca(OH) 2 + C0 2 = CaC0 3 + H2O. 



A pile of burnt lime slakes first on the outside and the 

 lumps fall apart covering the pile with fine material and 

 filling up the interstices so as to protect the interior of the 

 pile from rapid change to carbonate (Fig. 62). The outside 

 changes very quickly to the carbonate. Without a chemical 

 analysis it would be impossible to tell how much calcium 

 oxide there is in a given lot of air-slaked lime, unless, of course, 

 the amount of burnt lime originally present is known. The 

 process, however, is very slow. For example, a sample 

 taken at a depth of 4 inches from the surface of a heap 

 exposed ten years contained 27 per cent, calcium carbonate 

 and 37 per cent, calcium hydroxide. 



188. Availability of Lime. — Under the best conditions 

 chemically pure calcium oxide is soluble as calcium hydroxide 

 at ordinary temperatures to the extent of one part in 1000 

 parts of pure water. Chemically pure calcium carbonate, 

 freshly precipitated, is very slightly soluble in pure water. 



