BUILDING CONSTRUCTION 127 



a large amount of absorption should be discarded. A slate, set on 

 edge in water to half its height, should not absorb water in the 

 upper and unsubmerged part. In a really good slate no sign of 

 moisture will be seen above the water line. 



The following are the usual sizes in which slates are supplied 

 for ordinary building purposes : 



Doubles, 13 inches by 6 inches. 

 Ladies, 16 inches by 8 inches. 

 Countesses, 20 inches by 10 inches. 

 Duchesses, 23 inches by 12 inches. 



Thick slabs of slate are sometimes used to form cisterns or other 

 containers of water. Such cisterns are not to be recommended, 

 as the liability of the joints to leak make them unhygienic and 

 unreliable. 



LIMES, MORTARS AND CEMENTS. Lime for building purposes, 

 such as making mortar and plaster, is obtained by heating limestone, 

 limestone chalk, shells or any substance composed of calcium 

 carbonate. 



The limestone is burned in a kiln, along with fuel, in order that 

 the carbonic acid gas and any moisture are driven off and escape 

 into the open air. 



Pure chalk lime is burned in " flare kilns." These kilns are 

 arranged so that only the flame from the furnace comes into contact 

 with the stone. The common, old-fashioned kiln for preparing 

 ordinary lime " shells " is usually built on a hillside in proximity 

 to the quarry. This arrangement enables it to be easily filled from 

 the top, which is open and almost level with the high ground. The 

 firing and emptying is done from the draw-hole at the low level. 

 In these kilns the coal and limestone are piled up inside the stone- 

 built kiln in alternate layers, and finished off at the top in the form 

 of a cone. The firing of the kiln takes about a week, the time 

 depending upon the size of the kiln. After the kiln is cooled the 

 fire-bars at the bottom are removed, and the lime-shells or " quick- 

 lime " fall out and are removed in carts. Lime-shells should be 

 used as fresh as possible, as they deteriorate by exposure to the 

 atmosphere, from which they absorb moisture. 



For the preparation of ordinary building mortar without the 

 use of a mortar mill, one part of lime shells is placed on the ground 

 or on a cement floor and surrounded by about three parts sand, 

 and the whole thoroughly wet. The quicklime immediately absorbs 

 the moisture and begins to slake by effervescing violently, giving out 

 heat and falling into a fine powder. By this process the caustic 



