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soluble in water than the original gypsum, but only very slowly 

 takes up water to form the original compound. If, however, the 

 gypsum is only gently heated a different result is obtained. Three- 

 quarters of the water only will now be expelled and the product, still 

 containing about 7-8 per cent, of combined water, is known as 

 Plaster of Paris. This is more, instead of less, soluble than the 

 original gypsum and when mixed with water a firm and solid mass 

 is soon formed consisting essentially of long thin crystals inter- 

 laced. 



This is, perhaps, a convenient point at which to refer briefly 

 to mortar. Lime is slaked and made into a paste with which sand 

 (three or four parts to one of lime) is then thoroughly mixed. Let 

 us follow out the chemical aspect of the business. Chalk or lime- 

 stone taken from the quarry was converted by heating in a kiln 

 into calcium oxide or quick lime, 44 per cent, of carbonic acid 

 gas being eliminated in the process. This lime has been slaked 

 and thus converted into the hydroxide and the wet and plastic 

 mortar which the bricklayer manipulates with his trowel gets 

 stiffer as the excess of water evaporates. It is also left in a porous 

 state through which air can circulate. Now air contains carbonic 

 acid gas, and this is gradually absorbed by the lime. As time goes 

 on a greater and greater proportion of this is reconverted into the 

 substance from which it was in the first case made, namely, 

 calcium carbonate. A hard and crystalline mass thus results. 

 The process is slow and after the lapse of years, possibly of cen- 

 turies, some lime may remain uncombined. The sand and lime also 

 act upon each other to a small extent, but, even in very old mortar, 

 the proportion of silicate, as the compound of lime and sand is 

 called, is quite small, so that the function of the sand is mechanical 

 rather than chemical. 



Hydraulic mortars which will set under water have been 

 known from ancient times. They are made from limestones con- 

 taining a small proportion, say 8 per cent., of clay, or by mixing 

 good fat lime with a proper quantity of a suitable silicate. 



Cements differ from hydraulic mortars inasmuch as they 

 contain no free lime. They are obtained by calcining limestone 

 mixed with clay. These materials are ground together and the 

 mixture, after drying, burnt with the aid of coke in proper kilns. 

 The carbonic acid of the limestone is entirely expelled and the 

 resulting lime acts upon the clay, forming silicate and aluminate 

 of calcium. The clinker drawn from the kiln is broken up and 

 ground to a powder. When this powder is mixed with water 

 and left at rest it stiffens and continues to get harder and 

 harder for many months. Nothing but water is required and this 

 is completely absorbed. The chemistry is quite different from that 

 of ordinary mortar, there being no absorption of atmospheric 

 •carbonic acid. 



The dry cement consists essentially of a mixture of aluminates 

 and silicates of calcium. When water is added the calcium com- 

 pounds are decomposed, lime being set free ; this lime reunites with 



