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tar used in the great dykes of Holland. Substances 

 which will answer all the ends of puzzolana and tar- 

 ras are abundant in the British islands. An excellent 

 red tarras may be procured in any quantities from the 

 Giants' Causeway in the north of Ireland: and decom- 

 posing basalt is abundant in many parts of Scotland, 

 and in the northern districts of England in which coal 

 is found. 



Parker's cement, and cements of the same kind 

 made at the alum works of Lord Dundas and Lord 

 Mulgrave are mixtures of calcined ferruginous stones, 

 with hydrate of lime. 



The cements which act by combining with car- 

 bonic acid, or the common mortars, are made by mix- 

 ing together slacked lime and sand. These mortars, 

 at first solidify as hydrates, and are slowly converted 

 into carbonate of lime by the action of the carbonic 

 acid of the air. Mr. Tennant. found that a mortar of 

 this kind in three years and a quarter had regained 63 

 per cent, of the quantity of carbonic gas which con- 

 stitutes the definite proportion in carbonate of lime. 

 The rubbish of mortar from houses owes its power to 

 benefit lands principally to the carbonate of lime it 

 contains; and the sand in it; and its state of cohesion 

 renders it particularly fitted to improve clayey soils. 



The hardness of the mortar in very old buildings 

 depends upon the perfect conversion of all its parts 

 into carbonate of lime. The purest limestones are the 

 best adapted for making this kind of mortar; the mag- 

 nesian limestones make excellent water cements; but 

 act with too /ittle energy upon carbonic acid gas to 

 make good common morter. 



