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decomposing 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, 
siliceous, and aluminous matter, with hydrate of 
lime. 
The cements which act by combining with car- 
bonic acid, or the common mortars, are made by 
mixing 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 acid gas which constitutes 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 build- 
ings depends upon the perfect conversion of all its 
parts into carbonate of lime. The purest lime- 
stones are the best adapted for making this kind of 
mortar ; the magnesian limestones make excellent 
water cements ; but act with too little energy upon 
carbonic acid gas to make good common mortar. 
The Romans, according to Pliny, made their 
best mortar a year before it was used ; so that it 
was partially combined with carbonic acid gas be- 
fore it was employed. 
