1312. Different kinds of limestones have different effects. The limestones containing alumina and silica are less fitted for the purposes of manure than pure limestones; but the lime formed from them has no noxious quality. Such stones are less efficacious, merely because they contain less lime, and because they are apt to harden or vitrify in burning, and then do not fall to pieces well by slaking when spread over the land. There is very seldom any considerable portion of coaly matter in bituminous limestones; never as much as five parts in 100; but such limestones make very good lime. The carbonaceous matter can do no injury to the land, and may, under certain circumstances, become the food of the plant. Some limestones and kinds of chalk or marl contain a small proportion of phosphoric acid, and are in consequence valuable manures.