Tut. Such of them as are hard stone, as the marbles and many of the lime-stones, are used for the same purposes as other stones. But their great use is in the form of lime, which is a substance of many curious properties that I will now explain to you. When fresh burnt it is called quicklime, on account of the heat and life, as it were, which it possesses. Have you ever seen a lump put into water?
Geo. Yes, I have.
Tut. Were you not much surprised to see it swell and crack to pieces, with a hissing noise and a great smoke and heat?
Geo. I was, indeed. But what is the cause of this—how can cold water occasion so much heat?
Tut. I will tell you. The strong heat to which calcareous earth is exposed in making it lime expels all the water it contained, (for all earths, as well as almost everything else, naturally contain water,) and also a quantity of a peculiar kind of air which was united with it. If water be now added to this quicklime, it is drunk in again with such rapidity, as to crack and break the lime to pieces. At the same time a great heat is occasioned by the water combining with the lime, and this makes itself sensible by its effects, burning all the things that it touches, and turning part of the water to steam. This operation is called slacking of lime. The water in which lime is slacked dissolves a part of it, and acquires a very pungent harsh taste: this is used in medicine under the name of lime-water. If instead of soaking quicklime in water, it is exposed for sometime to the air, it attracts moisture slowly, and by degrees fails to powder, without much heat or disturbance. But whether lime be slacked in water or air, it does not at first return to the state in which it was before, since it still remains deprived of its air, and on that account is still pungent and caustic. At length, however, it recovers this also from the atmosphere, and is then mild calcareous earth as at first. Now it is upon some of these circumstances that the utility of lime depends. In the first place, its burning and corroding quality makes it useful to the tanner, in loosening all the hair from the hides, and destroying the flesh and fat that adhered to them. And so in various other trades it is used as a great cleanser and purifier.
Har. I have a thought come into my head. When it is laid upon the ground, I suppose its use must be to burn up the weeds?
Tut. True—that is part of its use.
Geo. But it must burn up the good grass and corn too?
Tut. Properly objected. But the case is, that the farmer does not sow his seeds till the lime is rendered mild by exposure to the air and weather, and is well mixed with the soil. And even then it is reckoned a hot and forcing manure, chiefly fit for cold and wet lands. The principal use of lime, however, is as an ingredient in mortar. This, you know, is the cement by which bricks and stones are held together in building. It is made of fresh slacked lime and a proportion of sand well mixed together; and, when used for plastering walls, some chopped hair is put into it. The lime binds with the other ingredients; and in length of time, the mortar, if well made, becomes as hard, or harder, than stone itself.
Geo. I have heard of the mortar in very old buildings being harder and stronger than any made at present.