[CHAP. V.]
Of Lime.
Any substance whatever, that has been roasted a considerable time in a strong fire without melting, is commonly called a Calx. Stones and metals are the principal subjects that have the property of being converted into Calces. We shall treat of Metalline Calces in a subsequent chapter, and in this confine ourselves to the Calx of Stone, known by the name of Lime.
In treating of earths in general we observed that they may be divided into two principal kinds; one of which actually and properly flows when exposed to the action of fire, and turns to glass; whence it is called a fusible or vitrifiable earth; the other resists the utmost force of fire, and is therefore said to be an unfusible or unvitrifiable earth. The latter is also not uncommonly called calcinable earth; though sundry sorts of unfusible earths are incapable of acquiring by the action of fire all the qualities of calcined earth, or Lime properly so called: such earths are particularly distinguished by the denomination of refractory earths.
As the different sorts of stones are nothing more than compounds of different earths, they have the same properties with the earths of which they are composed, and may, like them, be divided into fusible or vitrifiable, and unfusible or calcinable. The fusible stones are generally denoted by the name of Flints; the calcinable stones, again, are the several sorts of marbles, cretaceous stones, those commonly called free-stones, &c. some of which, as they make the best Lime, are, by way of eminence, called Lime-stones. Sea-shells, also, and stones that abound with fossile shells, are capable of being burnt to Lime.
All these substances, being exposed, for a longer or shorter time, as the nature of each requires, to the violent action of fire, are said to be calcined. By calcination they lose a considerable part of their weight, acquire a white colour, and become friable though ever so solid before; as, for instance, the very hardest marbles. These substances, when thus calcined, take the name of Quick Lime.
Water penetrates Quick Lime, and rushes into it with vast activity. If a lump of newly calcined Lime be thrown into water, it instantly excites almost as great a noise, ebullition, and smoke, as would be produced by a piece of red-hot iron; with such a degree of heat too, that, if the Lime be in due proportion to the water, it will set fire to combustible bodies; as hath unfortunately happened to vessels laden with Quick Lime, on their springing a small leak.
As soon as Quick Lime is put into water, it swells, and falls asunder into an infinite number of minute particles: in a word, it is in a manner dissolved by the water, which forms therewith a sort of white paste called Slacked Lime.