In order to finish what remains to be said upon the several sorts of saline substances, we should now speak of the Acids obtained from vegetables and animals, and also of the volatile Alkalis: but, seeing these saline substances differ from those of which we have already treated, only as they are variously altered by the unions they have contracted with certain principles of vegetables and animals, of which nothing has been yet said, it is proper to defer being particular concerning them, till we have explained those principles. 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_.