Thirdly, That all Metals (excepting Gold and Silver, which do not so much with the bare fire, unless assisted by other saline Bodies) do more or less _vitrifie_ by the strength of fire, that is, are corroded by a Saline Substance, which I elsewhere shew to be the true cause of fire; and are thereby, as by several other _Menstruums_ converted into _Scoria_; And this is called, _calcining_ of them, by Chimists. Thus Iron and Copper by heating and quenching do turn all of them by degrees into _Scoria_, which are evidently _vitrified_ Substances, and unite with Glass, and are easily _fusible_; and when cold, very hard, and very brittle. Fourthly, That most kind of _Vitrifications_ or _Calcinations_ are made by Salts, uniting and incorporating with the metalline Particles. Nor do I know any one _calcination_ wherein a _Saline_ body may not, with very great probability, be said to be an agent or coadjutor. Fifthly, That Iron is converted into Steel by means of the incorporation of certain salts, with which it is kept a certain time in the fire. Sixthly, That any Iron may, in a very little time, be _case hardned_, as the Tradesmen call it, by casing the iron to be hardned with clay, and putting between the clay and iron a good quantity of a mixture of _Urine_, _Soot_, _Sea-salt_, and _Horses hoofs_ (all which contein great quantities of Saline bodies) and then putting the case into a good strong fire, and keeping it in a considerable degree of heat for a good while, and afterwards heating, and quenching or cooling it suddenly in cold water. Seventhly, That all kind of vitrify’d substances, by being suddenly cool’d, become very hard and brittle. And thence arises the pretty _Phænomena_ of the Glass Drops, which I have already further explained in its own place. Eighthly, That those metals which are not so apt to vitrifie, do not acquire any hardness by quenching in water, as Silver, Gold, &c.