observed in those Bubbles which Children use to make with Soap-water, I did the less wonder; especially when upon Experiment I found, I was able to produce the same Phænomena in thin Bubbles made with any other transparent Substance. Thus have I produced them with Bubbles of Pitch, Rosin, Colophony, Turpentine, Solutions of several Gums, as Gum-Arabick in water; any glutinous Liquor, as Wort, Wine, Spirit of Wine, Oyl of Turpentine, Glare of Snails, &c.
It would be needless to enumerate the several Instances, these being enough to shew the generality or universality of this propriety. Only I must not omit, that we have instances also of this kind even in metalline Bodies and animal; for those several Colours which are observed to follow each other upon the polisht surface of hardned Steel, when it is by a sufficient degree of heat gradually tempered or softened, are produced, from nothing else but a certain thin Lamina of a vitrum or vitrified part of the Metal, which by that degree of heat, and the concurring action of the ambient Air, is driven out and fixed on the surface of the Steel.
And this hints to me a very probable (at least, if not the true) cause of the hardning and tempering of Steel, which has not, I think, been yet given, nor, that I know of been so much as thought of by any. And that is this, that the hardness of it arises from a greater proportion of a vitrified Substance interspersed through the pores of the Steel. And that the tempering or softning of it arises from the proportionate or smaller parcels of it left within those pores. This will seem the more probable, if we consider these Particulars.
First, That the pure parts of Metals are of themselves very flexible and tuff; that is, will indure bending and hammering, and yet retain their continuity.
Next, That the Parts of all vitrified Substances, as all kinds of Glass, the Scoria of Metals, &c. are very hard, and also very brittle, being neither flexible nor malleable, but may by hammering or beating be broken into small parts or powders.
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.
