And first, we shall find, that even Glass it self may, by the help of a Lamp, be blown thin enough to produce these _Phænomena_ of Colours: which _Phænomena_ accidentally happening, as I have been attempting to frame small Glasses with a Lamp, did not a little surprize me at first, having never heard or seen any thing of it before; though afterwards comparing it with the _Phænomena_, I had often 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.