of a violent jarring Motion, or a strong and nimble vibrative one, we may have from a piece of iron grated on very strongly with a file:  for if into that a pin screw’d so firm and hard, that though it has a convenient head to it, yet it can by no means be unscrew’d by the fingers; if, I say, you attempt to unscrew this whilst grated on by the file, it will be found to undoe and turn very easily.  The first of these Examples manifests, how a body actually divided into small parts, becomes a fluid.  And the latter manifests by what means the agitation of heat so easily loosens and unties the parts of solid and firm bodies.  Nor need we suppose heat to be any thing else, besides such a motion; for supposing we could Mechanically produce such a one quick and strong enough, we need not spend fuel to melt a body.  Now, that I do not speak this altogether groundless, I must refer the Reader to the Observations I have made upon the shining sparks of Steel, for there he shall find that the same effects are produced upon small chips or parcels of Steel by the flame, and by a quick and violent motion; and if the body of steel may be thus melted (as I there shew it may) I think we have little reason to doubt that almost any other may not also.  Every Smith can inform one how quickly both his File and the Iron grows hot with filing, and if you rub almost any two hard bodies together, they will do the same:  And we know, that a sufficient degree of heat causes fluidity, in some bodies much sooner, and in others later; that is, the parts of the body of some are so loose from one another, and so unapt to cohere, and so minute and little, that a very small degree of agitation keeps them always in the state of fluidity.  Of this kind, I suppose, the AEther, that is the medium or fluid body, in which all other bodies do as it were swim and move; and particularly, the Air, which seems nothing else but a kind of tincture or solution of terrestrial and aqueous particles dissolv’d into it, and agitated by it, just as the tincture of Cocheneel is nothing but some finer dissoluble parts of that Concrete lick’d up or dissolv’d by the fluid water.  And from this Notion of it, we may easily give a more Intelligible reason how the Air becomes so capable of Rarefaction and Condensation.  For, as in tinctures, one grain of some strongly tinging substance may sensibly colour some hundred thousand grains of appropriated Liquors, so as every drop of it has its proportionate share, and be sensibly ting’d, as I have try’d both with Logwood and Cocheneel:  And as some few grains of Salt is able to infect