Further, as these solutions and tinctures do alter the nature of these fluid bodies, as to their aptness to propagate a motion or impulse through them, even so does the particles of the Air, Water, and other fluid bodies, and of Glass, Crystal, &c. which are commixt with this bulk of the _Æther_ alter the motion of the propagated pulse of light; that is, where these more bulkie particles are more plentifull, and consequently a lesser quantity of the _Æther_ between them to be mov’d, there the motion must necessarily be the swifter, though not so robust, which will produce those effects, which I have (I hope) with some probability, ascribed to it in the digression about Colours, at the end of the _Observations_ on _Muscovy-glass._ Now, that other Stones, and those which have the closest and hardest textures, and seem (as far as we are able to discover with our eyes, though help’d with the best _Microscopes_) freest from pores, are yet notwithstanding replenish’d with them, an Instance or two will, I suppose, make more probable. A very solid and unflaw’d piece of cleer white _Marble_, if it be well polish’d and glaz’d, has so curiously smooth a surface, that the best and most polish’d surface of any wrought-glass, seems not to the naked eye, nor through a _Microscope_, to be more smooth, and less porous. And yet, that this hard close body is replenish’d with abundance of pores, I think these following Experiments will sufficiently prove. The first is, That if you take such a piece, and for a pretty while boyl it in Turpentine and Oyl of Turpentine, you shall find that the stone will be all imbu’d with it; and whereas before it look’d more white, but more opacous, now it will look more greasie, but be much more transparent, and if you let it lie but a little while, and then break off a part of it, you shall find the unctuous body to have penetrated it to such a determinate depth every way within the surface. This may be yet easier try’d with a piece of the same _Marble_, a little warm’d in the fire, and then a little Pitch or Tarr melted on the top of it; for these black bodies, by their insinuating themselves into the invisible pores of the stone, ting it with so black a hue, that there can be no further doubt of the truth of this assertion, that it abounds with small imperceptible pores. Now, that other bodies will also sink into the pores of _Marble_, besides unctuous, I have try’d, and found, that a very Blue tincture made in _spirit of Urine_ would very readily and easily sink into it, as would also several tinctures drawn with _spirit of Wine_.