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 body it acts upon. As when Goldsmiths burnish a Plate or Vessel of Silver, that having been lately boil'd lookt white before, though they deprive it of the greatest part of its colour, and give it a new power of reflecting the beams of Light and visible Objects, in the manner proper to specular bodies; yet all this is done by the intervention of a burnishing Tool, which often is but a piece of Steel or Iron conveniently shap'd; and all that this Burnisher does, is but to depress the little prominencies of the Silver, and reduce them, and the little cavites of it, to one physically level or plain Superficies. And so when a Hammer striking often on a Nail, makes the head of it grow hot, the Hammer is but a purely Mechanical Agent, and works by local motion. And when by striking a lump of Glass, it breaks it into a multitude of small parts that compose a white powder, it acts as Mechanically in the production of that Whiteness as it does in driving in a Nail to the head. And so likewise, when the powder'd Glass or Colophony lately mention'd is, by the fire, from a white and opacous body, reduc'd into a colourless (or a reddish) and transparent one, it appears not, that the fire, though a natural Agent, need work otherwise than