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 Mechanically, by colliquating the incoherent grains of powder into one mass; wherein, the ranks of pores not being broken and interrupted as before, the incident beams of Light are allow'd every way a free passage through them.

Fifthly, the like Phænomena to those of a Quality to be explicated, or at least as difficult in the same kind, may be produc'd in bodies and cases, wherein 'tis plain we need not recurre to Substantial Forms. Thus a varying Colour, like that which is admired in a Pigeons Neck, may be produc'd in changeable Taffety, by a particular way of ranging and connecting Silk of several Colours into one piece of Stuff. Thus we have known Opals casually imitated and almost excell'd by Glass, which luckily degenerated in the Furnace. And somewhat the like changeable and very delightful Colour I remember to have introduced into common Glass with Silver or with Gold and Mercury. So likewise meerly by blowing fine Crystal-Glass at the flame of a Lamp to a very extraordinary thinness, we have made it to exhibit, and that vividly, all the Colours (as they speak) of the Rainbow; and this power of pleasing by diversifying the Light, the Glass, if well preserved, may keep for a long time. Thus