of these Tryals, that till I can
procure small Colour'd Prisms, or Hollow ones that may be filled with
Tincted Liquor, or obtain Some better Pigments than those I was reduc'd to
imploy, I shall forbear to Build any thing upon what has been delivered,
and shall make no other use of it, than to invite you to prosecute the
Inquiry further.

_EXPERIMENT XVI._

And here, _Pyrophilus_, since we are treating of Emphatical Colours, we
shall add what we think not unworthy your Observation, and not unfit to
afford some Exercise to the Speculative. For there are some Liquors, which
though Colourless themselves, when they come to be Elevated, and Dispers'd
into Exhalations, exhibit a conspicuous Colour, which they lose again, when
they come to be Reconjoyn'd into a Liquor, as good Spirit of _Nitre_; or
upon its account strong _Aqua-fortis_, though devoid of all appearance of
Redness whilst they continue in the form of a Liquor, if a little Heat
chance to turn the Minute parts of them into Vapour, the Steam will appear
of a Reddish or deep Yellow Colour, which will Vanish when those
Exhalations come to resume the form of Liquor.

And not only if you look upon a Glass half full of _Aqua-fortis_, or Spirit
of _Nitre_, and half full of _Nitrous_ steams proceeding from it, you will
see the Upper part of the Glass of the Colour freshly mention'd, if through
it you look upon the Light. But which is much more considerable, I have
tried, that putting _Aqua-fortis_ in a long clear Glass, and adding a
little Copper or some such open Metall to it, to excite Heat and Fumes, the
Light trajected through those Fumes, and cast upon a sheet of White Paper,
did upon that appear of the Colour that the Fumes did, when directly Look'd
upon, as if the Light were as well Ting'd in its passage through these
Fumes, as it would have been by passing through some Glass or Liquor in
which the same Colour was Inherent.

To which I shall further add, that having sometimes had the Curiosity to
observe whether the Beams of the Sun near the Horizon trajected through a
very Red Sky, would not (though such rednesses are taken to be but
Emphatical Colours) exhibit the like Colour, I found that the Beams falling
within a Room upon a very White Object, plac'd directly opposite to the
Sun, disclos'd a manifest Redness, as if they had pass'd through a Colour'd
_Medium_.

_EXPERIMENT XVII._

The emergency, _Pyrophilus_, of Colours upon the Coalition of the Particles