e reduc'd into Films
so extremely thin, that being kept clean they constantly shew'd on their
Surfaces (but after the manner newly mention'd) the varying Colours of the
Rain-bow, which were exceedingly Vivid, as I had often opportunity to
observe in some, that I caus'd purposely to be made, to keep by me.

But lest it should be objected, that the above mentioned Instances are
drawn from Transparent Liquors, it may possibly appear, not impertinent to
add, what I have sometimes thought upon, and several times tried, when I
was considering the Opinions of the _Chymists_ about Colours, I took then a
Feather of a convenient Bigness and Shape, and holding it at a fit distance
betwixt my Eye and the Sun when he was near the Horizon, me thought there
appear'd to me a Variety of little Rain-bows, with differing and very vivid
Colours, of which none was constantly to be seen in the Feather; the like
_Phaenomenon_ I have at other times (though not with altogether so good
success) produc'd, by interposing at a due distance a piece of Black
Ribband betwixt the almost setting Sun and my Eye, not to mention the
Trials I have made to the same purpose, with other Bodies.

_EXPERIMENT XX._

Take good Syrrup of Violets, Impraegnated with the Tincture of the flowers,
drop a little of it upon a White Paper (for by that means the Change of
Colour will be more conspicuous, and the Experiment may be practis'd in
smaller Quantities) and on this Liquor let fall two or three drops of
Spirit either of Salt or Vinegar, or almost any other eminently Acid
Liquor, and upon the Mixture of these you shall find the Syrrup immediatly
turn'd Red, and the way of Effecting such a Change has not been unknown to
divers Persons who have produc'd the like, by Spirit of Vitriol, or juice
of Limmons, but have Groundlessly ascrib'd the Effect to some Peculiar
Quality of those two Liquors, whereas, (as we have already intimated)
almost any Acid Salt will turn Syrrup of Violets Red. But to improve the
Experiment, let me add what has not (that I know of) been hitherto
observ'd, and has, when we first shew'd it them, appear'd something
strange, even to those that have been inquisitive into the Nature of
Colours; namely, that if instead of Spirit of Salt, or that of Vinegar, you
drop upon the Syrrup of Violets a little Oyl of Tartar _per Deliquium_, or
the like quantity of Solution of Potashes, and rubb them together with your
finger, you shall find the Blew Colour of