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.