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If at every drop of Spirit of Urine you shake the Vial containing the Red Tincture, you may delightfully observe a pretty variety of Colours in the passage of that Tincture from a Red to a Blew, and sometimes we have this way hit upon such a Liquor, as being look't upon against and from the Light, did seem faintly to emulate the above-mention'd Tincture of Lignum Nephriticum.
Annotation upon the twentieth Experiment. The use of what we lately deliver'd concerning the way of turning Syrrup of Violets, Red or Green, may be this; That, though it be a far more common and procurable Liquor than the Infusion of Lignum Nephriticum, it may yet be easily substituted in its Room, when we have a mind to examine, whether or no the Salt predominant in a Liquor or other Body, wherein 'tis Loose and Abundant, belong to the Tribe of Acid Salts or not.
But this brings into my mind, that having sometimes for Curiosity sake, brought a round Vial with a long Neck fill'd with the Tincture of Lignum Nephriticum into the Darken'd Room already often mention'd, and holding it sometimes in, sometimes near the Sun-beams that enter'd at the hole, and sometimes partly in them, and partly out of them, the Glass being held in several postures, and look'd upon from several Neighbouring parts of the Room, disclos'd a much greater Variety of Colours than in ordinary inlightn'd Rooms it is wont to do; exhibiting, besides the usual Colours, a Red in some parts, and a Green in others, besides Intermediate Colours produc'd by the differing Degrees, and odd mixtures of Light and Shade.
'We have sometimes, he says, 'found in the shops of our druggists certain wood which is there called Lignum Nephriticum, because the inhabitants of the country where it grows are wont to use the infusion of it, made in fair water, against the stone in the kidneys.
Reflections upon the XL. Experiment Compared with the X. and XX. For as well the one as the other of these Salino-Sulphurous Salts will restore the Cæruleous Colour to the Tincture of Lignum Nephriticum, and turn that of Syrrup of Violets into Green. Wherefore this XL. Experiment does opportunely supply the deficiency of those.
This wood may afford us an experiment which, besides the singularity of it, may give no small assistance to an attentive considerer towards the detection of the nature of colours. Take Lignum, Nephriticum, and with a knife cut it into thin slices: put about a handful of these slices into two or three or four pounds of the purest spring water.
The like also is certain of all sensible ideas peculiar to each sense; for whatever of each kind is present in any subject, excludes all other of that sort: v.g. no one subject can have two smells or two colours at the same time. To this, perhaps will be said, Has not an opal, or the infusion of LIGNUM NEPHRITICUM, two colours at the same time?
And here I must not omit, Pyrophilus, to inform You, that we can shew You even in a Mineral Body something that may seem very near of Kin to the Changeable Quality of the Tincture of Lignum Nephriticum, for we have several flat pieces of Glass, of the thickness of ordinary Panes for Windows one of which being interposed betwixt the Eye and a clear Light, appears of a Golden Colour, not much unlike that of the moderate Tincture of our Wood, but being so look'd upon as that the Beams of light are not so much Trajected thorough it as Reflected from it to the Eye, that Yellow seems to degenerate into a pale Blew, somewhat like that of a Turquoise.