Theophrastus's History of Stones/On the Colours of the Sapphire and Turquoise
LETTER I.
On the Colours of the Sapphire
and Turquoise.
SIR,
WHEN my Notes on Theophrastus were mentioned Yesterday, some objected to the Sapphire's being coloured by Particles of Copper, and seemed very firm in the Opinion, that that Gem owes its Dye to a Native Zaffer.
I am sorry I have only Room to name Things in those Notes, without Opportunities of entering into a Detail of the Experiments. Thro' the Course of those Notes I have not tied myself down to the Sentiments of any particular Author, but have, as my own Experiments and Observations directed, at Times agreed to, and in other Places dissented from, the Opinions of the whole Number. And how I have succeeded in this Example, the fairest Way of judging will be first, fairly to give the Arguments used in Support of the other Opinion; which are principally three, and have the Appearance of being of some Weight. They are:
1, That the Turquoise is evidently coloured by the same Matter with the Sapphire, and that the Matter of its Colour is known to be a Native Zaffer.
2. That Copper is not capable of giving the deep Blue of some of the deeper Sapphires; and of the Veins and Striæ of the rough native Turquoises.
3. That Zaffer is the Substance which colours the common blue Glass; and that it is capable of giving it the Colour of the deepest native Sapphires; as is evident from the counterfeit ones which are coloured with it, and are of all the Degrees of Colour of the genuine.
To which permit me to answer,
First, That it was incumbent on the Assertors of this Doctrine, to have proved the Existence of this Native Zaffer, before they attributed such great Effects to it. I am not ashamed to say, that I don't know what Native Zaffer is; that I never yet saw any such Fossil, nor believe I ever shall: And, notwithstanding that Dr. Woodward, and some other able Naturalists have ventured to name some of their unknown Specimens native Zaffers, I cannot bring myself to think that Nature ever formed any Substance that could be properly so called; all that I have been shewn as such, having been Things which a little Chemistry was able to shew that Naturalists ought to have been ashamed of calling by such a Name: Not that I would pretend to limit the Operations of Nature within the Bounds of our narrow Understandings; or declare any Thing impossible, because it has not yet been seen to be effected: But I think the Assertors of such great Effects from so very uncertain a Substance, ought, if ever they had seen it, to have given a more rational Account of it than any we have at present.
The Zaffer we know, and with which the blue Glass and counterfeit Sapphires are stained, is a Preparation which seems to owe its present Mode of Existence to the extreme Force of Fire; and is perhaps no genuine Production of Nature, even in a latent State, except in its constituent Principles. It is prepared from Cobalt, affording, by the Assistance of Fire, the Arsenics, this Substance; and Smalt, with the Addition of a fixed Alkali. After the Fire of a reverberatory Furnace has driven off the arsenical Particles, the remaining Mass is powdered and calcined three or four Times over; and then being mixed with three Times its Quantity of powdered Flints, affords us the common Zaffer.
But it may be proper to examine what Weight, even allowing the Existence of a native Zaffer, there is in the Arguments founded on its supposed Effects.
And to the First, That the Turquoise and Sapphire are coloured by the same Matter, and that this Matter is universally allowed to be a native Zaffer: I shall answer, That I allow the Sapphire and Turquoise to be coloured by Particles of the same Kind; that I know it to be the common Opinion, that the Turquoise is coloured by Zaffer, and not by Copper: But that I also know it to be an erroneous one. I know the Turquoise owes its Colour to Copper only; having succeeded in a Course of Experiments, by which I have been able to divest the Turquoise wholly of its Colour; to precipitate and preserve that Colour separate and alone; to prove that Colour, by the Effects of different Menstruums, to be absolute Copper; and, by Experiments founded on this Process, to give, by a Solution of Copper in a volatile Alkali, the true Turquoise Colour to the Substance of the native Turquoises, which is absolutely no other than animal Bone; and to make, by that means, those factitious Turquoises, which have been put, before a judicious Assembly, to the severest Trials; and gave all the Marks of the genuine.
To the Second Argument, That Copper is not capable of giving so deep a Blue as that of some of these Gems; I have a Solution of Copper, the very one with which I tained the factitious Turquoises, which is of the true Colour of the deepest Male Sapphires, and deeper than the commonly called black Veins of the rough native Turquoises, if carefully examined.
The Authors of this Objection might, indeed, have known, from the excellent Mr. Boyle's Experiments, that Copper is the last Thing to be, with any shew of Reason, suspected of wanting this Property; for that Gentleman has proved, that a Grain of that Metal is capable of giving a blue Colour to 530,620 Times its Bulk of Water.
In regard to the Third Argument, That the genuine Sapphires are probably coloured by Zaffer, because blue Glass, and the counterfeit Sapphires are so; I cannot but observe, that external Appearances are of little Weight in Philosophy; and I am sorry to say, that it was only a very superficial View of these Things, that could start an Objection to Copper's colouring the Sapphire, from them: For a more careful Examination of these very Bodies, must afford Arguments for the contrary, as it will evidently prove, that the Colour of the Sapphire cannot be owing to the same Substance with that of these Glasses: Since the very Heat necessary for forming them, would, in a few Minutes, wholly divest the finest Sapphire in the World of all its Colour.
The common blue Glass is made from the common or crystal Frit melted with Zaffer; and the finest counterfeit Sapphires, with a crystal Glass, worked with an Admixture of Lead, and this Zaffer, in the Proportion of about one fiftieth Part. The Lead gives, in this Case, an additional Density to the Glass, which adds greatly to the Lustre of the counterfeit Gem; as the more dense the transparent Matter is, the more bright and vivid the metalline Tinge appears through it; but while Lead thus increases the Density, it debases the Glass in another respect of equal Consequence, in that it makes it softer. Whichever of these Substances, however, is made the Subject of this Experiment, the Effect will be the same; for if we bring to the Trial of only a clear Charcoal Fire, a genuine Sapphire, and either of these factitious Substances, and throw them together into it, we shall soon see that they owe their Colours to Particles of a very different Kind; for the Genuine will be seen to emit a fine clear blue Flame, the Counterfeit not so much as the least Vapour; and when, after this, they are taken out together, the true Sapphire shall be found wholly colourless and transparent as a Piece of Crystal, and the Counterfeit or Glass, unaltered.
Fire, which is thus able to divest the Sapphire of its Colour, has also the fame Effect on the Turguoise; as the Workers on it well know: And this is easily accounted for, if they are coloured, as I am convinced they are, by a fine metalline Sulphur. But I will venture to affirm, that it could not be the Case, if those Gems were coloured by a Zaffer.
Let it not be here objected, that the Workers on the native Turquoises are obliged to have Recourse to Fire to give them their Colour; and that therefore it is not probable, the same Power should be able to take it away; for the Truth of this, is only, that the Colour of the native Turquoises of some Countries is not equally spread through the whole Mass, but lodged in different Parts of it in Form of Veins and Striæ: It is to dislodge the Colour from these Veins, and diffuse it equally thro' the whole Mass, that they have Recourse to Heat: A very gentle Fire is all they dare trust on this Occasion, and is always found sufficient. What I would observe from the Whole of this is, that this Effect of Fire on the rough Turguoises, is a Proof that their Colour is owing to the fame Particles with that of the Sapphire; and that this dislodging and diffusing it through the whole Mass, is the first Step toward the dissipating and entirely driving it off; for a little too long Continuance in the same Heat, will, as the Workmen too often find to their Sorrow, wholly drive off the Blue, and leave the Matter colourless, as the Sapphire when taken from the Fire,
- I am,
- SIR,
- Your humble Servant,
- SIR,
- I am,