# Theophrastus's History of Stones/On the Effects of different Menstruums on Copper

Addressed to Martin Folkes in the 1746 edition
LETTER II.
On the Effects of different Menstruums on Copper.
SIR,
IN a Letter of the 19th of last Month, which you did me the Honour to read before the Royal Society, I endeavoured, principally by means of some Experiments I had been lately making, to settle the Question so much disputed among the present Naturalists, Of, what the blue Gems in general are coloured from. What engaged me in the Dispute, was an Objection raised against the Opinion I had declared myself of in this Case, in my Notes on Theophrastus: And I am very happy to find, that even the Gentlemen who made that Objection are now convinced, that it is to Copper alone that the Sapphire and Turquoise owe their beautiful Blue.
For myself, I must acknowledge, that tho' I have long been convinced of the Fact, the Manner in which it was effected, was long a great Difficulty to me: The Menstruum in which my Tincture of Copper, (which proved ​to the Senses, that Copper was capable of giving the deepest and finest Blue imaginable) was made, was a volatile alkaline Spirit: And where Nature could find, in the Bowels of the Earth, any Thing analogous to a volatile urinous Alkali, produced by Chemistry, was a Question not easily answered. The particular Salt of the mineral Waters seems to approach, indeed, something to a Menstruum of this Kind; and Dr. Hoffman has proved, that it is at least much fitter to be classed with the Alkalies than with the Acids. But the System of the Colours of the blue Gems being from Copper, must stand upon a very precarious Basis, if there could be found no other Menstruum than one we are so very uncertain about, to strike their Colour from that Metal.
Copper, however, is, in Truth, perhaps the farthest of all the Metals from being subject only to the Power of one appropriated Menstruum; and a Course of Experiments on it, have now shewn me, that we need not have Recourse to so uncertain a mineral Substance as this latent Alkali, for producing a Blue from it; but that Menstruums of another Kind, even Acids, and those the very Acids, whose Principles are the commonest of all others in the Earth, can afford us the same Colour; and are every where to be found in great Abundance. Gold is soluble only in Agua regia; for all the other Menstruums that are talked of for it, have a genuine Sea-salt for their Basis, and are therefore only so many ​Kinds of Aqua regia; Silver, in Aqua fortis, but not in Agua regia, or Spirit of Salt, or Oil of Vitriol, or, in short, in any but the nitrous Acids: whence it may very properly be said, that Sea-salt is the true Dissolvent of Gold, and Nitre of Silver. Lead is readily dissolved by the weaker Acids, but not at all by Agua regia, and but difficultly by many of the stronger; Iron by most of the acid Salts; and Tin by Aqua regia, and not easily by any other Menstruum, unless first divested of its Sulphur by Calcination; but Copper is to be dissolved by every Kind of Salt; and, in short, by almost every thing that ever had in Chemistry the Name of a Menstruum; and produces, with its different Solvents, an almost infinite Variety of very beautiful Colours: So that it may indeed have been the Basis of the Colour of, perhaps, more of the Gems than has yet been imagined.
Filings of Copper dropt into the Flame of a Lamp, thrown into an horizontal Direction by a Blow-pipe, emit a very beautiful green Flame.
Mixed with three Times their Quantity of corrosive Sublimate, and afterwards divested of the Mercury by Fire, they form, with the remaining Salts, a transparent Resin of a beautiful Hyacinth Colour, which will melt and burn in the Fire, emitting also a fine green Flame.
Exposed to the Fumes of Quicksilver, they become white and shining like Silver.
​Melted with Zink, they make an uniform Mass of a fine gold Colour, as they do Brass with Calamine.
Held over melted Orpiment, they become not only white but brittle.
And by extreme Violence of Fire, are converted into a hard, dense, glassy Matter, of a deep Red; transparent, and in some Degree resembling the Sorane Garnet.
It has been the general Opinion of the Chemists, that Solutions of this Metal in Acids were green, and in Alkalies blue: Some, however have altered, from a few Experiments of their own, or perhaps only from what they imagined must have been the Success of Experiments, this general Account; and particularly among certain of the more modern Writers, it has stood, that Copper, dissolved in Acids or fixed Alkalies, affords a green Colour; and in volatile Alkalies, a fine Blue: But you will observe, by the following Experiments, that these Accounts are neither of them to be depended on: And, indeed, whoever has Disquisitions of this Kind to attempt, will always find, that it must be a Knowledge of Nature, and not of Books, that will afford him what he can depend on; and that Systems built on any Body's Experiments but his own, will be found to stand on a very infirm Basis.
What I have been able to learn, by repeated Experiments on this Metal in Menstrums of all Kinds, is, that the Solutions of it in different Fluids, cannot be, in regard to Colour, ​determinately reduced into Method at all: The different Acids having the Properties talked of in the Alkalies, of producing different Colours; and even the same Acid being sometimes capable of affording either a green or a blue Solution, according to the different Quantity of the Metal dissolved in it. In Cases of this Kind, however, I have every where judged the most perfect Solution the properest to describe the Effect of the Menstruum by: And of what I have principally learnt by these Experiments, be pleased to accept the following Account.
A Solution of Copper in Oil of Olives, is of a fine grass Green; in white Wax, of a bluish Green, approaching to the Colour of our Aqua marine; and in pure Water, of a dead whitish Green. In regard to these Menstruums it is, however, to be observed, that the expressed vegetable Oils do not dissolve Copper, as Oils, but by means of certain other heterogene Particles which they contain; for all expressed vegetable Oils contain in them Water, and a latent acid Salt: of both which, I am pretty certain, they may be wholly divested by Fire, and rendered, by that Means, incapable of acting as Menstruums on this Metal: For I have found, that Oil of Olives, after long boiling, has been capable of extracting scarce any Colour at all from Copper; and make no doubt but that it might be so perfectly deprived of its Acid, as well as Water, by long boiling with Litharge, or some similar Substance ​proper to imbibe its Acid, as to have no Power of dissolving this Metal at all. Nor is this latent Acid peculiar to the expressed Oils alone; those procured by Distillation evidently contain it also, as the excellent Dr. Hoffman has proved, who by grinding the distilled Oils of Lavender and Turpentine with Salt of Tartar, obtained thence a neutral Salt.
Wax, in like Manner, dissolves Copper no otherwise than by a true, genuine, and pretty sharp Acid, which it evidently contains, and which is easily separated from it by Distillation with a very gentle Heat. And in regard to Water, it may not be improper to observe, that though it is but a poor Dissolvent of Metals with us, yet it may in the Bowels of the Earth, do Wonders: For we find evidently, that the Power of Water, as a Menstruum, depends, in many Cases, exactly on its Degree of Heat; and as it is capable of the greater Heat, the greater Weight of the Atmosphere it is pressed by, we know not to what Height its Heat and dissolving Power may be raised at great Depths in the Earth.
Of the mineral acid Menstruums, Spirit of Sea-salt, Spirit of Nitre, and Agua regia, all afford green Solutions of Copper, but with this Difference, that the Spirit of Salt gives a yellowish Green; the Spirit of Nitre a deep Green, with no Yellowness at all; and the Agua regia, a bright vivid Green, but there is some Admixture of Yellow in it, about in the same Measure that it is in some of the Gems which ​Pliny describes by, Quorum extremus igniculus in flavedinem exeat. The Solution in Spirit of Nitre is of the true Emerald Colour, and extremely bright and vivid; and each of the others resembles very exactly the Colour of a particular Gem of the same Class; the first of them being perfectly of the Colour of the yellowish green Prasius, and the third of the Smaragdo-prasius.
These Colours are each of them very beautiful; and that of the Solution in Agua regia is no other than what must be expected, when we know the Colours of the other two, the Spirits of Salt and Nitre being simple Menstruums, and affording a green, and a yellowish green Solution; and the Agua regia, a compound Menstruum, partaking of the Nature of both the others, it must naturally give a Solution of a Colour between both, that is a Green with less Yellow than that of the Spirit of Salt.
But though these three acid Menstruums afford green Solutions of this Metal, it is too hasty a Conclusion to infer from thence, that all the Acid Menstruums will therefore do the same; for Solutions of Copper in Oil of Vitriol, Oil of Sulphur, and Agua fortis, are all blue. They are in different Degrees, tho' all nearly approaching to each other, and the deepest of them not darker than that of the common Turquoises. These Solutions have also this peculiar Property, that they immediately precipitate their Copper on Iron, if immersed in ​them; and may serve to explain the Effects of those vitriolic Waters which are said to convert Iron into Copper. A Piece of Iron Wire dipped into any of these Solutions, and taken almost immediately out again, is seen covered with Copper so far as the Menstruum has touched it; and by drawing the Fingers carefully over it, a fine thin Tube of pure Copper may be taken off from it: This may serve to shew us of what Kind the Menstruum is which Nature uses to produce the blue Vitriol from Copper, which in Solution has the same Effect; and proves that the Ziment or vitriolic Water, so famous for its supposed Virtue, of turning Iron into Copper, is no other than a blue Vitriol in a fluid State, because suspended in too large a Quantity of aqueous Matter; perhaps, indeed, containing Particles of many other Kinds, but evidently owing its characteristic Quality, to Particles of Copper, in a State very nearly resembling that of blue Vitriol, though at present in Solution.
That the natural Colour of Solutions of Copper in the vitriolic Acids is blue, is evident from only leaving a Drop of any of them on a Plate of Copper, which is presently covered with blue Crystals: And any one a little acquainted with Chemistry will know, that no Difference is to be expected in Solutions made with Oil of Sulphur from those with Oil of Vitriol; for these Acids differ scarce sensibly when both well rectified, and indeed appear, on strict Examination, to be really the same ​Thing; the same universal mineral Acid, exeistent every where in the Earth, and sometimes perceivable by the Senses, in the suffocating Damps of Mines, being the certain Basis of both; as also of a third, that of Alum: And though the different Matter it meets with in Alum, Vitriol, and Sulphur, gives it a different Appearance in the Concrete, yet when freed from that Matter by Chemistry, and rendered as pure as that Art will make it, it appears the same Thing, whether drawn from one or the other of these Substances.
That Oil of Vitriol, therefore, and Oil of Sulphur, should produce a Solution of Copper of the same Colour, is no other than what must naturally be expected: But that Agua fortis, which is a compound Menstruum, and made, though partly from Vitriol, which affords a blue Solution, yet partly also from Nitre, which we have seen before affords a fine green one, should give a simply blue Solution, as it evidently does, without the least Admixture of Green, may seem, at first View, something strange. But here I must observe, that Spirit of Nitre is the Menstruum I hinted at in the Beginning of this Letter, as capable of affording different Colours, from different Quantities of the Metal dissolved in it. And nothing, indeed, is more certain, than that the greenest Solution of Copper in Spirit of Nitre, may be turned into a pale Blue, only by adding more and more Filings of the same Metal, up to the proper Quantity for the Change.
​These, of all my Experiments on Copper, are what have afforded me the greatest Satisfaction in the Subject of the present Enquiry; as they shew, that Nature is so far from being tied to one single Menstruum for producing the Sapphirine Colour from Copper, that instead of the Colours of the blue Gems being owing only to the Effects of a single, scarce, indeed uncertain Menstruum on that Metal, we find they are producible from the Action of others, and those the most common, most abundant, and indeed, universal Menstruums of the fossile World. We need be no longer at a Loss to find where Nature could meet with a sufficient Quantity of a proper Menstruum to extract from Copper the Colour necessary for the various blue Gems, when we see, that the universal native fossile Acid, whether in Form of Vitriol, Sulphur, or Alum, and unquestionably not less when alone; and even the nitrous, under proper Limitations, are able abundantly to produce it.
Of the vegetable Acids, distilled Vinegar, Lemon-juice, and Spirit of Verdigrease, all give green Solutions of Copper; but with this Difference, that the first gives some faint Bluishness with the Green; the second is a pale whitish Green; and the third, the true, pure, and unmixed Green of the Emerald.
The fermented vegetable Acids, therefore, have more Effect on this Metal than the native; this is evident from the deeper Colour, and from the much greater Quantity of the Metal separable from Solutions with them, ​made in the same Proportions: And the Spirit of Verdigrease may very naturally excel both, as it is the strongest vegetable Acid that Art can any way produce; though it is truly no other than a Vinegar absorbed by Copper, and afterwards driven from it again by the Force of Fire; little altered, except as rendered more pure. It is remarkable, that Copper will thus part with this Acid in its proper and natural Form; whereas no other Metal will; for Iron and Lead, the only other Metals that will admit this Acid, alter it in the Mixture from its original Nature; for it can never be produced from them again in its natural State, but is in both Cases quite a different Thing: When separated from Lead, it appears in Form of an oily fat Liquor; and from Iron, little other than insipid Water. The Spirit of Verdigrease is, however, the strongeft of all vegetable Acids; and accordingly, extracts from Copper the Colour nearest approaching to that of the Solutions of that Metal in some of the strongeft mineral Acids.
Of the fixed Alkalies, Salt of Wormwood, Potashes, and Oil of Tartar per deliquium, all afford Solutions of Copper of a glorious, deep, celestial Blue, and no way distinguishable from one another, if the Solutions are made in exact Proportions. An Ærugo, of a greenish Colour, is indeed producible on Copper by these Menstruums; and a small Quantity of a similar Substance is sometimes found swimming on the Surface of these very Solutions: But this ​is not purely the genuine Effect of the Menstruums, but a Change wrought in the Solutions made by them, by Particles of adventitious Salts floating in the Air; and mixing with a small Quantity of them. These Changes of Colour in the Solutions of Copper from an Admixture of Salts of a different Kind, tho' but in small Quantities, we shall see hereafter in this Letter are very natural and easily producible Effects; and we need not wonder at a small Quantity of an Ærugo of this Kind floating on the Surface of the Menstruum, or affixed to a Plate of Copper, wetted with it, and exposed to the Air, tho' the true Solution of Copper in the Menstruum is blue; when we confider, that a Solution of the blue Vitriol in a Water impregnated with Sal Armoniac is green, notwithstanding that a simple Solution of Copper in that Salt is blue; (sfuch is the endless Variety resulting from Mixtures of Salts as Menstruums) and that the natural Ærugo produced on Copper by the Salts floating in the Air, is green.
It is not to be wondered at, that the Solutions of Copper in the fixed Alkalies produced from different vegetable Substances, are no way different from one another, since these Bodies act in these Solutions, not as the peculiar Salts of this or that Plant, but as a Body made, not by any Operation of Nature, but by the Effect of Fire; which has strongly united the essential Salt, the Earth, and some small Portion of the Oil of the Vegetable they have ​been prepared from: For all these fixed Alkalies of Plants may be resolved into a bitter saline Substance, a stronger fixed Alkali, and a pure simple Earth; and in the Operation there will a small Quantity of an oily Matter always be discovered.
Of the volatile Alkalies, Spirit of Sal Armoniac, Spirit of Urine, and Spirit of Hartshorn, all afford Solutions of Copper of the most beautiful and vivid celestial Blue: This is of different Degrees, according to the different Quantity of the Metal dissolved; but in equal Proportions, and with the Spirits of equal Strength, the Colour is exactly the same in them all. The volatile Alkalies have in their Operations on this Metal, therefore, a great Analogy to the fixed. These Menstruums consist only of a very fine, subtle, volatile, alkaline Salt, suspended in a small Quantity of Water, which has no Share in extracting this glorious Colour; for the dry volatile Salts of the same Substances, mixed with Copper Filings, and corked up in a Vial together, acquire, in a Day or two, the very same Colour.
Of the neutral Salts, a Solution of Copper with crude Sal Armoniac, is of a glorious Blue; with native Borax, of a fine deep Green; and with Sea-salt, of a pale whitish Green: Of these, the Sal Armoniac dissolves it the soonest, the Sea-salt takes more Time, and the Borax is slowest of all. The rest of the Solutions also mentioned here, require different Time and different Methods to produce them; the ​Spirit of Nitre dissolves the Metal almost instantaneously, Agua fortis is nearly as quick in its Operation, and Agua regia requires only a little Time: But of the others, some require long and tedious Processes, and others act best, or perhaps only, by Vapour; and one of these Processes shews, that where Mr. Boyle says, he knew a Menstruum which by its Vapour would dissolve a certain Metal, though it would scarce work on it at all in Substance; he is only talking of Copper and Vinegar. Sal Armoniac, it is to be also observed, affords us another Instance whence Nature may be supplied with a Menstruum for giving a blue Solution of Copper; since, tho' the Sal Armoniac common among us now is factitious, there is no Question but that there is, and ever has been, a true native Sal Armoniac; and there needs no more than Copper dissolved in Water impregnated with it, to give the different Blues of all the deepest Sapphires in the World; it being most easy to procure a Solution of Copper of any Degree of Blue, only from a Solution of this Salt in Water, digested for a few Days on Filings of that Metal.
Nature therefore is not tied to one Menstruum for the producing Blue from Copper; and that but a very scarce and uncertain one: Since it is evident, that the Bodies necessary to give it are many; and those, many of them, common and every where abundant. That the common and universal mineral Acid, so abundant every where in all the Kinds of Pyrites, ​the Acid of Sulphur, Vitriol, or Alum; which are, indeed, the same with the former, and with each other, in different Combinations, can do it: And even no better a Menstruum than common Water running over a Quantity of native Sal Armoniac, is able to produce from Copper, all the different Degrees of Blue, from that of the palest to that of deepest oriental Sapphires.
    I am,
       SIR,
          Your humble Servant,
John Hill.