As these waters are so long in separating their mineral contents, they appear particularly well adapted for being transported to distant places: for by this quality they are fitted to undergo a long carriage, and to be kept a considerable time, without any diminution of their medicinal virtues. It must also be noticed, that the water of the under well is by much the best of the two for carriage, or for being long kept, as it is longer in separating its mineral contents than the upper one.
From these experiments it is evident, that there is a considerable difference betwixt the waters of the two springs. The upper one contains a much larger quantity of ochrous earth, and metalline cremor, than the under one; which is the reason, why it yields a deeper colour with galls, as may be observed in the first experiment. I suspected, on the other hand, that the under water contained a greater proportion of alum, than the water of the upper spring; but this I cannot affirm, as I find I had neglected to make the experiment, which would have determined whether it be so or not. Tho' the mineral contents of these two waters be similar, yet, if they be thus mixed in them in different proportions, this must certainly create a difference between them, which deserves to be attended to, as it may be sufficient to disallow of their being used promiscuously, since their medicinal effects may be thereby different.
But now, to sum up the evidence, which these experiments, taken all together, do afford, concerning the mineral ingredients of this Spaw; I think they determine, with some degree of certainty, that it contains two different principles of iron, both of which are fixed. The one, which is the ochrous earth, is a true minera ferri, and, altho' it be a crude mineral, exists in the water in a very fine and subtile form; the other, which is the cremor or pellicle, whose parts are also extremely attenuated in the water, appears to be iron, not in its mineral, but in its metalline form, and, when thrown up upon the surface of the water, shews itself like an extreme thin lamina of that metal. There seems also to be some small proportion of sulphur joined with the metalline cremor. The other mineral ingredient, which enters into the composition of this Spaw, is a considerable proportion of an aluminous salt, which is conjoined with a small quantity of a light brown-coloured earth (probably a part of the matrix whence the salt is formed), and still more intimately connected with some of the chalybeat parts of the water, which are not separable from it either by elixation or evaporation. Whether these be saline or terrestrial, I cannot determine.
Having thus endeavoured to discover, by some plain and simple experiments, the mineral principles, with which this medicinal water is impregnated; I shall now only add some observations, with respect to the origin of steel waters, and particularly of this Spaw, whose origin, I think, is thereby discovered and ascertained in a very obvious manner.
Among several things, that are still deficient in the history of mineral waters; an exact knowlege of their origin seems to be the chief; that is, from what fossils, and in what manner, these waters do acquire the mineral substances, with which they are impregnated. As this happens in the bowels of the earth, and is therefore far removed from our view, it is not surprising, that there has been so little discovered concerning it; tho' indeed there have been many elaborate hypotheses framed in order to account for it.
The writers on mineral waters have been of very different and opposite opinions concerning their origin. They have disagreed widely amongst themselves; and I very much suspect, that the accounts, which most of them give of this matter, are not agreeable to truth: particularly with respect to chalybeat waters, I have seen none, who have given a satisfactory account of their origin. They have all agreed, that iron, or the vitriol of that metal, does exist in mineral waters; but they have never yet agreed, how they came to exist in them, or in what manner mineral waters come to be imbued with these fossils.
Some of the more ancient writers cannot comprehend, how simple water should be intimately impregnated with so many different kinds of minerals, except by the means of some powerful agent. And as they thought nothing more proper for communicating and mixing mineral substances with water, than violent heat, they therefore termed all mineral waters, of whatever kind, by the name of thermæ. They saw some spring from the earth extremely hot, others moderately hot, others tepid, others excessively cold: they concluded from this, that all such various degrees of heat in these waters were owing either to the different degree of subterranean fire, which they had undergone; or else to the great distance, which some of them had run in the earth, after they had been sufficiently heated. They therefore maintained, that those waters particularly termed acidulæ (the greatest part of which are impregnated with iron), or those, which, tho' intensely cold, contained a large proportion of mineral matter, had in some part of the earth been impregnated with it, by means of an intense heat, which they had been gradually deprived of by a long passage thro' the colder parts of the earth.
Some naturalists again, of a later date, having exploded the former notion as chimerical, have thought, that a vapour rising from vitriolic minerals, and mixed with the neighbouring streams of water in the bowels of the earth, has imbued them with some of the parts, and with the properties, of vitriol.
Others are of opinion, that the exhalations of vitriolic minerals, passing thro' the cavities of the earth, are there condensed by the subterraneous cold into a limpid fluid, containing the very finest parts of that mineral salt: which fluid, mixing with the præterlabent streams of water, and issuing out of the earth with them, produce those mineral springs called vitriolic.
The last opinion I shall mention on this subject, and which indeed appears the most plausible, is of those, who think, that the iron is corroded and dissolved in these waters by means of an acid: for, as they imagine simple water incapable of doing this, they suppose, that it is first imbued with an acid in the bowels of the earth; and then, by the corrosion of the chalybeat minerals, thro' which it runs, it comes to be impregnated with them. I once received this opinion, as the most probable I could then observe, concerning the origin of these springs: yet not as being satisfactory; for there are many objections against it, which it is difficult either to elude or to answer.