There must needs be a very great quantity of iron in this water, when it yields as deep a colour with galls as a strong solution of sal Martis. I was indeed at first apprehensive, that this perhaps might not be owing so much to a large and uncommon proportion of chalybeat parts, as to the commixture of alum, which I judged to be in the water. But we see the contrary appears by these trials: for alum of itself affords no tincture with astringents, and, instead of rendering a solution of sal Martis with galls of a more intense colour, rather makes it lighter and more diluted.
We see here, that the ferrugineous matter is intirely separated from the water by an infusion of galls. The like also happens by elixation; after which it is almost deprived of its tinging quality. Yet other chalybeat waters lose this quality by much less degree of heat.
As there is an ochrous earth separated from all steel waters, when exposed to the air, which subsides to the bottom, and a metalline scum or cremor, which swim on their surface; we shall next consider the appearances, which they make in this water.
Exp. 8. A solution of saccharum Saturni being dropt into common spring-water, left the upper parts of the water clear and colourless, but formed a lactescency towards the bottom. The same solution being added to the mineral water, soon turned it of a turbid yellow colour, which afterwards subsided, and formed a deep yellow cloud in the bottom of the glass; and below this yellow sediment there adhered to the bottom of the glass a whitish substance, which I took to be the metalline parts of the saccharum Saturni separated from the purer parts of the salt, which were still suspended in the water, and which made it of a muddy whitish colour.
9. Forty drops of oleum tartari per deliquium being added to an ounce of the water, made it of an uniform light yellow colour; but in an hour afterwards there were many small yellow terrene nubeculæ formed in it. These the next day were more conspicuous, being thoroughly separated from the water, and precipitated to the bottom, leaving the water quite clear, as it was before the mixture. A small quantity of this limpid water being taken, it would afford no tincture with galls. It was then all poured off, except so much in the bottom of the glass as contained the above-mentioned clouds: to this there were some galls added, which in half an hour turned these clouds from a light yellow to a deep red colour, but did not change the colour of the water, in which they swam.
10. Immediately after the affusion of ol. tart. p.d. to the water, galls were added to the mixture, which tinged it of a deep and bright red colour. After standing for some time, there were red clouds precipitated to the bottom, and the water continued of a dusky opake red colour.
11. There is a small brook, formerly mentioned, which runs near by these springs; into which the water, that flows from them, is discharged. I observed the stones and channel of this brook all tinged with ochre of a deep yellow colour, so far up as the water of these springs flowed into it; but the channel, which the mineral water ran over before it was mixed with the water of the brook, was very little or nothing discoloured with ochre. As I conjectured what this was owing to, I afterwards took two equal quantities of the mineral water, into one of which I put an equal quantity of common water. In two hours the mixture became less transparent, and appeared yellowish, while the simple mineral water retained its clearness. Next day there was much ochre separated from the mixture, which subsided to the bottom of the glass: but the unmixed mineral water remained still clear and colourless, as at first.
All chalybeat waters separate their ochrous parts, when exposed some time to the air; but this separation is made sooner by the commixture of several kinds of salts. Thus we see the ochre in this water is immediately separated and precipitated by the solution of saccharum Saturni.
The oil of tartar causes a precipitation of these ferrugineous parts in the same manner. Which parts must be the sole cause, that the water receives a tincture from galls; since, after they are precipitate, it loses that quality, which they notwithstanding retain even after they are separated from the water. This precipitation of the ochrous parts of the water were the only visible effects that I could perceive to follow from the affusion of the ol. tart. p.d. I remember indeed, when I was at Moffat, I saw the manuscript of Dr. Horsburgh's experiments upon this mineral water; which appeared to be very accurate; and which I understand are since printed, in a volume lately published by the Philosophical Society at Edinburgh. Amongst these I observed one, which I thought so very remarkable, that I particularly adverted to it. It was the effects of the affusion of ol. tart. p.d. to the water, producing in it clouds, or a coagulation of a green or grass-green colour. I think these were the words; and I own I was something surprised at them. A solution of vitriolum Martis, mixed with this alkaline oil, does indeed produce a green coagulum: but I could scarcely think, that this, or any other chalybeat water, contained so large a proportion of that vitriol, as to be sufficient to produce these effects, when I considered, that so many writers, which I had seen, upon this subject, have all failed in their attempts of extracting a conspicuous martial vitriol from such mineral waters. I had tried this experiment upon four or five chalybeat springs in Scotland, and likewise upon the Spa and Pyrmont waters, which had been well preserved; but there never resulted any such effects from the mixture of these with oil of tartar, as are related in the above experiment. All the alteration it produced in these waters was the precipitation of an ochrous earth, but without the least appearance of any green colour. As I looked upon this as a leading experiment in the history of vitriolic waters; as I had often tried it, and as often seen the green coagulum produced with the solution of the factitious vitriol, and never could observe it produced in any of the above water; I began to suspect, that these waters were either not possessed of a vitriolic salt at all, or else, that it was in some respects very different from the factitious vitriol. For these reasons, Dr. Horsburgh's experiment appeared very extraordinary; tho' at the same time I was greatly pleased, that I should have the opportunity of repeating it, and of observing those phænomena in this ferrugineous water, which I had sought for in vain in several others. But when I came to make the trial, I was yet more surprised, when I found it misgive, and that the ol. tart. p.d. produced no green colour or coagulum in this mineral water, nor caused any other alteration in it, than the separation of a large quantity of ochrous earth of a yellow colour, exactly the same with what I had observed in the other steel waters. This failure made me immediately conclude, that I had somehow or other committed an error in the experiment: and tho' I was pretty sure, that the mineral water, which I had used in it, was quite fresh, yet I could not be so positive as to the oil of tartar, which I suspected to have been long kept. Yet that this could have been the cause of my being so unsuccessful, I could scarcely believe, tho' indeed I could assign no other. I was sorry, that I had not afterwards an opportunity of repeating this experiment with more accuracy, from which I might have expected to reap more success, as it is perhaps one of the most consequence, that can be performed on this mineral water, as it is capable of demonstrating the existence of a substantial vitriolum Martis in it; which is more than has been hitherto done, or perhaps ever will be done, concerning any one of the vast number of chalybeat waters, which have been yet discovered.
When galls are added to the water, at the same time with oil of tartar, instead of its deep blue colour, it affords only a red tincture.