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.