which occasions no change, while caustic potash precipitates it abun dantly. By evaporation to dryness, the quantity of earthy salt contained in the water may be accurately ascertained.

Concerning artificial Mineral Waters.-The numerous processes we have' prescribed for examining the residues of mineral waters by evaporation, serve to ascertain, with the greatest precisiou, all the several matters held in solution in these fluids. Another process remains to be made to prove the success of the analysis, viz. That of imitating nature in the way of synthesis, by dissolving in pure water the different substances obtained by the analysis of mineral water which has been examined. If the artificial mineral water has the same taste, the same weight, and exhibits the same phenomena with re-agents as the natural mineral water, it is the most complete, and the most certain proof that the analysis has been well made. This artificial combination has likewise the advantage of being procured in all places at pleasure, and at a trifling expence; and is even in some cases superior to the natural mineral waters, for their whole properties may be changed by carriage and other circumstances. The most celebrated chemists are of opinion, that it is possible to imitate mineral waters. Macquer has observed, that since the discovery of the carbonic acid, and the property it is found to possess of rendering many substances soluble in water, it is much more easy to prepare artificial mineral waters. Bergman has described the method of composing waters which perfectly imitate that of Spa, Seltzer, Pyrmont, &c. He likewise informs us, that they are used with great success in Sweden, and that he himself has experienced their good effects. Duchanoy has published a work, in which he has given a number of processes for imitating all the mineral waters usually employed in medicine. We may therefore hope, that chemistry may render the most essential service to the art of healing, by affording valuable medicines, whose activity may be increased or diminished at pleasure.

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In order to present the reader, under one point of view, with the most conspicuous features in the composition of the mineral waters of this and some other countries, the following Synoptical Table is subjoined, from Dr. Saunders' work on mineral waters.

The reader will please to observe, that under the head of Neutral Purging Salts, are included the sulphats of soda and magnesia, and

the muriats of lime, soda, and magnesia. The power which the earthy muriats possess of acting on the intestinal canal, is not quite ascertained, but from their great solubility, and from analogy with salts, with similar component parts, we may conclude that this forms a principal part of their operation.

The reader will likewise observe, that where the spaces are left blank, it signifies that we are ignorant whether any of the substance at the head of the column is contained in the water; that the word none implies a certainty of the absence of that substance; and the term uncertain, means that the substance is contained, but that the quantity is not known.

* That is, 2.94 contained in the sulphat of iron, (this salt, when crystallized, containing 28 per cent, of oxyd of iron, according to Kirwan)

and 1.875 additional of oxyd of iron.

uncertain uncertain

uncertain 2.5

5. uncertain

4.75

none

none

0.5

SECTION IX.

Cataracts and Inundations.

1. Cataracts of the Nile.

THE bed and course of this river we have described in a previous chapter. Through its long and fertile range of about two thou. sand British miles, it has often to wind through abrupt and preci. pitous countries; and is not unfrequently strengthened by other rivers, as well as occasonal lakes and rapids. In consequence of which it often exhibits very considerable cataracts or water-falls. With respect to the number of these the different travellers are not agreed, some having placed in the catalogue, several of not more than seven or eight feet of perpendicular height, which others have rejected as too diminutive to deserve notice. We may safely affirm, however, that it contains not less than ten or twelve whose descent may be estimated at upwards of twenty feet before it reaches the level of Egypt. One of the chief occurs to the north-east of Morcho, where the Nile, having traversed a long and rugged chain of mountains, throws itself down to a considerable depth in a wide precipitous sheet, and forms the cataract of Jan Adel at Janadil, constituting the seventh fall in its regular course. But that which, by way of eminence, is called the Cataract of the Nile, occurs in a different country and a very different part of the river, at the village of Atalata, near Dara. It constitutes the eighth cataract, and is thus described by Mr. Bruce:

Our horses were immediately fed; bread, honey, and butter served: Ali had no occasion to cry, Drink; it went about plenti. fully, and I would stay no longer, but mounted my horse, thinking every minute that I tarried might be better spent at the cataract. The first thing they carried us to was the bridge, which consists of one arch, of about twenty-five feet broad, the extremities of which were strongly let into, and rested on, the solid rock on both sides; bút fragments of the parapets remained, and the bridge itself seemed to bear the appearance of frequent repairs, and many attempts to ruin it; otherwise, in its construction, it was exceedingly.commo. dious. The Nile here is confined between two rocks, and runs in a deep trough, with great roaring and impetuous velocity. We were told no crocodiles were ever seen so high, and were obliged to remount the stream above half a mile before we came to the cataract,

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through trees and bushes of the same beautiful and delightful ap pearance with those we had seen near Dara.

The cataract itself was the most magnificent sight that ever I beheld. The height has been rather exaggerated. The missionaries say, the fall is about sixteen ells, or fifty feet. The measuring is, indeed, very difficult; but, by the position of long sticks, and poles of different lengths, at different heights of the rock, from the water's edge, I may venture to say, that it is nearer forty feet than any other measure. The river had been considerably increased by rains, and fell in one sheet of water, without any interval, above half an English mile in breadth, with a force and noise that was truly terrible, and which stunned me, for a time, perfectly dizzy. A thick fnme, or haze, covered the fall all round, and hung over the course of the stream both above and below, marking its track, though the water was not seen. The river, though swelled with rain, pre. served its natural clearness, and fell, as far as I discern, into a deep pool, or bason, in the solid rock, which was full, and in twenty different eddies to the very foot of the precipice; the stream, when it fell, seeming part of it to run back with great fury upon the rock, as well as forward in the line of its course, raising a wave, or violent ebullition, by chaffing against each other.

Jerome Lobo pretends, that he has sat under the curve, or arch, made by the projectile force of the water rushing over the precipice. He says he sat calmly at the foot of it, and looking through the curve of the stream, as it was falling, saw a number of rainbows of inconceivable beauty in this extraordinary prism. This, however, I, without hesitation, aver to be a downright falsehood. A deep pool of water, as I mentioned, reaches to the very foot of the rock, and is in perpetual agitation. Now, allowing that there was a seat, or bench, which there is not, in the middle of the pool, I do believe it absolutely impossible, by any exertion of human strength, to have arrived at it. Although a very robust man, in the prime and vigour of life, and a hardy, practised, indefatigable swimmer, I am perfectly confident I could not have got to that seat from the shore through the quietest part of that bason. And, supposing the friar placed in his imaginary seat, under the curve of that immense arch of water, he must have bad a portion of firmness, more than falls to the share of ordinary men, and which is not likely to be acquired in a monastic life, to philosophise upon optics in such a situation, where every thing would seem, to his dazzled eyes, to be in motion, and