The vegetable alkali is rarely found in mineral waters; yet as it has been discovered in some granites we may expect to meet with it. The solitary instances in which it has yet occurred, scarcely however support the. expectation. The water at Doway in France is the only uncontroverted one which occurs to us; for Mon-net's discovery of it in the waters of Aix la Chapelle has not been supported by subsequent experiments. The volatile alkali, which we should with less reason expect to find, has been discovered by Mr. Cavendish in the waters of Rathbone place in London, and by M. Malouin in France. If the kali might occur from burnt woods, the ammonia would not be a less probable impregnation from decayed animal substances.

Lime is a very common ingredient, seldom however, if ever, uncombined. Aerated lime frequently occurs, and the air in these instances acts as a solvent, and is not combined with the earth. Bergman supposes, that about 100 cubic inches of water, saturated with its own bulk of fixed air at the temperature of 50°, will dissolve nearly 29 grains of aerated lime; and about 1504 parts of fully aerated water, dissolve about one of aerated lime. Mr. Cavendish however found that this proportion was too low, and that a quarter of a grain at least might be added (Phil. Transactions, for- 1767); and Berthollet has even advanced the quantity of lime dissolved in water to nearly 3 to 1500. This question is rather a chemical than a medical one, and we need not pursue it further.

Magnesia, as an ingredient in sea water, will of course appear a frequent impregnation of mineral waters. Magnesia, fully aerated, is soluble in 900 parts of water; but fully aerated water takes up a larger proportion of this earth at the period of its precipitation by a mild alkali. Of common magnesia about 1/300 is dissolved in aerated water, and by management a larger proportion. Even pure water, we are told, will dissolve 1/577 of its weight of the common earth. It is remarkable, that the impregnation of water with vitriolated kali, common salt, and Epsom salt, will render it a better menstruum of magnesia.

Argil is rather suspended than dissolved in water; but if Wcstrumb found nine grains in five quarts of Pyrmont water, it must have been really dissolved. We are not now speaking of aluminous waters, but of argillaceous ones, where the earth is suspended by fixed air: with its assistance, magnesia is very much more soluble than argil.

Aerated iron is very common, and 100 cubic inches of fixed air may dissolve four of iron. If in the waters of Medevi, analysed by Bergman in th,e first volume of his Opuscula, four grains of iron arc suspended with only six cubic inches of this air, we may suspect the intervention of some other solvent.

Of all the impregnations of mineral waters, the neutral salts are the most frequent: they occur in all their varieties, and in every possible combination. The selenite is a very common impregnation; and in Monro's system, compiled when chemistry had scarcely assumed a scientific form, it is called calcareous glauber. The Epsom and Glauber's salts are more rare, but frequent. Vitriolated ammonia, and vitriol of iron, are the production chiefly of volcanos. The latter occurs in the waters of Mulino near Latera (Lavoisier, Mem. de Paris, 1777); in those of Horsley Green (Garnet); and in Denmark (Bergman, vol. i. p.176). Vitriol of copper is contained only in waters issuing from the copper mines.

We have mentioned the acid of nitre seldom occurring in a combined state; but Margraaf found some traces of nitre in the wells of Berlin, and Bergman in the waters of Nidda, near Altendorf. Nitrat of lime and magnesia occasionally occur, but these springs are not medicinal.

Of the muriats, we find muriated natron, lime, and magnesia, in many mineral springs. Muriated ammonia has occurred in some of the Italian lakes, and in Siberia (Herman, vol. ii. p. 346). Muriated barytes has been found in water by Bergman and Schoeffer; muriated argil by Withering (Translation of Bergman's Sciagraphia, p. 31); and muriated manganese by Bergman; and, in our own country, by Mr. Lambe in the water of Lemington Priors.

The sulphureous waters owe their peculiar properties to the union of alkalis or lime; but Mr. Kirwan remarks that the sulphur is never in the form of hepars, since these are always coloured, or, when diluted, become so on standing; then ceasing to have any effect on nitrated silver, lead, or any other test. Bituminated alkali occurs only in one or two solitary, perhaps doubtful instances.

Petroleum was found floating on the waters of Dri-burg, and in some of those of Pyrmont, by Westrumb. Vegetable and animal extractive matters are accidental, and never medicinal impregnations.

The ferruginous waters we have already mentioned: the iron is held by the sulphuric and the carbonic acid, or mechanically suspended. Of the first we have already spoken: the carbonate of iron occurs chiefly in the acidulous waters, and the mechanical suspension occurs in the forge waters sometimes used as topical astringents, to be afterwards noticed. The medicinal are, in some measure, ascertained by the sensible qualities of waters. To the sight, pure water is clear, like crystal: when less so, some heterogeneous matters are certainly present. Water that has run through a gravelly soil is generally very pure: that which has passed through argillaceous or muddy land much less so. Though good water is without colour, it does not follow that every colourless water is good. An obscure muddiness verging to yellow or red is found in stagnant waters, and may arise from the extractive vegetable matter found in marshes, or from fatty substances. A blue colour points out copper; a green, vitriol of iron. The aerial acid, when uncombined, becomes obvious by the bubbles separated on standing, or on agitation. The purest water, however, when standing in the sun, will discharge numerous air bubbles.