to light in such regions, in which volcanic mountains are found near the primary.

In Sweden, with its prevalence of primitive rocks, not one considerable therma is found. At the northern declivity of the Alps, as in Baden, Würtemberg, Bavaria, comparatively few hot springs are found, the greater number being on the southern.

Formed out of melted granite or analogous stones, and violently raised, like lava, from their subterranean volcanic birthplace by vapours, basaltic masses burst the crusts of different kinds of mountains, and frequently attend active or extinguished volcanoes. The warm springs of Bertrich arise out of greywacke slate, interspersed with lava and basalt; the therma of Aix-la-Chapelle out of grey wacke. This connection of thermal character with basaltic eruptions is clearly shown in the thermæ of North Bohemia, of the Taunus and Eifel mountains (near the Rhine).

As regards the cause of the inherent heat, curious notions were formerly entertained. Some explained it from a peculiar process of fermentation in the interior of the earth, others imagined that the springs came in contact with caustic lime, by which heat was produced; according to others, through the action of water on sulphur pyrites. Some ascribed it to subterranean conflagration of strata of coals or peat. It is more rationally assumed, that as the temperature lowers in proportion to the rarefaction of the air in ascending upwards from the surface, the temperature must become higher the deeper the earth is entered and the more the air is condensed.

Others maintained that a peculiar heat constantly exists in the centre of the earth, independent of extraneous influences. That the temperature rises in proportion to depth without reference to geographical latitude, or to season, is an acknowledged fact. The frequent presence of cold springs in the neighbourhood of hot, demonstrating the deep origin of the latter, has been already pointed out.

Let us now consider the composition of the Therma. Alkalies prevail in combination with carbonic, hydrochloric, or sulphuric acid. The non-volcanic springs possess more earthy salts. Carbonate of soda is very frequently met with in volcanic mountain masses, as in klingstone, basalt, pearlstone, pumice, different kinds of lava, &c. Sometimes sublimated carbonate of soda is found in volcanic regions.

Sulphuric and hydrochloric acids are frequently found near volcanoes. The same holds good of carbonic acid, though it is

sometimes difficult to determine whether its evolution is due to the action of heat or of a stronger acid, by which it may be expelled.

Hydrochloric acid gas escapes from Etna. The surface of the lava ejected from Vesuvius in 1794 was covered after a few days with crystals of chloride of sodium and hydrochlorate of ammonia. Chloride of sodium is the prevailing constituent of volcanic productions; the next is sulphate of lime, then sulphate of potash, then hydrochlorate of lime and of potash. Moreover the quantity of nitrogen found in several therma seems to refer to their volcanic origin, this gas being often met with in gaseous volcanic evolutions.

Potash is frequently found in volcanic mountains, and also in granite, mica, and other kinds of primary rocks. In a similar manner, the presence of lithia, manganese, and iron, can be explained, as also the phosphates and fluates.

The Riesengebirge and the mountains of the county of Glatz, have only two therma; but numerous cold acidulous springs. The same is the case with the mountains of North Bohemia, which form, as it were, one chain extending as far as the Rhine, Taunus and Eifel mountains.

Most acidulous springs originate out of transition lime, variegated sandstone, clay slate, gneiss, greenstone, marl, and very frequently near basalt, and other volcanic products.

Their temperature is remarkable, from its stability being very slightly influenced by atmospheric changes, which shows them to possess a much deeper origin than common springs, and a great analogy to the characteristics of the thermæ.

In the Ragoczy spring of Kissingen, for instance, the temperature was 43° F., when the thermometer stood at the freezing point; when in hot summer, the temperature rose to 104° F., the spring had only 52° F. Thus the temperature of the spring only changed 9° to 72° of the atmosphere (from 32 to 104).

As in the thermæ, their ingredients are most intimately combined, carbonic acid and soda being the media of solution. Even the proportionate quantity of solid ingredients is analogous to the thermæ. It is inconsiderable, when they arise in primary mountains. In both, soda is mostly present, with carbonic, sulphuric, or hydrochloric acid. The quantity of free carbonic acid deserves particular attention.

Mighty evolutions of gas take place periodically in volcanic eruptions, or appear as results of continued volcanic processes in

the interior. In Italy, this not only happens in the celebrated Grotto del Cane, but also in other parts. Permanent evolutions of gas are also met with in the volcanic mountain ranges of the Rhine, especially of the Eifel, near Birresbronn; at Daun, near the shore of the lake of Laach, which furnishes by its environs unmistakable proofs of a former active volcano; at Kissingen and its environs; in the neighbourhood of Pyrmont and Driburg; and near Franzensbad and Marienbad. The gas mostly consists of carbonic acid gas, but sometimes contains sulphuretted hydrogen, and nitrogen mixed, as in Marienbad and Franzensbad: the temperature of the gas naturally corresponding with that of the spring: the quantity somewhat depends on the pressure and temperature of the air, on its movement or quietude, or its electric condition.

The circumstance that carbonic acid gas may become liquid by high pressure, induces the belief that the gas may have originally existed in a liquid state, but become transformed into gas by its issue into the external regions, under diminished pressure.

A very important consideration in the choice of a spa is, further, the variation of the climate. Geographical latitude often fixes the character of a watering-place, though not invariably so. The more easterly or westerly position must also be looked upon as exerting some influence. Thus you find, under nearly the same latitude, Wiesbaden, which is renowned for its temperate and mild climate, and Steben on the Saale (tributary of the Elbe), and Alexandersbad (on the Main), known for the inclemency of their climate.

The quality of the ground, the nature of the surrounding mountains, the cultivation of the soil, standing or flowing waters, all exercise their influence. In spas surrounded by marshy fields or stagnant waters, intermittent fever is sometimes found endemic. How this injurious influence may be removed by cultivation is shown in Franzensbad, where intermittent fever was formerly endemic. But now, the marshy meadows and ponds having been dried up, this disease is very seldom met with, even in very hot weather.

LECTURE III.

THE mysterious nature of these remarkable outpourings induced many persons to attribute accidental circumstances to their influence. Thus, for instance, the vapours arising out of the springs at Pfäfers were thought a preservative against the plague, because in 1611 and 1629, when the cantons of St. Gallen and Appenzell were devastated by the plague, all those that resided at the time in Pfäfers were spared. This, however, could not furnish a proof of the preserving power of the spa. The non-attack may have been due to the high and isolated situation of the place; being, perhaps, beyond the reach of atmospheric miasma. On the other hand, the salutary effect of sea air was well known and appreciated by the ancients. The cause is, no doubt, the diminished carbonic acid of the atmosphere, and its impregnation with saline exhalations. The same influence is also shown in the character of the surrounding vegetation. A favourable effect, similar to that of sea air, may be expected from the vicinity of salt and graduation works, for a considerable quantity of free hydrochloric acid has been found in the air near the salt-works of Halle. The same acid has also been detected in the rain water of Salzufeln (lat. 52° on the Weser).

If snow-covered mountain peaks are situated in the neighbourhood, the climate will be characterised by a certain bleakness, though the snow may be present only a part of the year.

This is, for instance, perceived in the spas at the foot of the Fichtelgebirge, Thüringer Forest, Erzgebirge and Black Forest.

The climate is further modified, according as the spa is situated on the southern declivity of the mountains, and thus protected against violent north and east winds, or on the northern declivity just exposed to them.

Look at Wiesbaden, being on the southern declivity of the Taunus, below the reach of north and east winds, and enjoying an extremely pleasant and mild climate. Now, behold Schwalbach, only a few leagues to the north, unfavourably known for

the bleakness and severity of its climate. What is the reason? It lies on the plateau of the mountain, and the same height that serves as protection to its fortunate neighbour exposes it to the full attack of the north and east winds.

In Barèges, the celebrated French spa of the Pyrenees, the climate is so severe, that it cannot be well inhabited more than six months of the year, whilst in its neighbourhood mineral wateringplaces enjoy the most lovely climate. A still greater difference exists between the places lying north and south of the St. Bernardino.

Though in the northern situation the climate is very severe, on the southern, luxuriant meadows are seen at a height of 5000'. At Misox (4900'), corn cultivation begins: at Soazzo (3000'), the vine is cultivated; and farther down fig and mulberry-trees flourish.

The circumstance whether the valley of the respective spas be narrow, encircled by steep and high cliffs, or broad and open, becomes of high importance; as, for instance, in diseases of the chest. The partial impediment to the entering rays of the sun causes the narrow valley to be comparatively colder and moister, and patients with weakened thoracic organs will feel oppressed by a long sojourn in it. Invalids of this description occasionally complain of a somewhat impeded respiration, even in the valley of the Tepl, at Carlsbad, or of the Lahn, at Ems. How oppressed and inconvenienced would these persons feel at Pfeffers, which, though it lies 2000' above the level of the sea, is so enclosed by high and steep rocky mountains, that the beneficent influence of the sun is not admitted more than seven hours in the longest days. Thus, supposing the present disease should strongly mark out Pfeffers as the most appropriate spa, you must not send a sufferer there, if his thoracic organs are at all liable to irritation, or disposed to derangement.

The influence of the greater or smaller elevation I have already treated of. I have said that the purity and tenuity of the atmosphere belonging to high situations, possesses tonic and stimulating properties, whilst that of lower situations, being denser and impregnated with more impurities, must produce corresponding depression of the vital functions, or vitiate the circulating fluids, and thus lay the seed of actual disease.

You will have observed some analogy between acidulous and hot springs, as regards their volcanic origin. But you may further notice, that heat and carbonic acid seem respectively to perform