was so low as 28°, the temperature of the ensuing night sunk to 30°; nor did the self-registering thermometer cease to approach the freezing point, until the absolute humidity of the atmosphere became permanently and considerably augmented. From the remarks which I have made on this important subject, a method of determining the average hygrometric state of the air, by means of the minimum temperature, readily suggests itself, it being evident, that places, where there is little difference between the greatest heat of the day and the extreme cold of the night, must be exposed to a moist atmosphere, while those situations which possess a wider range, with respect to the diurnal returns of temperature, must enjoy a corresponding degree of dryness, and probably, too, of salubrity. But it is to the influence of moisture, as the means of checking the diminution of temperature during the absence of the sun, the great source of light and heat to our globe, that I wish chiefly to draw the attention of meteorologists. After the sun has sunk below the horizon, the temperature of the air, which had been previously declining with his altitude, undergoes a more rapid diminution, and reaches its minimum state generally about midnight; after which it suffers little or no change till towards the hour of sunrise. From midnight the progress of the cold is most completely arrested, the transition of the vapour to the liquid state evolving a sufficient portion of heat to counteract the loss of temperature by radiation, and the other physical causes by which the cold of the night-air is induced. Before sunset, a considerable portion of moisture, it must be admitted, is discernible on the blades of grass, and other spicular substances on the ground; but the dew thus formed is rather a condensation of the moisture exhaling from the surface of the earth, by the heat it had acquired during the day, than a precipitation of vapour from the atmosphere, which it would seem does not begin to take place until the temperature of the air has reached the point corresponding to complete dampness. This is the reason why from midnight to the time when the heat of the sun begins to be sensibly felt next morning, the temperature of the night is nearly stationary ; and were it not for the constitution of things which I have described, the reduction of temperature would proceed to a much greater extent than is actually observed, seeing that the night-cold, even in the months of summer, when the season happens to be very dry, is always excessive, and frequently descend below the point of congelation, at the very time when the temperature of the day rises to 75°.

ART. L.-Analysis of the Water of the Rio Vinagre, in the Andes of Popayan, by M. Mariano de Rivero; with geognostic and Physical Illustrations of some Phenomena which are exhibited by Sulphur, Sulphuretted Hydrogen, and Water, in Volcanoes. By M. A. DE HUMBOLDT. [Phil. Mag.]

[Extract of a Letter, dated the 8th October 1823.]

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"In compliance with the desire of M. de Humboldt, I procured some of the water of Rio Vinagre. It was sent to me by M. Torrés, who takes an interest in all that can contribute to scientific researches. This water has yielded per litre sulphuric acid, 1.080; muriatic acid, 0.184; alumine, 0.240; lime, 0.160; and some indications of iron.* The presence of muriatic acid confirms the observations made on the vapours and the stony productions of Vesuvius and of several other volcanoes."

I had made known, at the time of my return from America, the presence of the sulphuric and muriatic acids in the water. of the Rio Vinagre, which the aborigines call Pusambio. (See Views of the Cordilleras, and Monuments of the People of America, vol. II. p. 166; Barometric Levelling of the Andes, No. 126; Caldas, Samanario del Nuevo Reyno de Granada, t. i. p. 265); but not being furnished with the salts of barytes, I had engaged MM. Rivero and Boussingault, when they departed from Bogota, to verify these facts. The analysis, which we owe to one of these expert chemists, is the first which has been attempted on the water of the Rio Vinagre. I shall give some extracts from the journal of my travels, in great part still unpublished, explanatory of the local circum

stances.

The town of Popayan is situated in the beautiful valley of the Rio Cauca, on the Bogota road to Quito, at the foot of the two great volcanoes of Puracé and Sotarà.

It cannot be doubted that the indications are by grammes and fractions of grammes; a litre of the water of the Rio Vinagre includes 1.080 grammes of sulphuric acid, and 0.184 grammes of muriatic acid. This proportion of sulphuric acid is nevertheless very sensible to the taste, and is manifest by an abundant precipitate with the salts of barytes.

[The litre being 2.113 pints, the contents of the water in English grains will be as follows: sulphuric acid 16.68; muriatic acid 2.84; alumine 3.7; lime 2.47. Edit.]

These volcanoes, almost extinct, and exhibiting only the phenomena of solfataras, form part of the central chain of the Andes of New Granada. At 1° 55′ and 2° 20′ of north latitude, the group of mountains which incloses the sources of the Magdalena is divided into three branches, of which the eastern is continued towards Timana and the Nevados of Chita and of Merida; the intermediate and central one towards the Paramos of Guanacas and of Quimdiù; the western towards the platiniferous district of the Choco and the isthmus of Panama. In ascending from the town of Popayan to the summit of the volcano of Puracé, M. Bonpland and I found at the height of 8672 feet a little plain (Llane del Corazon), inhabited by poor Indian husbandmen. This plain is separated from the rest of the acclivity, with which it would otherwise be continuous, by two ravines extremely deep: it is at the edge of these precipices that the village of Puracé is built. Springs rise every where from the trachytic rock; each garden is surrounded with a quickset hedge of narrow-leaved euphorbium (lechero) of the most delicate green. This beautiful verdure contrasts in a striking manner with the background of black and arid mountains, which surround the volcano, and which are rent by the effects of the earthquakes.

The site of the village is celebrated in the country on account of three beautiful cascades [choreras] of the river of Pusambio, whose water is acid, and which the people, who know no other acid than vinegar, call Rio Vinagre, sometimes Gran Vinagre. This river takes its rise at the height of nearly 10,871 feet, in a very inaccessible spot. Although the temperature of the water be little different in the lower cascades from that of the surrounding atmosphere, it is no less certain that the sources of the Pusambio or Venagre are very hot. This fact was attested to me by the natives and by the missionary of the village of Puracé. In going to the summit of the volcano I saw a column of smoke rise at the place where the acid waters make their appearance. I have drawn the second of the falls of the Vinagre (plate xxx of the Views of the Cordilleras): the water which opens itself a passage across a cavern, is precipitated more than 383 feet in depth. The fall has a very picturesque effect; but the inhabitants of Popayan would be better pleased if the river, instead of throwing itself into the Rio Cauca, became engulfed in some other crevice; for such is the delicacy of constitution of animals which breath by gills, and which absorb the oxygen dissolved

1

in the water, that the Cauca, during a course of four leagues, is destitute of fish, on account of the mixture of its waters with those of the Rio Vinagre,* which are charged both with oxide of iron and with sulphuric and muriatic acid. After staying a considerable time on the craggy wall of rock which borders the cascade, a pricking sensation is felt in the eyes from the minute spray in the atmosphere. Fish re-appear in the Rio Cauca at the point where it becomes enlarged by the influx of the Pindamon and of the Placé.t

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A little to the north of the sources of the Pusambio rise two other rivulets charged in like manner with free sulphuric acid, which the people call the Little Vinegars (los dos Vinagres chicos) they throw themselves into the Rio de San Francisco, which is itself but a tributary of the Gran Vinagre. During my stay at Popayan it was an opinion generally received that all these acid waters contained some iron dissolved by a great quantity of carbonic acid. When it was merely remembered that the sources of the Vinagre are very hot, this opinion ought to have been abandoned. I boiled some water taken from the cascade; and I found, after the ebullition, the same acid taste and the precipitates as in the unboiled water. At this period I had very few re-agents left.

The nitrate of silvert gave a white and milky precipitate, indicating the presence of muriates. The presence of iron was shown by the prussiate of lime, that of lime by the oxalate of potash. When the water was weighed with great care in the office of the mint of Popayan, the weight of an equal quantity of the water of the Vinagre was found to be to that of distilled water as 2735 gr. to 2731 gr.; that is to say, that the specific gravity of the water of the cascade was 1.0015.

The waters which I describe, and of which M. Rivero has given the first analysis, must not be confounded with those of the two subterraneous lakes which we have found near the summit of the volcano; one is 14,356 feet high, the other, above the snows, 15.475 feet. This volcano of Puracé is a dome

* M. Caldas has even attributed to this mixture, doubtless with little reason, the absence of goitres in the valley of Rio Cauca.-Semanario, t. 1. p. 265. See my Memoir on the Goitres in the Cordilleras. (Magendie, Jour. de Physiol. t. iv, p. 109 )

+ Journal de Physique, t. lxii, p. 61.

The conjoint presence of the sulphuric and muriatic acids has also been observed by M. Vauquelin in the water which M. Leschenault had taken from the crater-lake of Mount Idienne in Java (Journal de Physique, t. lxv. p. 406.) See Phil. Mag. vol. xlii. pp. 126, 182.

of semivitreous trachyte of a bluish grey, and having a conchoidal fracture. It does not present a great crater at its summit, but several little mouths. It differs very much from the neighbouring volcano, the Sotarà, which is of a conical form, and which has thrown out an immense quantity of obsidians. These masses, covering the plains of Julumito, are balls or tears of obsidian, the surface of which is often tubercular. They present, what I have seen no where else in the two hemispheres, all the shades of colour, from deep black to that of an artificial glass entirely colourless. It may appear surprising to see that this deprivation of colour has not been accompanied by any inflation or porosity. The obsidians of Sotarà are mixed with fragments of enamel which resemble the porcelain of Réaumur, and adhering to which I have found masses of felspar which have resisted fusion.

Here, as in the Andes of Quito, as at Mexico, and at the Canary Islands, the system of basaltic rocks lies far from the trachytes which form the volcano of Puracé and of Sotarà. The basalts of the Tetilla of Julumito belong only to the left bank of the Cauca. They rise from transition porphyries free from augite, containing some hornblende, a very little quartz in small crystals embedded in the mass, and a felspar which passes from the common to the vitreous variety. This porphyry is covered, near to Los Serillos, with a blackish-grey limestone, traversed by veins of carbonate of lime, and so much overcharged with carbon that in some parts it stains the fingers like an aluminous schist, or like the lydian* stone of Steeben in the Fichtelgebirge.

The trachytic dome of Puracé which gives birth to the little river of sulphuric acid, rises out of a porphyritic syenite (with common felspar), which in its turn is superimposed on transition granite abounding in mica. This observation,† very important for the position of volcanic rocks, may be made near to Santa Barbara in ascending from Popayan to the village of

*M. Vauquelin has recently proved by a direct analysis the presence of carbon in the purest lydian stones. I had found, in a series of experiments made on the galvanic exciters in 1798, that the lydian stones of the transition schists of Steeben produced jointly with zinc the same effect as graphite or carburet of iron. I have since made some trials to prove chemically the presence of carbon in several varieties of lydian stone. See my Experiments on the Nervous and Muscular Fibre (in German), t. ii. p. 163.

+ See an account of the whole of these phenomena of the volcanoes of Popayan in my Essai sur le Gisement des Roches, 1823, pp. 129, 139, 340.