into a test tube, containing a few drops of a solution of the perchloride of mercury. The arseniuretted hydrogen produces first a yellow precipitate, which, after passing through various shades of brown, at last becomes black, and decomposes the perchloride gradually into calomel*. This test is so nice, that 33, even Tobo, of a grain of metallic arsenic dissolved (either in the form of arsenious acid or of arseniate of potash, though the latter requires a longer period for its decomposition) in 100 parts of a diluted muriatic acid, was still indicated, although the greater part of the arsenic was precipitated as a black powder. This effect is not prevented if the arsenical liquid contains, at the same time, much organic matter in solution, even though the latter surpass by 2000 times the weight of the arsenic present. Nitric and nitromuriatic acid prevent the disengaging of arseniuretted hydrogen, almost entirely, even if of a grain of arsenic be present. When a copper wire is made use of, arsenic with metallic lustre is precipitated on that part of the wire which is above the surface of the liquid; and if the surface of the wire is augmented by being bent into spiral windings, the gas thus disengaged, precipitates the mercurial salt less copiously. The arsenic is not carried up mechanically, for a disc of thin pasteboard sealed to the wire, at some little distance from the surface of the liquid, did not impede the effects. The arsenic adhering to that part of the wire which dips into the liquid, appears in the form of a black powder. With platinum wire I could not observe this phenomenon. In a similar galvanic circuit, the interior of the tube being filled with diluted muriatic acid, the exterior one with a solution of muriate of ammonia or soda, the copper wire becomes tarnished, and that part of it which is in the liquid, is covered after some time with a film of a black substance. This change, however, cannot be at all confounded with the appearance which the wire assumes from the of arsenic; nor is the solution of the perchloride of mercury changed in the least, as was to be anticipated by the hydrogen gas.

presence

The phenomena attending this decomposition will vary in some measure if an excess of the arseniuretted hydrogen should happen to be present. Berzelius has examined them, and he states the perchloride of mercury to be such a delicate test for the arseniuretted hydrogen as to indicate even Toto of it in a gaseous mixture.

I have a few words to add, touching the nature of the galvanic circle above described. It appears to me, that its effects are as much depending upon the electromotive power of the liquids, as on that of the metals: for if both cylinders are filled with a solution of chloride of sodium or muriate of ammonia, no hydrogen gas is evolved at the negative metal*; and the briskness with which the gas is disengaged, increases with the greater difference in the electro-chemical nature of the liquids employed. Thus, the interior of the cylinder being filled with diluted muriatic acid, hydrogen gas will be more rapidly disengaged, if the interior one be filled with a solution of caustic alkali, in preference to a neutral salt. If, on the contrary, the electro-negative metal be in contact with the electro-positive liquid, scarcely any evolution of gas is visible— and the less, the more the liquids differ in their electro-chemical relation.

On the Limits of the mutual Reaction of Iodide of Potassium and Chloride of Platinum.

Iodide of Potassium, with an additional quantity of diluted sulphuric acid, renders a solution of the perchloride of platinum, containing Too of the chloride, at the first moment, brownred, the liquid becomes afterwards dark green, and a black precipitate is ultimately produced. If the solution of the chloride of platinum is still more diluted, it turns red by the reagent, and it is still very distinctly so when the chloride is 1,000,000 of the weight of the liquid. The limit where iodide of potash is still indicated by perchloride of platinum, and some additional sulphuric acid, is at 100 of the iodide. Starch, with an addition of fuming nitric acid, indicates the iodide very distinctly, if it amounts to 10 of the solution, its colouring being limited at 500,000 of the salt present. The reaction was not impeded when the solution contained at the same time a quantity of chloride of potassium, equal to 10,000 times that of the iodide.

*If both metals are at the same time in contact with a diluted acid, hydrogen gas is evolved at the negative wire; but in this case, the instantaneous formation of a metallic salt on the surface of the metal must be taken into account. In a paper published in Poggendorff's Annalen der Physik, 1825, I have treated more at large on the electromotive power of the binary combinations and their solutions.

Nitrate of silver and chloride of gold are very readily reduced by the protoxide of iron, just precipitated from its solution. Thus, a diluted solution of nitrate of silver in ammonia indicates the crystallized protosulphate of iron, if only amounting to Tooooo of the liquid.

SIR,

German Spa, Brighton, June 1st, 1828.

Letter from Dr. Nicholas Mill.

Bogota, Colombia, November 23, 1827. I TAKE the liberty to forward you, for insertion in the 'Quarterly Journal,' an account of the earthquake which has just desolated this city, and also of a new mineral substance consisting of sulphuric acid and alumine. This mineral salt is unknown, I believe, except as an artificial production; I shall, therefore, take leave to call it Davite, in honour of Sir Humphry Davy. I also send you an analysis of a salt spring in the Andes, containing iodine in the shape of hydriodide of potassium. I have also discovered a new vegetable alkali in the Quina Blanca of Mutis, (Cinchona Ovalifolia, Cinchona Macrocarpa of Vahl,) which I call Blanquinine, to distinguish it from others, and to convey an idea from whence it proceeds. I am now engaged in examining its salts, the results of which I shall forward you upon another occasion.

Earthquake in Colombia, November 16, 1827.

On the 16th of November, 1827, at a quarter past six o'clock in the evening, the inhabitants of Bogota in Colombia were thrown into the greatest consternation and alarm by the severest shock of an earthquake which has ever been known to visit that city.

At the moment of its occurrence, a subterraneous noise was very distinctly heard, resembling the noise of a carriage passing briskly over the pavement, and a white thin transparent cloud was seen to hang over the city; this cloud has been noticed in Italy, as generally, if not always, present near the volcanic commotions of that country, previously, and at the time of these commotions. This cloud is entirely unlike any other which I have ever noticed, and resembles a thin gauze veil.

I noticed it not only upon this occasion, but also in the earthquake of June 17th, 1826, in this city*.

The earthquake took a direction from S. E. to N. W., in which it could plainly be traced by the havoc which it made. Its effects on the city were partial in the above direction, but every part was convulsed.

The confusion and affliction which such a calamity occasions, particularly in a catholic country, can neither be imagined nor described. I was sitting reading in a small house of one story above the ground-floor, when the trembling commenced; the table on which my book lay, first shook, and almost at the same instant, the chair on which I sat; I immediately got on my legs, but found much difficulty in sustaining myself without holding by some fixture; the house all this time rocking to and fro as in a hurricane, but not a breath of air stirred. After passing ten or more seconds in this way, I collected my reason sufficiently to run down the steps into the street; all this time the earth was in motion. When I arrived at the portal of the door, I found it impossible to stand without holding very tight by the doorway, and many persons fell on their faces. During these moments, part of the house adjoining mine fell with a terrible cree, and the street was filled with a cloud of dust, out of which emerged a man distorted with horror, but who had almost miraculously escaped immolation, without any other hurt than what his fright had occasioned. After continuing a minute or more, the trembling ceased, and nothing could now be heard but the cries of the people; with that exception all was still and silent, and the stars appeared with all their brilliancy, as if smiling at this scene of human distress. Some persons asserted, that there were two distinct shocks, but I must confess I felt the earth in motion during the whole period of a minute or more; and being situated over the direction which the earthquake took, was, therefore, better able to judge of this, than others who were more distant, and particularly as I retained my presence of mind. Fortunately

*If I may be allowed to offer a conjecture on the cause of this singular white veil, or cloud, I can only attribute it to the vapour of water which escapes from the earth from the heated mass below, and which is condensed on rising into the cold air, aud thus rendered visible. Bogota, according to my measurement, which corresponds very nearly with that of Baron Humboldt, is 9600 feet above the level of the sea, and is distant at least one hundred miles from any known volcano.

for me my house was well built, for had it fallen, I should inevitably have been buried in the ruins. To describe the scene which ensued is difficult; the streets were filled with despair; some entirely and others half naked were seen on their knees imploring divine protection with all the energy which the catholic religion empowered them, each hailing his favourite saint to stretch forth his hand and help them, with the enthusiasm of madmen; no one knew what to do or where to fly, for all were in the same consternation and distress. After this had a little subsided, the city became soon deserted, and a fresh scene presented itself; all those who had horses, were seen scampering through the streets towards the plain, to elude the terror of another shock, others on foot with their beds on their backs; and the sick, wrapped up in blankets, were conveyed in armchairs, with two sticks passed underneath them to form sedan chairs, and some were conveyed in hammocks. This afflicting sight, accompanied by the cries of the distressed and the melancholy chaunt of their progress, was painful in the extreme; and hard, indeed, must be that heart, who could view it with indifference; yet such was the apathy occasioned by terror, that scarcely any one offered assistance to his neighbour, and frequently neglected his own safety. When all was quiet, I went out to examine the city. The first thing which attracted my notice was the turret of the stately cathedral partly demolished, and the building split and cracked in various places; the precious stones, consisting of diamonds, emeralds, and topazes, which adorned the interior, were scattered in all directions, and many of them broken, particularly a very large emerald weighing some ounces. edifice had but just been repaired from the effects of the earthquake in the preceding year, and was, by this last, reduced to a tattered ruin. In all the streets which ran in the direction of N. W. and S. E., many houses were "levelled with the dust," and others "rent in twain," and some of the unfortunate inhabitants buried beneath their ruins. In all, fourteen persons have lost their lives; and the damage done to the city is estimated to be at least six millions of dollars, although it did not contain a larger population than 30,000 souls. Deserted streets, heaps of ruins, and tottering houses, threatening to APRIL-JULY, 1828.

This