Puracé. The volcano, like the most part of the great volcanoes of the Andes, presents layers or mantles of melted stony matter, not real currents of lava. Some fragments of granular limestone, probably magnesian, which I found at more than 12,790 feet high, seem to have been thrown up through crevices which have since become closed. They are like those of the Fosso Grande of Vesuvius, which owe their granular texture to volcanic fire. It is not possible to go on horse back further than the cascades of the Rio Vinagre. From thence we were eight hours in mounting on foot to the summit of the volcano and in descending from it. The weather was dreadful; snow and hail fell. I had a great deal of difficulty in lighting the tinder the point of the conductor of Volta's electrometer; the balls of elder pith separated from 5 to 6 lines, and the electricity passed often from positive to negative without there being any other symptom of storm': for thunder and lightning are (according to my experience) generally very rare when we are above 12,800 or 14,000 feet high. The hail was white ;* the hailstones, from five to seven lines in diameter, composed of layers varying in translucency. They were not only much flattened towards the poles, but so much increased in their equatorial diameter, that rings of ice separated themselves on the least shock. I had already twice observed and described this phenomenon, in the mountains of Bareuth, and near Cracow, during a journey in Poland. Can it be admitted that the successive layers which are added to the central nucleus are in a state of fluidity so great that the rotary motion can cause the flattening of the spheroids?

When the barometer indicated that we were come very near the limit of perpetual snow, we found the masses of sulphur disseminated in imperfectly columnar trachytic rocks augmented. This phenomenon struck me the more, as 1 knew how rare sulphur is on the sides of inflamed volcanoes: a column of yellowish smoke and a frightful noise informed us of the neighbourhood of one of the mouths (bocas) of the volcano. We had some trouble to approach its edge; the declivity of the mountain being very steep, and the crevices only covered by a crust of sulphur, of whose thickness we

I have already remarked elsewhere in the Ann. de Chimie, that at Paramo de Guanacas, where the road from Bogota to Popayan passes to the height of 14,700 feet, there has been seen fall, no snow, but red hail. Did it inclose those same germs of vegetable organization which have been discovered above the polar circle?

were ignorant. We believed we might rate the extent of this crust, which is often interrupted by rocks, at more than 12,000 square feet. These little ridges of trachytic rocks act strongly on the magnet. I tried to keep at as much distance from them as possible to determine the inclination of the needle. It was at the town of Popayan (height 5,825 English feet) 23.05, centesimal division; at the village of Puracé (height 8671 feet) 21°.81; near the summit of the volcano of Puracé (height 14,542 feet) 20°.85. The intensity of the magnetic force varied very little at Popayan and at the village of Puracé; and the diminution of the inclination is certainly not the effect. of the height, as is proved by so many other observations which I have made on the summit of the Andes, but the effect of local attractions depending on certain centres of action in the trachytes. The mouth of the volcano of Puracé is a perpendicular cleft, the visible opening of which is only 6 feet long and 3 broad. It is covered in form of a vault by a layer of very pure sulphur, which is 18 inches thick, and which the force of the elastic vapours has split on the north side. At the distance of 12 feet from the mouth we felt an agreeable heat. The centigrade thermometer, which had kept till then at 6°.2 (43° F.) (a cold not at all considerable in a time of hail, and at a height of 14,356 feet), rose to 15° (61° F). Placed in such a manner as not to be incommoded by the vapours, we had the pleasure of drying our clothes. The frightful noise which is heard near this opening has almost always the same intensity: it can only be compared to that which would be caused by several stearn-engines together, were the dense steam suffered to escape from all at the same moment. We threw great stones into the crevice, and we discovered on this occasion that the opening communicated with a basin full of boiling water. The vapours which escape with so much violence are of the sulphurous acid, which is indicated by their suffocating smell. We shall soon see that the water of the subterraneous lake is charged with sulphuretted hydrogen; but the odour of this gas is not smelt at the summit of the volcano, because it is disguised by the much stronger smell of the sulphurous acid vapours. I had not any means of determining the temperature of these vapours, which seem to undergo a prodigiously strong pressure in the interior of the volcano. The Indians pretend that the opening has several compartments which are not all filled with water, and that the noise which is heard at times in the interior of the

crevice is the forerunner of flames. I ́introduced by means of a long pole, some papers coloured with the tincture of violets, under the vault, where I could be sure of not touching the surface of the water. Drawing back the pole I found the papers strongly reddened, but not at all inflamed, as was easy to be foreseen.

We succeeded after several vain attempts in obtaining some water from the crevice: this was by tying a tutuma (the fruit of the Cresentia Cujete) to a stick 8 feet long. The water was directly poured into a bottle and hermetically stopped. We examined it on our return to the village of Puracé; it exhaled a strong smell of sulphuretted hydrogen; it had no acid taste, but some weak precipitates caused by the nitrate of silver showed the presence of muriatic acid. The crust of sulphur which forms above the mouth arises without doubt from the contact of vapours of sulphurous acid with the sulphuretted hydrogen which the subterraneous lake disengages. Even the water of this lake is covered with a coat of sulphur which disappeared in the places where we threw the stones. It results from these observations, that only the presence of the muriatic acid, or of combinations of this acid with salifiable bases, indicates a feeble analogy between the waters of Rio Vinagre and those of the lakes. The first, which spring much lower, at the declivity of the volcano of Puracé, are charged with free sulphuric acid; the others, which are found at the summit of the volcano, contain sulphuretted hydrogen. As the upper mouths are found at very different heights above the level of the sea, it may be supposed that their subterraneous waters are owing to the melting of the snows, and that they do not communicate. The Rio Vinagre receives its acid in the interior of a volcano which abounds in sulphur, and the temperature of which appears extremely elevated, although for centuries no luminous phenomenon has been perceived at its

summit.

[To be continued.]

ART. LI.-Biography of Professor Playfair. [New Edin. Encyclopædia.]

JOHN PLAYFAIR was the eldest son of the Rev. James Playfair, minister of the united parishes of Liff and Benvie, in the

county of Forfar. He was born at Benvie, on the 10th of March, 1748; and after receiving a classical education under the roof of his father, he entered the university of St Andrews at the age of fourteen. At this ancient seat of learning Mr Playfair soon distinguished himself by the excellence of his conduct, as well as the ardour of his application; and so great was his progress in natural philosophy, that Prof. Wilkie, (the author of the Epigoniad) who taught that branch of knowledge in the university, selected him to teach his class during his indisposition.

In the year 1766, upon the death of Mr Stewart, Professor of Mathematics in Marischal College, Aberdeen, seven candidates appeared for the vacant chair. Among these were the Rev. Dr Trail, Dr Hamilton, and Mr Playfair. The professorship was a private foundation, by Dr Liddel, and a clause in the deed of foundation was considered as a direction to fill up the vacancy by a disputation or comparative trial. Dr Reid of Glasgow, Mr Vilant of St Andrews, Dr Skene, of Marischal College, and Professor Gordon of King's College, accepted the office of judges on this occasion; and after a severe examination, which lasted for a fortnight, Dr Trail was appointed to the chair. Mr Playfair was excelled only by two, out of six candidates, viz. Dr Trail and Dr Hamilton, who now fills the same chair; but when it is considered, that Mr Playfair was two years younger than Dr Trail, the result of the election must have been greatly affected by that circumstance alone; and Dr Trail himself has modestly remarked, in a letter to the writer of this article, that he has always attributed his own success to this disparity of years.

In the year 1772, when the chair of natural philosophy became vacant by the death of Dr Wilkie, Mr Playfair again cherished the hopes of a permanent appointment; but his expectations were a second time frustrated; a disappointment which was the more severe, as the death of his father in the same year had devolved upon him the charge of his mother and her family. This circumstance appears to have determined Mr Playfair to follow the profession of his father, to which he had been educated, but which his ardent attachment to mathematical pursuits had induced him to think of abandoning. Having been presented by Lord Gray to his father's living of Liff and Benvie, of which, however, he did not obtain possession till August, 1773, in consequence of a dispute respecting the right of patronage, Mr Playfair devoted his

time to the duties of his sacred office, to the education of his younger brothers, and to the occasional prosecution of his own favourite studies. In 1774 he went to Schehallien, where Dr Maskelyne was carrying on his interesting experiments on the attraction of the mountains; and while he was enjoying the acquaintance of that eminent astronomer, he was little aware that he should himself contribute, at some distant period, to the perfection of the result which it was the object of this experiment to obtain.

In the year 1777, Mr Playfair communicated to the Royal Society of London an essay On the Arithmetic of Impossible Quantities, which appeared in the Transactions for that year, and which was the first display of his mathematical acquirements. In this ingenious paper, which is strongly marked with the peculiar talent of its author, Mr Playfair has pointed out the insufficiency of the explanation of the doctrine of negative quantities given by John Bernouilli and Maclaurin, viz. that the imaginary characters which are involved in the expression compensate or destroy each other; and he has endeavoured to show that the arithmetic of impossible quantities is nothing more than a particular method of tracing the affinity of the measures of ratios and of angles, and that they can never be of any use as an instrument of discovery, unless when the subject of investigation is a property common to the measures both of ratios and of angles.

The late Mr Ferguson of Raith having held out to Mr Playfair a very advantageous offer to superintend the education of his two eld st sons, the present Robert Ferguson, Esq. of Raith, and Sir Ronald Ferguson, he was induced, in 1782, to resign his living for this purpose, and we believe he never afterwards exercised any of the duties of the clerical office.

In the year 1785, when Dr Adam Ferguson exchanged the chair of Moral Philosophy for that of Mathematics, which was then filled by Mr Dugald Stewart, Mr Playfair was appointed joint Professor of Mathematics, a situation which had been the highest object of his ambition, and which he was in a peculiar manner qualified to fill.

As Mr Playfair was a member of the Philosophical Society of Edinburgh, he became one of the original fellows of the Royal Society at its institution by royal charter in 1783, and, by his services as an office-bearer, as well as by his communications as a member, he contributed most essentially to promote the interests, and to add to the renown, of this distin