ful study of tracts of country once convulsed by volcanic action.

In the descriptions, and more especially in the examination, of volcanic regions, there is much, apart from geology, that cannot fail to gratify the curiosity, and awaken the deepest interest in the minds of us all;-as when we descend into the streets and enter the deserted edifices of once populous and flourishing cities, that have for centuries been buried beneath the consolidated ashes, or streams of compact lava. With what feelings do we learn that here the very inmates of the houses were discovered, the husband, and wife with the infant in her arms, the master and slave, the prints of the soldiers' feet in the stocks, and the remains of groups that fled for safety to the cellars! What are our emotions, when we read the writings scribbled on the walls by the loungers of the guard room, or trace the baker's name stamped upon the loaf, more than a thousand years ago, and follow the deep ruts in the pavement! when we are told of the figure with the uplifted axe petrified, as it were, in the very moment of forcing a passage for all he held dear on earth; of the miser with his keys in his shrivelled hand, hastening to secure his treasure, and arrested on the threshold by the suffocating vapors! Effects and traces like these are calculated, we think, to awaken a general interest, not inferior to that of any of the modern discoveries in fossil geology.

Nor is this interest limited to the circumstances or discoveries we have just mentioned, to those of the works of art, or of other objects that are from time to time brought to light from beneath the present surface. The surface itself is not wanting in interest. Nowhere do we find a scene that offers so many objects to the senses, the memory, and the imagination, as a true volcanic region. The inhabitant of more favored climes, whose repose is never broken by these tremendous convulsions of nature, when he visits a volcanic country, while he inhales the grateful and balmy atmosphere, with which such regions are so often blessed, and luxuriates beneath the orange and the lemon trees, bending under their golden fruit, or plucks the delicious grape from the clustering vines above his head, looks around with feelings that cannot be described. He sees the smiling faces and happy people, the playful children and the cheerful grandsire, at their evening repast beneath the aged chestnut, or in merry dance and song upon the verdant turf that decks the spots that but a few years ago were the very centres of volcanic eruptions, and with fearful anticipation of a similar catastrophe, is lost in amazement

at all this indifference; -he reads their fate upon every stone in his path, and hears a warning voice from every hillock and naked rock.

The traces of volcanic action are not confined to those parts of the earth's surface, which are covered with the usually well known lavas, or where volcanic fires can still be discerned; they are perceived by the eye of the geologist in every part of its crust, at the greatest elevations, and in the lowest depths. To volcanic action he now looks for the solution of many difficulties, which were not long since incapable of being removed by any of the prevailing theories of the earth. In the infancy of geological science, the present physiognomy of a country, the order and arrangement of the materials composing its crust, were, it is well known, accounted for in the most crude and fanciful manner. At one time fire was the great agent, at another water; the collision of comets, a change in the axis of the globe itself, and various other causes, were assumed as all sufficient. While one was satisfied with the examination of the present order of rocks and strata, another looked only to the petrifactions contained in them, and each conceived that he had solved the great problem. But as observations multiplied, and the light of other sciences was reflected over the chaos of geological speculation, it could not but be perceived that the great revolutions of the globe must be referred to no single operation, or secondary cause. Geology began to free itself from assumptions and conjectures, and has since been rapidly advancing to the rank of an experimental science. It seems hardly necessary, at this day, to allude to the aid which it has derived from the higher branches of physical science, from the discoveries in botany, zoology and chemistry; or to the vast number of observations of all kinds that have, within the past ten years, been made not only at the surface of the earth, but at great depths below. What had been "a sealed book" to those who have gone before us, has been laid open, and the symbols and hieroglyphics on its pages have become legible. The effects of heat, which had been so important an agent to all theorists, have been experimentally determined; the temperature both of the surface and of the interior of the earth has been carefully observed, the physical and mathematical difficulties of the old theories have been investigated, the all dissolving power of water has evaporated; and we have come to look at heat as the great agent in bringing about the present order and arrangement of mountains and strata, and to water as modifying and smoothing down the

rugged physiognomy of the globe. We have still, however, to seek for the cause or source of subterranean heat; nor can we as yet say that the original heat, which the earth had at its formation, is not still preserved in its central parts, or that it is not developed by the galvanic actions which may be established among the dissimilar materials of its strata. From all the observations that have been of late made in deep mines and wells, it appears to be pretty well established, that as we descend from the surface, the temperature increases about one degree of Fahrenheit for every hundred feet. From this it may be calculated that, providing the increase of temperature be progressive, we should have a temperature, at about three miles, equal to that of boiling water.

Facts of this kind have been applied to the explanation of THERMAL waters, or the high temperature of warm and hot natural springs, the frequent occurrence of which is not the least interesting of the phenomena connected with the geology of volcanoes. It is not uncommon to meet with hot springs even under the surface of streams of cold water; and we have ourselves enjoyed the luxury of a cold bath in a stream, beneath which, the temperature of spots at the bottom was too high to be borne. Thermal waters are, however, not confined to districts in which volcanoes are now active; they are met with in almost every country that has been examined in the polar regions, as well as in the temperate zone, and under the equator. Nor are they peculiar to any particular rock; they occur in granite as at Baden and Carlsbad, in gneiss as at Landeck, in limestone as at Aix, in clay slate as at Barège, in the coal formation as at Vichy, in sandstone as at Bath, and in trachyte as at Mont D'Or. Their origin has given rise to many theories, and much ingenious and often wild speculation. Some, with Klaproth, have found a cause for their high temperature in the combustion of beds of coal; and others, with Becher, in the decomposition of iron pyrites. The most satisfactory theory at present, seems to us to be that which looks to chemical action-the oxidation of the bases of the alkalies, earths, and metallic oxides, constituting the crust of the globe, by the agency of air and water. This last is the ingenious theory adopted and advocated by Dr. Daubeny, in his work on volcanoes, published in 1826; and which he has since that time had many opportunities of bringing to the test of experiment, in his subsequent visits to many of the most remarkable and instructive localities of such phenomena.

The theory that regards the temperature of thermal springs, as arising merely from the internal heat of the globe, has been sanctioned by the high authority of La Place, and is that which has been adopted by Professor Bischoff of Bonn, a translation of whose prize essay on the temperature of the interior of the earth, and of springs, is now publishing in the Edinburgh Philosophical Journal. This essay contains much experimental and other evidence in support of the theory, and a mass of valuable and important details, of which our limits will not permit us to attempt an abstract, but which deserves the careful study of geologists. Not less valuable and important is the memoir of Dr. Daubeny, on mineral and thermal waters, which we have already mentioned. In this, memoir, the author has presented us with all that is known of value on this interesting department of physical geography, and has replied to the objections which have been urged against the theory he adopted in his former work, and which he has applied, as we have just said, to the explanation of the origin of thermal waters. To the memoir is appended a tabular view of the geographical position of all the most important hot springs of Europe, of their chemical constituents, temperature, &c.

Dr. Daubeny's memoir is not confined to an account of those waters to which the term "mineral" is usually limited, but embraces every description of water-in the state of vapor as in the atmosphere, the water of the ocean, and that which is distributed over the surface of the earth. Atmospheric water is first noticed as being the purest presented to us by nature, but even this is found to contain various foreign substances; thus, there are said to have been detected in it small quantities of several metals, especially iron and nickel, ammonia, and a peculiar organic matter to which the name pyrrhine has been given. This last substance was first detected in a red shower of rain at Giessen, in 1821. "The water that contained it was of a peach red color, and flakes of a hyacinthine tinge floated on its surface." This latter was the substance designated by the name pyrrhine. The same matter has since been detected in other water, and Dr. Witting of Hoxter, on the Weser, has found in the air collected on the Hartz mountains, the same principle. He has also found that the atmosphere of a place contains generally the same foreign substances which are brought down by the first fall of rain, as muriates in minute proportions, muriatic and carbonic acids, and carburetted hydrogen gas. Phosphoric acid was detected in rain which fell during a north

west wind, and in that which fell during particular states of the weather. Snow, hail, and sleet, contained a large quantity of an organic coloring matter, and in dew were traces of nitric and muriatic acid. Hoar frost was found to exhibit no signs of foreign matters. From these and similar observations, the existence of organized matter in atmospheric water seems to be the fact best established. The philosopher to whose microscopical observations we have before alluded, ascribes this to a particular class of Infusoria, (the Polygastrica) "which being raised by currents and by evaporation, fill the atmosphere, and thus produce the pyrrhine." Dr. Daubeny does not consider the existence of metallic bodies in the atmosphere as well established; and in support of his opinion adduces the experiments of Dr. Faraday, which go to prove that every substance has a certain fixed point, below which none of its particles pass into the state of vapor; and we think with him that there is great force in the objection to the existence of such bodies, in the fact that the point of temperature at which metals are volatilized, is greatly above the highest which the atmosphere ever attains. But then the question arises, whether such bodies may not owe their presence in the atmosphere to their affinity for others: some experiments recently made seem to show that this may be the case. Thus, it has been stated that electrical light is capable of taking up incandescent metallic bodies, and that lightning is known to deposite sulphur and iron in the state of oxide. From this Fusineri has explained the smell which accompanies thunder, and the deposition of pulverulent matter at the fractures occasioned by lightning. Observations and experiments on this highly curious and important subject are richly deserving of attention, as connected with the origin of those masses which occasionally fall from the atmosphere, known as meteoric stones, We cannot quit this subject without directing the attention of our scientific readers to the theory which has lately been advocated by Von Hoff, which is, that meteoric masses are not originally solid fragments, but bodies "which, at the instant of the occurrence of the meteoric phenomena of the light and the explosion, are, by the agency of a great physico-chemical process, newly formed from incoherent, and probably gaseous materials, and by the same cause solidified, and which descend to the earth's surface when this, still to us obscure process, takes place within the sphere of attraction of our globe.

• Becquerel Traitè d' electricité, Vol. 3.

Poggendorff's Annalen XXXVI.