not sensibly affected. We may thus suppose that | The ore is poured into the muffle by a funnel at f, arsenic is capable of being fused, if, instead of heating Fig. 61, and is then spread uniformly over the sole it in an open tube, it is heated in a stout glass tube, by means of a rake. In the course of 12 hours hermetically sealed: the increased pressure within the ore is decomposed, with the production of sulphate the tube ought to prevent the ebullition of the metal, of iron and arsenious acid. The latter, in the form and thus cause it to melt long before it boils. Pro- of a vapour or of a light powder, is conducted by the fessor Brande states that the experiment has been channel c, Fig. 61, into condensing-chambers, o 0, tried, and that the tension of the vapour burst the Fig. 63, where it gradually condenses, in following the tube before the arsenic showed signs of fusion. direction of the dotted line, while the heated air escapes by the flue. Every five or six weeks these

Vapour of arsenic is colourless; it has an odour of garlic, which is characteristic; the density of the vapour is about 10.37. The atomic weight of arsenic is 75, and its chemical symbol As.

Arsenic combines with the oxygen of the air at ordinary temperatures; its surface tarnishes, and it becomes covered with a blackish dust; but its metallic lustre is restored by leaving it for some hours in a solution of chlorine.

Arsenic is combustible: it burns with a livid flame, with the production of white arsenic. This substance is prepared on a large scale by decomposing, by means of heat, arsenical pyrites. In Silesia there are two mines of this ore, which supply the works at Reichenstein and Altonberg, where large quantities of metallic arsenic, arsenious acid, realgar, and orpiment, are manufactured. The ore is roasted in a muffle furnace, the sole of which m is somewhat inclined, as in Fig. 61.

chambers are cleared out, for which purpose, doors are opened at d, and the different chambers can be made to communicate by uncovering the openings at o. The purest acid is found in the lower chambers; that in the upper ones contains sulphur. The removal of the acid from these chambers is a dangerous occupation. The workmen wear a leathern dress, carefully fastened round every part of the body, and also over the head, which is further protected by a leathern helmet, furnished with glass eyelits. Under the helmet the mouth and nostrils are covered with a wet sponge or moist linen, for the purpose of filtering the air necessary for respiration.

The crude arsenious acid obtained by the above process is refined by a second sublimation, in iron pots covered with drums of sheet-iron, terminating in condensing chambers. A number of these pots are placed in a furnace, and raised to a full red heat, when the acid sublimes into the drums, and forms an internal crust 2 inches thick, the exterior of which is still impure and of a brown colour, and requires to be refined a third and even a fourth time. Flowers of arsenic are condensed in the chambers.

In the treatment of certain metallic arsenio-sulphurets, the principal object is to obtain the metal which is combined with the arsenic; in this case the ore is roasted on the hearth of a reverberatory furnace, and the sulphur, which is converted into sulphurous acid, is allowed to escape up the chimney; but the

(2) Dumas, Tome Quatrième. The same excellent authority states that the health of the workmen engaged in the arsenic works requires particular care. Two small glasses of olive oil are administered to each man daily. His food consists chiefly of

numerous openings for the flame. Fig. 62, is a transverse section of the furnace, showing the form of the muffle m; f is the fire, and a, the ash pit. leguminous vegetables eaten with plenty of butter, with very

little meat, but that which is eaten is very fat. Alcoholic drinks are especially to be avoided.

arsenic, which is converted into arsenious acid, is | other objects in Natural History. Dumas gives the condensed in conduits placed between the furnace following recipe :—

zontal plains subsequently elevated, and circum- | from end to end by a rectangular opening 150 feet scribed by the upheaving force of the primary rocks which now form their hill or mountain boundaries. These basins are arranged in layers or strata, some of which are of great thickness, and consist of loose and very permeable sand, and which in rising from the extremity of the basins project or crop out on the sides of hills and mountains; the rain-water filtering through these out-cropping strata, may form within them extended sheets of water; but when they are inclined, or dip at a high angle, this water rushes towards lower levels, carrying with it by degrees portions of sand, and even of the surrounding rocks, thus forming subterranean rivers which displace portions of the original massive strata, and excavating large caverns where previously there were none.

The tertiary formations are also stratified, or composed of a greater or less number of overlying beds, which, like the courses of a wall, are separated from one another by distinct and well-marked joinings. This formation, like the preceding, is usually basin-shaped, but commonly of far less extent. This shape appears to be due to some alteration in the position of the strata, during which the constituent elements of the tertiary series formed the ridges of the slopes and hills which surround them. During this change in the various strata, they were all more or less violently torn, broken, and detached, so that some are exposed and crop out on the sides and summits of hills.

About two-thirds of the habitable portion of the earth consist of stratified rocks, and geological inquiries obtain much of their precision from the circumstance that the various series of strata are always arranged in a given order. In the tertiary series beds of porous sand occur at various elevations, and the surface-water first penetrates those in which the inclination is great; it then, by its lateral pressure, finds its way into the horizontal branches; so that wherever a succession of sand beds occurs resting on and alternating with impervious strata, we may also expect to find as many subterranean sheets of

water.

There are many geological differences between the secondary and tertiary strata, but it will suffice for our present purpose to state, that in secondary formations all the phenomena are exhibited on a much grander scale on account of the prodigious thickness of the strata, of their less frequent alternations, and the greater velocity of the subterranean currents. Hence in the secondary formations natural springs are rare, but when they do occur they are very abundant, and form great chasms and caverns, as in the celebrated rock of Torghat in Norway, which is pierced

(1) These strata were originally deposited horizontally, but sometimes two and three deposits are found in preexisting basins bounded by older formations. These more recent beds extend horizontally until they come in contact with the older rocks, which alone receives directly the rain. The surface-water cannot reach the older strata except across the beds which cover it, conditions not very favourable to the formation of subterranean lakes compared with those which exist in basins the boundary ridge of which is composed of strata which have undergone the alterations noticed above.

enclose them as in a circus. The upper bed only is visible, and it

|

It

high, and upwards of 3,000 feet long. So, also, the cavern of Guacharo, in the valley of Caripe, in South America, has for its vestibule a vault 72 feet high by 80 feet wide, near the summit of a vast rock of secondary limestone, known as Jura limestone. For a length of 1,455 feet it has all the characteristics of the vestibule. The superstitious Indians would not allow Humboldt to advance further than 2,400 feet from the entrance, but he found along this extent a river 30 feet broad rolling along the floor of this magnificent cavern. In the cavern of Adelsburg in Carniola, the river Poick engulfs itself. appears and disappears many times, and has been traced underground through an extent of 6 miles, as far as a large lake. The Fountain of Vaucluse also issues from subterranean rocks, and pours forth a volume of 13,000 cubic feet per minute, even under ordinary circumstances, and this is sometimes increased to 40,000 cubic feet. But one of the most striking examples of these subterranean sheets of water of a varying level is that of the Lake Zirknitz in Carniola. This lake is about 6 miles long, by 3 broad. Towards the middle of summer, if the season be dry, its surface falls rapidly, and in a few weeks it is completely dry. The openings by which the waters retire beneath the soil may then be seen, in some places quite vertical, in others sloping towards the caverns of the surrounding mountains. Immediately after the retreat of the waters, all the extent of the surface which they covered is put under cultivation, and at the end of a couple of months the peasants are mowing hay, or reaping millet and rye, in the very spot where, some time before, they were fishing for tench and pike. Towards the end of autumn, and after the rains of that season, the waters return by the same natural channels which had opened a passage for them at the time of their departure. This is the regular and usual course; but sometimes a very heavy fall of rain on the mountains which surround Zirknitz occasions the return of the waters to the surface at an earlier period than was expected.

But it is not in hilly and mountainous districts alone that these abundant supplies of subterranean waters are to be observed; for even in flat countries there are caverns in which whole rivers are engulfed. Thus the Guadiana loses itself in a flat country, in the midst of an immense meadow. This explains the fact that, when speaking with admiration of some superb bridge in England or France, the Spaniards remark that they have one in Estremadura upon which 100,000 head of cattle can feed at the same moment. Thus the Drôme in Normandy is lost in the midst of a meadow, in a pit about 30 feet in diameter, knowr. as the Fosse de Soucy. In the same district of France other rivers are lost by degrees. There are from one point to another in the beds of these rivers great gaps, called bétoirs, each of which absorbs a portion of the stream. On its arrival at the last bétoir the stream is usually reduced to the size of a trifling rivulet.

There is often in these stratined formations distinct

This simple experiment is realized on a grand scale in the conduit-pipes which serve to distribute the water of elevated springs or reservoirs to the different parts of a town, or in the subterranean pipes destined to produce jets d'eau in gardens and public squares. When the Romans wished to conduct water from one hill to another, they constructed in the intermediate valleys immense aqueduct bridges, such as the Pont du Gard; and they did so, as we believe, not in ignorance of the hydrostatic principle of fluid levels, but in order to obtain a far more abundant supply than could be furnished by a pipe or series of pipes. The Turks were among the first to adopt the more economical plan, of carrying metal pipes, or a brick or stone tunnel, along the intermediate valley, making it follow the different inflections it might encounter, and finally causing it to ascend the slope of the second hill. The water flowing down this pipe rises, after crossing the

sheets of water at different depths. For example, in | scended from the level of the reservoir which conthe works which have been undertaken in search of stantly supplies the opposite branch. In practice, coal near St. Nicholas d'Aliermont, near Dieppe, the height of the jet will be somewhat less than the seven great sheets of water were passed through; level of the reservoir, on account of friction, the the first of which was at the depth of 76 feet; the resistance of the air, and opposing currents of ascendsecond at 307 feet; the third at 537 feet; the fourth ing and descending particles; but the slight deducat 645 feet; the fifth at 768 feet; the sixth at 880 tion which must on these accounts be made interferes feet; and the seventh at the depth of 1,030 feet. in no way with the principle of fluid level which Wells which have been sunk in the neighbourhood of we are now considering. London and elsewhere have illustrated the same fact. But, in addition to these stationary sheets of water in the heart of stratified rocks, water-courses have been found, true subterranean rivers, flowing rapidly in the empty spaces which exist among the impermeable rocks. For example, in boring near the barrier of Fontainebleau, at Paris, the progress had for some time been slow and tedious, as is usually the case in such works; but suddenly the boring tool escaped from the hands of the workman, who saw it fall rapidly upwards of 20 feet, and the transverse handle at the top of the first joint of the borer prevented it from falling any further. When the men endeavoured to raise the instrument, it appeared as if a strong current was carrying it on one side, and causing it to oscillate; but the rapid ascent of the waters of this deep stream prevented further observation. At Port St. Ouen five distinct sheets of water were observed. On penetrating the third sheet, the borer sank sud-valley, to nearly the same height as it had descended. denly more than a foot; and the current must have been strong, since it impressed the whole of the instrument with a very sensible oscillatory movement. When the end of the borer, filled with debris of the rocks, came, in drawing it up, to the spot where this third sheet of water was, it was not necessary to raise it further, for the current washed away the whole of the debris, an effect which could not have been produced had the water been stagnant.

It will be seen then, that on making an opening from the surface down to these subterranean waters, the water in many cases gushes up to the surface, and forms a perpetual fountain. Now, the question is, What is the power which causes these subterranean waters to rise to the surface and form a constant jet d'eau ?

Fig. 65.

If water be poured into a tube of the shape of the letter U, Fig. 65, it assumes a level, and maintains itself in the two branches a, b, at heights which are exactly equal. Suppose the left branch of this tube to open towards the top into a large reservoir, which can maintain itself always full; and that the right branch is cut across towards its lower part, so that only a portion of its vertical part remains, and that this portion is fitted with a stop-cock; when the stop-cock is opened, the water will be projected into the air to the same height to which it would have risen had this branch remained entire. In other words, it will ascend as far as it has de

U

Now suppose the tube to be carried only to the middle of the valley, and that a single opening on its upper side is made for its escape, the water will in such case be projected perpendicularly upwards, and this jet will rise higher in proportion as the descending current has a great fall. This is the construction of all jets d'eau. The form of the pipe in which the water is conducted is quite a matter of indifference. It may be circular or elliptical, square or polygonal, straight, and of great length, or having many windings and ramifications; and yet the water will equally rise to the height nearly of the level of the reservoir, whenever it has free course to obey the pressure to which it is subjected. Fig. 66 is a section of the reservoir or head of water at New York used for dis

tributing the water of the Croton Aqueduct over the city. (AQUEDUCT.) It will be seen that this reservoir is situated above the level of the houses, and the pipe commencing at its base, being conducted into one of the public squares of the city, forms a beauti ful jet d'eau, which rises nearly to the level of the water in the reservoir.

Some of the most remarkable ornamental jets d'eau are constructed on this principle. That at Versailles rises to the height of 90 feet; the old fountain at Chatsworth rose to the height of 94 feet; that of Peterhoff, in Russia, 120 feet; that at St. Cloud 160