« ΠροηγούμενηΣυνέχεια »
ON PHYSICS, OR NATURAL PHILOSOPHY.
[Continued from page 572.)
Electro-Dynamical Attraction and RepuUion.—We owe our knowledge of this interesting subject almost entirely to the ingenious and fertile investigations of M. Ampere and Mr. Faraday. It constitutes by far the most important portion of electro-dynamics, by reason of the fact, thit it is the bond of connexion between electricity and magnetism, and the foundation of the mathematical theory which embraces and accounts for all the known phenomena of electro-magnetism. These phenomena, like those of ordinary electricity, are so numerous, and susceptible of such infinitely varied modifications and combinations, that a theory is not only useful, but necessary. Indeed, we may here remark in general, that when the facts which constitute any branch of physical science become so numerous that it is difficult, if not impossible, to remember them independently, a theory, by serving to connect them, is useful as an artificial means of memory. And when that theory is capable, not only of generalising known facts, but also of leading to the discovery of new truths, by suggesting experiments and truths of exact reasoning, it becomes an important auxiliary to experimental philosophy. We should not, however, fall into the error of mistaking the scaffolding for the edifice to be erected; and, still less, treat with contempt all such artificial aids, as is the habit of some, who not unfrequently confound together the known and unknown, and speak of the electrical and magnetic; fluids as realities, instead of as mere hypotheses to connect isolated facts.
The first discovery in electro-dynamics which served to draw particular attention to the subject was made in 1819, by Professor (Ersted of Copenhagen, who found that a magnetic needle acted upon by a galvanic current assumes a position at right angles to the direction of the current. Other new and important facts were promptly discovered by Ampere, Anigo, Sir Humphry Davy, Faraday, and others; and all the observed phenomena were satisfactorily referred by Ampere to the laws which govern the mutual action of electrical currents, by means of the ingenious hypothesis, that magnetism consist* in electrical currents revolving round the molecules of a magnet, in planes perpendicular to its axis.
The laws of electro-dynamical attraction and repulsion, experimentally established by M. Ampere, and which serve to explain all known phenomena, may be succinctly stated in a few general propositions.
Prop. 1. Parallel currents flowing in the same direction attract each other; and they mutually repel when their directions are opposite.
Prop. 2. Two currents attract each other when they both flow towards or from a point—or the line of shortest distance between them—if they are not in the same plane; and they repel each other, if one approaches and the other recedes from it.
Prop. 3. The forces vary in intensity in the inverse ratio of the square of the distance. And the action of a ourrent flowing in a given direction is, in all coses, equal and contrary to that which it exerts when it passes in the opposite direction, the current acted upon and the distance remaining constant.
Prop. 4. The attraction or repulsion exerted by a current passing through a tortuous conductor, however numerous its flexures, is exactly equal to that which is produced by the same current when it flows in a straight line between the same points.
To deduce from Ampere's theory the phenomena of magnetic attraction and repulsion, it becomes necessary first to ascertain the direction, which the above laws of electro-dynamical action require us to assume to be that of currents supposed to revolve round the axes of the magnets. And it has been found that to render the theory accordant with observed facts, the motion of the hypothetical currents must be contrary to the apparent daily motion of the sun, or to that of the hands of a watch, when the north pole of the magnet is elevated.
All the effects produced by the mutual action of electrical currents and magnets flow from this theory, as consequences
of the above principles of electro-dynamical attraction and repulsion Thus the fact discovered by (Ersted, that a magnet assumes a polar position transverse to that of an electrical current, or tangential to circles described about the latter as an axis, is a simple instance of the tendency of currents to assume parallel and similar directions.
(Ersted's Experiment.—The discovery made by (Ersted, is the directive action exerted by a fixed current at a distance upon a moveable magnetised needle. To make this experiment, stretch a copper wire horizontally in the direction of the magnetic meridian above a moveable magnetised needle, as fig. 432 represents. As long as the wire is not traversed
by the current, the needle remains parallel, but directly the ends of the wire are put in communication with the electrodes of a pile or battery, the needle it deflected, and-becomes more and more nearly perpendicular to the current as the latter increases in intensity.
With regard to the direction in which the poles are deflected, there are several cases, which may be reduced to a single principle. Before doing this, we will recall the convention already stated, with regard to the cause of the current, that it U conceived as always going from the positive to the negative poie along the wire. Bearing this in mind, the preceding experiment presents the four following cases.
1. If the current passes above the needle and goes from south to north, the south pole is deflected towards the west. It is this position that is represented in the figure. 2. If the current passes below the needle still from south to north, the south pole is deflected to the east. 3. When the current passes above the needle in the direction from the north to the south, the south pole turns towards the east. 4. Lastly, the deflection is towards the west when the current goes from north to south below the neeedle.
If we conceive, as Ampere did, an observer placed in the wire in such a way that the current, entering at his feet, goes out at his head, and his face is constantly turned towards the needle, we shall easily see that in the four cases just mentioned the south pole is deflected towards the left of the observer. The current being thus personified, as it were, we may include the above four cases in the enunciation of the following general principle.
In the directive action of currents upon maynels, the south pole it constantly deflected towards the left of the current.
It was in the month of July, 1850, that CErsted made known his celebrated discovery, and in the succeeding September, M. Arago communicated to the Academy of Sciences at Paris the second electro-magnetic discovery, that a galvanic current possesses the power of rendering iron or steel magnetic.
It therefore appears, that the power of electrical currents to induce other currents and develop magnetism, required for the verification of Ampere's theory, is found by experiment to be a physical fact.
The established principles of electro-dynamical attraction and repulsion, combined with Ampere's theory, not only furnish a ready explanation of every electro-magnetic machine which has been or may be invented, but also enable any one to foresee the result of new combinations, and therefore to contrive without difficulty an unlimited number of arrangements for the purposes of illustration. When the nature and laws of physical forces are once determined, it is the office of mere mechanical ingenuity to apply those forces; but little credit, therefore, can be any loriger claimed for the invention of an electro-dynamical toy.
Galvanometer, Eltrtro-Mapnetie Multiplier.—It is evident that, if a number of similar currents act in like manner upon a given current or magnet, the sum of the forces which they exert ■will be directly proportional to their number. It also appears from the four principles stated above, that any two currents flowing in opposite directions should act alike upon a magnet placed between them. Upon these principles the eleetra-magnetie multiplier, or galvanometer, of Schweigger, a German philosopher, is constructed. It is an extremely delicate apparatus, serving to show the existence, direction and intensity of currents. It was invented by Schweigger shortly after the discovery of CErsted.
To understand the principle of it, let U9 consider the case of a magnetised needle suspended by a silk untwisted thread, fig. ■133, and surrounded, in «iit? ulane of the magnetic meridian, with
a copper wire forming a complete circuit round the needle in the direction of its length. When a current passes along this wire, it follows, from the convention which we have already stated, that in every part of the circuit an observer lying along the wire in the direction of the arrows, and looking at the needle a b, would have his left turned towards the same point of the horizon, and, consequently, that the action of the current everywhere tends to deflect the needle in the same direction. In other words, the actions of the four branches of the circuit contribute to deflect the south pole in the same directions therefore, by winding the wire round the needle as the figure represents, we have multiplied the action of the current. If, instead of a single circuit, we have several, the action is multiplied still further, and the deflection of the needle is increased still more. However, we should not multiply the action cf the current indefinitely by continuing the circumvolutions 01 the wire, for we shall soon see that the intensity of a current becomes feeble when the length of the circuit through which it passes increases.
As the directive influence of the earth tends incessantly to keep the needle in the magnetic meridian, and thus opposes the action of the current, the effect of the latter may be rendered more perceptible by making use of a system of two astatic needles, as seen in fig. 434. The action of the earth upon the needles is then very feeble, and besides the actions of the current on the two needles are added together. In fact, the action of the complete circuit tends, according to the direction of the current marked by the arrows, to deflect the south pole of the inner needle a b towards the west. With regard to the upper needle a' b', it is subject to the action of two contrary currents m n and p g, but the former being nearer, its action prevails over the other. Now this current passing below the needle from the south to the north pole, evidently tends to deflect the pole a' towards the east, and consequently the pole V towards the west, that is to say, in the same direction as the pole a of the other needle.
These principles having been premised, it is easy to explain the theory of the multiplier. This apparatus, represented in fig. 435, is composed of a cylindrical copper case, round which is rolled a copper wire covered with silk along its whole length, to isolate the circuits from one another. Above this case is a horizontal dial-plate graduated, the zero point of which corresponds to the diameter parallel to the direction of the copper wire upon the dial. This dial has two graduations, the one on the right of the zero point, and the other on the left, but only as far as 90 degrees. By means of a support and an extremely fine silk fibre, an astatic system is suspended, consisting of two sewing needles a b and A, the former above the dial, and the latter within the circuit. These needles, which are
connected together by means of a copper wire, like those in fig. 371, p. 457, and cannot be deflected separately, or one without the other, cannot have exactly the same magnetic intensity unless every current, whether strong or feeble, renders them always transverse to itself.
The bent branches K and H which are connected, below the apparatus, with the two ends of the circuit, are intended to receive the conductors which transmit the current that we wish to observe. The foot screws c, serve to place the apparatus in a perfectly vertical position, so that the suspended thread corresponds exactly to the centre of the dial. Lastly, a button E transmits the motion to the plate n and the dial—both of which are moveable about a vertical axis, in such a manner as to bring the wires of the circuit in the direction of the mag
netic meridian without displacing the apparatus. The galvanometer represented in our illustration is the work of M. Billant, an ingenious manufacturer of such articles.
When the galvanometer is intended for observing currents arising from chemical action, the wire of the circuit, as we shall soon see, ought to be of a small diameter, and to make from four hundred to six hundred revolutions, or even more. On the contrary, for thermo-electrical currents, of which we shall speak on a future occasion, the wire ought to be of large diameter, and to make a much smaller number of revolutions.
Thus constructed, the galvanometer gives no indication of any current when we pass the electricity of an electrical machine along the wire, by connecting one of the ends with the conductors and the other with the earth. We can only render the current which then passes along in the apparatus perceptible, by making use of a very fine wire coiled round two or three thousand times, and by completely isolating the circuits from one another by means of silk, varnish and gum lac. If these conditions are observed, the needles will be deflected by the electricity of an electrical machine, which shows the identity of statical and dynamical electricity.
Graduation of the Galvanometer.—The galvanometer which we have just described is an extremely delicate apparatus for establishing the fact of the presence of currents, but it does not make known their intensity. To make it serve this purpose, it is necessary to construct tables, by means of which the intensity of the current may be deduced from the deflection of the needle.
The simplest method of forming these tables is that of the double wire multiplier. Coil two copper wires at the same time round the apparatus, each being covered with silk, and both of the same length and diameter. Then selecting a constant, but very feeble, source of dynamical electricity, pass the current along one of the wires, thus causing a certain deflection, say of five degrees. Then by means of an electric source identical with the above, pass a current of the same intensity along both wires at the same time, thus obtaining a deflection of ten degrees, owing to the simultaneous action of the two currents, or—which is the same thing—to a current of twice the intensity of the first. If you then pass along one of the wires a current capable of producing by itself a deflection of ten degrees, and along the other one of the currents which gave a deflection of five degrees—which is evidently the same thing as tripling the first current—you will obtain a deflection of fifteen degrees. Lastly, by passing along each of the wires at the same time a current capable of producing a deflection of ten degrees, you may get a deflection of twenty degrees. In other words, up to twenty degrees the deflections increase in proportion to the intensity of the current. Beyond that point they increase less rapidly; but by the same process you may gradually determine from point to point the deflections which correspond to known intensities, and complete the table by the method of interpolations. Each galvanometer requires a table of its own, because the relation between the intensity of the current and the deflection of the needle varies with the degree
in which the needle is magnetised, its length, its distance from the current, and, lastly, with the extent of the circuit.
The double wire multiplier may also serve to establish the difference of the intensities of two currents. This is effected by passing a current along each wire at the same time, but in contrary directions. The apparatus then bears the name of the differential galvanometer.
Uses of the the Galvanometer.—The galvanometer, on account of its extreme sensibility, is one of the most valuable philosophical instruments. It serves not only to establish the fact of the presence of the feeblest currents, but to make known their direction and intensity. It was by means of this apparatus that M. Becquerel was able to prove that a disengagement of electricity accompanies all chemical combinations, and to determine the laws which regulate these combinations.
For example, if you fasten two platinum wires to the extremities of the circuit of the galvanometer, and plunge them into a small vessel containing nitric acid, you discover no deflection of the needle, as might have been easily foreseen, because the platinum is not acted upon by nitric acid. But if you pour a drop of hydro-chloric, chloro-hydric, or muriatic acid, near one of the submerged wires, the needle of the galvanometer is immediately deflected, which proves that there is a current in the circuit. In fact, we know that nitric and chloro-hydric acid by their mutual reaction produce chloronitric acid, which acts upon platinum. We gather also from the direction of the deflection, that the platinum is electrised negatively and the acid positively.
LESSONS IN GEOLOG Y.-No. LVIIL
By Thos. W. Jenkyn, D.D., F.U.G.3., F.G.S., Etc
ON THE CLASSIFICATION OF ROCKS
ON THE COAL FORMATION.
61. THE LITHOLOGICAL CHARACTER OF THE COAL ROCKS
The rocks which embrace in one group mineral coal, iron ore and limestone, are the most important in the whole world. By opening up sources of industry and wealth, and by calling forth the energy and skill of a people, the carboniferous rocks tend to confer upon a country far greater power than it could derive from veins of silver or valleys of gold dust. Coal, iron and limestone have made England what it is.
The whole series of rocks which contain sand rocks, clay shales, seams of coal, beds of grits and bands of limestone, take their name from the coal only, and are therefore called the carboniferous system. This is somewhat remarkable, and must be taken as an indication of the great worth of coal, for if all the seams of coal found in the group were measured, their united thickness would not be 130 feet, while the depth of the entire series, from the bottom of the Permian to the upper surface of the Old Red Sandstone, is more than twelve thousand feet.
The series of rocks which contain coal are divided into three classes.
I. The Coal Measures.
II. The Millstone Grit.
III. The Mountain Limestone.
I. THE COAL MEASURES.
1. The rocks which are specially called Coal Measures, consist of deep beds of gritty sandstones, layers of soft and pitchy clays called, on account of their being laminated, shales, seams of coal in an almost crystalline state, bands of clay rocks containing iron-stone, and alternating with sand rocks. The uppermost beds of the coal measures generally consist of gritty stones which give very little indication of coal. These grits are sometimes three thousand feet thick, are for the most part destitute of fossils, and are generally called "the upper coal grits." They are represented in fig. 14.
2. Coal is a pitchy mass of vegetable matter mineralised. The vegetable matter is the decomposed remains of cone-bearing trees, gigantic ferns and club mosses, with sometimes the branches, and even the bodies, of entire trees of immense size. The coal beds are found in seams, separated from each other by layers of clayey shales, beds of hardened clay and thick masses of sand rock. The position of the seams of coal in what are called Coal Basins is represented in the accompanying engraving, fig. 15.
3. Every seam of coal has an under-bed of sandy shale and a roof of slate clay, or clay shale.
The Floob, or the bed on which a seam rests, is called the underclay, which is always co-extensive with every seam of coal. It consists of a sandy or gritty shale, which is sometimes called firestone, or rather fire-clay, on account of its
they are completely blackened with it. Though this underclay always forms a floor for the coal scams, yet there are occasionally bands of it without any coal at all. When masses of this tough band are exposed to the atmosphere they become a gray friable earth.
This underclay bed contains, almost invariably, an abundance of the fossil vegetable termed Sliginaria, which were once supposed to be aquatic plants, but are now found to be the roots of some of the plants, whose decomposition and mineralisation formed the seams of coal.
The Ruov of the coal seams is generally a slaty clay called shale. It seems to consist of the waterworn detritus of other rocks. It abounds with the leaves, branches and fruits of plants. It contains also layers and nodules of ironstone, which enclose leaves, insects and the remains of animals allied to the crab.
In some places, its beds contain fresh-water shells, and in others, sea shells, and these beds are interstratitied with pitchy shales and fine slaty clays, micacious sands, and grits and pebbles of limestone and sandstone.
Hence you learn, First, that the underclay was the natural Boil in which the Sligmaria, the roots of the coal forest, grew. Secondly, that the coal is the foliage, branches and trunks of such forest, decomposed during milleniuins of ages, and then mineralised. Thirdly, that the Roof or the shaly stratum was formed by the detritus worn from other rocks, and either transported by a flood, or brought by the sea after a submergence that overwhelmed the decomposed forest.
4. The decomposed vegetable matter which forms coal is capable of various modifications.
First. Peat. When vegetable matter is exposed to a certain amount of moisture, combined with a certain degree of temperature, it rots, decomposes, and forms peat, which is digged for fuel from swamps and bogs in mountainous districts.
Secondly. Lionite.—Lignite seems nohting but a bed of Peat that has been buried for ages under the soil of the earth, where it has undergone those chemical changes which produce bitumen or pitch. Fragments of it generally show the vegetable structure of the mass, on which account it is called lignite, or wood-coal, and sometimes brown-toul. The most perfect specimen of it is the Jet. Lignite is chiefly found in the tertiary beds, and sometimes in the higher divisions of the secondary rocks, but never in the true coal measures.
Thirdly. Pitchy on Bitumixovs Coal.—Pitch coal consists of the same vegetable or woody matter as lignite, and differs •from it only by laving been buried in the earth a longer time, during which it has undergone different and greater chemical modifications, by which it has been mineralised.
This is sometimes called "Black Coal," to distinguish it from the lignite, or brown coal. It is more or less combined with earthy particles, which accordingly, after burning, yield a proportionable quantity of ashes or cinder. The more earthy bands of coal are most generally employed in making gas, and the remains of such coal, after its distillation, form the mass called coke.
Fourthly. Anthracite.—Anthracite coal is called, on account of its compactness, " Stone Coal," and on account of its brilliant hues, "Glance Coal." It consists, like pitch coal, of
carbonised vegetable matter—occasionally in the proportion of ninety per cent.—and also of a small proportion of earthy matter. It burns without either luminous flame or dense smoke. Anthracite is only common or black coal that lies in deeper seams in the earth, and that by its contiguity to some heated volcanic rocks has lost its pitch.
CciM, is coal lying nearer to volcanic rocks than Anthracite coal—so near as not otdy to be charred by the heat, but alsocrushed and shivered by the pressure.
Sixthly. Steam Coal.—Coal used to be divided into the bituminous and the anthracite, but lately a third kind has been discovered in the neighbourhood of Swansea, South. Wales, an intermediate one called "Steam Coal," on account of its peculiar fitness for the engines of steam-packets. It is extremely compact, and hard, burns without any visible smoke, and contains so little pitch as not in the least to be liable to spontaneous combustion.
Seventhly. Blacklead.—Blacklead is more properly called Plumbago or Graphite. It ought never to have been called blacklead, for though in drawing pencils, etc., it marks like lead, it has not a panicle of lead in its composition.
Plumbago is coal that has undergone more chemical changes than culm or anthracite, by which processes it has become more mineralised. In many instances, when coal is found contiguous to igneous rocks, it is converted into Plumbago, or Graphite, a name derived from the Greek ypafu, grapho, to write.
The manner, in which graphite is formed from coal may be in some measure understood from the following statement of the different relations of gases. Hydrogen is the most abundant in fluid pitch, and pitch and carbon abound in coal. In anthracite the pitch is wanting; but in graphite both the pitch and the hydrogen are absent, and nothing but the carbon remains.
Eighthly. The Diamond.—At present the real origin of the diamond is involved in great obscurity, but the prevalent opinion is that it is of vegetable origin. It forms the hardest substance known to chemists. It seems to be but pure vegetable matter perfectly oarboiiited, or pure mineral carbon crystallised. Diamonds are generally found in the detritus of beds that have been contiguous to igneous rocks, but they are beds that are associated with the carboniferous system. Thus, at Bundel Kund in India, the diamond is found imbedded in the New lied Sandstone j for 400 feet of that rock underlay the lowest diamond beds, and beneath this rock are clear indications of coal measures. That diamond is pure coal is evident from the fact that its combustion produces the same results and yields the same elementary residuum as charcoal. Liebio, however, thinks that diamonds owe their origin, not to fire, but to decay, and that they have been formed in the humid way, that is, as pure carbon in a liquid state.
II. THE MILLSTONE GRIT.
1. Beneath the sandstones, coal seams and shales of the coal measures, are thick strata of sandstones more or less compact, called the Millstone Grit. There are, however, some coal districts in which this stratum is absent, in others it appears as a chertzy and sandy rock. Out of England this gritrock is generally absent.
2. The most characteristic bed of the millstone grit is the quartzy conglomerate, which consists of rolled fragments and pebbles of quartz, limestone, and granite, of various sizes, from a pea to a good-sized apple, all of which are cemented together by a clayey paste, or, in some instances, by a flinty paste. Such a piece, when tooled as a hand specimen, has the following appearance, fig. 16.
Fragments of these beds appear sometimes in large masses, some of which are dressed and tooled into millstones, from which the rock derives its name of millstone grit.
3. There are also associated with this grit beds of sandstone composed of the fine detritus of other and older sandstone rocks. In such beds of sandstone are found water-worn fragments of shale, red sandstone, and even stems of plants, all of which bear the marks of having been transported and deposited by currents.
4. As to the real position of this rock in the carboniferous system, the general rule is that it is under the coal measures; but yet, in some coal fields, the seams of coal are found interstratified in the beds of this grit. In other districts the lowest beds of the millstone grit, instead of being gritty, consist of shale containing coal plants and nodules of iron ore, just like the shales which we have described as forming the roof of the coal in other places. When shales appear in this composition, they have often satin spar, naphtha, petroleum and other substances of a pitchy character. There are also instances in which the lower beds of the millstone grit have veins of lead and copper ore.
Fig. Id. A small Fragment of Conglomerated Millstone Grit, tooled on the surface.
5. In parts of Derbyshire and Yorkshire, the high ridges of the mountain limestone are capped by thick beds uf millstone grit. In these cases the lower portions of the grit tire sometimes represented by a series of laminated clays or pitchy shales, which rest immediately on the limestone. These shales ■contain some bands of iron-stone, and are interstratiSed with fine thin block limestone. The upper part of this grit, that which forms the surface of the ridges, consists of beds, many hundred feet thick, of pebbly grits and other sandy rocks, which alternate with thin seams of bad coal.
III. THE MOUNTAIN LIMESTONE.
The Mountain Limestone has been sometimes called "the carboniferous j" but this designation is not truly descriptive of it, because in no district but in the Berwickshire coal-field do these limestone beds bear or contain coal.
1. This limestone goes by different names. On account of its abounding occasionally with ores of lead and other metals, it has been called "metalliferous limestone." As in some districts it abounds with the remains of fossil lilies or encrinites, it has been called "encrinal or encrinital limestone." Because it Bometimes forms elevated chains of hills, as at Matlock, in Derbyshire; St. Vincent's Rocks, near Bristol; Taff Vale, between Brecon and Merthyr Tydfil, in South Wales, it is called the mountain limestone. Even this is not an invariable feature of this rock, for in Ireland and on the continent it is most frequently found in valleys.
2. The mountain limestone is the basis of the coal formation, as it lies between the coal beds and the more ancient rocks, the old red sandstone—or where this group is absent, the Silurians, In the south-western extremity of England an
imperfect coal measure called culm takes the place of the mountain limestone, and lies between the millstone grit and the old red sandstone.
These calcareous beds are composed, for the most part, of subcrystalline gray limestones, in beds of considerable thickness, nearly three hundred feet, and separated only by very thin partings of clay. The stone is capable of very high polish, and forms beautiful marble. It is supposed to have been originally a coral reef, for it abounds with bands of shelly and encrinital remains, and also with fragments of fishes.
Some of its beds are indeed destitute of fossils, but the rock itself is, for the most part, made up of the remains of coral, shells, encrinites, etc., which often form three-fourths of the mass, as may be seen in the different marbles, especially those of Derbyshire.
3. This group of limestone is remarkable for the variety of substances found in it. In some districts it is interstratifled with shales, and grits, and amygdaloidal rocks, or volcanic rocks which have cells in them, and which cells are filled with a different substance. In other districts these limestone beds have in them layers and nodules of chert, just as beds of chalk have those of Hints.
These rocks are also distinguished for their spars. The fluor spar, which is the celebrated Derbyshire spar, is a filiate of lime; and it occurs among these limestones both in crystals and nodules. The blue John spar is very celebrated, as the best material for making vases and other ornaments. This is found tj occur both as veins and also as large and irregular masses.
4. Most of the caverns of the earth occur in this limestone. Wherever it occurs, it abounds in deep chasms and fissures, by which it is traversed in all directions, and also in deep subterranean caverns like those of Somersetshire, Derbyshire, and Yorkshire.
In the north-western parts of Yorkshire the mountain limestone is one thousand eight hundred feet thick, and may be di vided into two groups. The upper forms the Yoredale Rocks, and the lower is what is called the Scar Limestone. Both of these abound in large natural caverns. Though these two arenas of limestone are true mountain limestone, they differ from the mountain limestone of the south of England, by having occasionally thin layers of coal among them, and by being divided into several thin beds by partitions of grit and shale.
6. Many lead mines have been opened in the mountain limestone, especially in the counties of Somerset, Derby, York, Durham and Northumberland. The metal occurs generally in veins. Sometimes chasms or hollows of many hundred feet wide in the rock are found to contain metallic ores and spars. Manganese, copper, zinc and iron are found in this limestone; but the predominating metal is galena, or the sulphuret of lead, which occurs often in cubes and in eight-sided crystals. The same ore is found also in thin layers and in veins, frequently accompanied with fluor spar, lime spar, barytes (barrettes) and iron pyrites.
B I 0 G R A P H Y.—No. XVI.
By J. R. Beard, D.D.
It is possible to be a poet of nature without being a natural poet. To some extent the fact finds illustration in Thomson, whose poetry, with all its attractions, wants simplicity and true sentiment. James Thomson -was born on the 11th of September, 1700, at Ednatn, near Kelso, in Roxburghshire. His father was minister at that place at the time of James's birth, but removed to Southdean, as the income there was less insufficient for the wants of a family of nine children. The young Thomson soon manifested tokens of poetic talent, and his mother, a woman of a tender heart, encouraged the propensity. In Edinburgh, whither he had gone in order to continue his studies, he did not distinguish himself beyond other students. Nay, under the impulse of a natural call to the poetic office, he seems to have undervalued learned pui