are called when they are thus employed, the currency of a country. At first it is probable that these pieces of metal were melted into rough ingots or bars, and stamped with some symbol used by the person who first melted them. Both gold and silver are susceptible of an easy assay or test, by rubbing them on a hard stone, called a touchstone, and by looking at the streak which is made. The issue of rude masses of silver, stamped with the trade-mark of some well-known and reputable merchant, is still practised in China. The central Government and its officials are too weak and too corrupt to issue coins on their own authority, with any hope that the Chinese public would take them. From stamped lumps to coins is an easy step. A coin is a mass of metal of specific fineness and weight, issued by some public authority. If the authority is honest and intelligent, such an issue is of considerable advantage, more for certifying the fineness than the weight, since it is much easier to weigh a quantity of coins, than it is to interpret the fineness of each coin. Unfortunately, Governments have not been either honest or intelligent in this matter. They have frequently issued base money, and frequently lowered the quantity in the coin. The latter, though an evil, is comparatively a slight inconvenience; the former is a terrible mischief, and inflicts lasting injury on the community, the greater part of the suffering being endured by the poor. They who trade in money can easily make themselves masters of the extent to which the fraud has been committed, and may even make a profit out of their acuteness and knowledge; but they who are poor and live by wages, are muleted by the fraud, and have great difficulty in obtaining that advance in the price of their labour which dealers in other articles of value speedily effect. We have now brought our society-and the progress is verified by the experience of economical history-to such a state as that of a sufficient supply of metallic money. It is found convenient to employ gold for large values, silver for transactions of a smaller kind, copper for the least exchanges. In order, however, to prevent confusion, only one of these metals should be taken as a standard, and the others should be overvalued. We have adopted gold and have overvalued silver and copper, that is, have assigned the pieces issued a higher proportion to gold than their intrinsic value. Thus the silver in twenty shillings is worth a good deal less than a sovereign; the copper in 240 pence very much less. No inconvenience ensues, for any person who has to receive a sum in liquidation of a debt can refuse to take more than forty shillings in silver, and twelve pence in copper. In France a debtor can pay either in gold or silver, a fixed ratio of value being established by law between these metals. Most communities, however, use only silver as the fixed measure of value. My readers will now see that money is the machinery, the necessary machinery, by which exchanges are effected. The true or complete exchange is between the producers. But the use of money enables one of the producers, who receives money, to postpone his part in the completion of the exchange till such time as it may be convenient to himself. Hence a sale effected by the payment of money has been called half an exchange. It will also be seen that a person who takes money takes it in order to get rid of it. Its utility is not immediate, but derivative. Except in so far as gold and silver are employed in the arts, they are of no direct service to mankind. Their indirect value is very great, because, as I have said, they who take them accept them on the understanding that they are, of all articles of value, those which are most easily exchanged, which are most serviceable, because they enable their possessor to satisfy his own wants with the greatest certainty and ease. But except for such purpose nobody would take, retain, or store them. Now my reader will remember that gold and silver have been adopted as the measure of value for this reason among others, that they represent great value in small compass. In other words, it has cost great labour and pains to obtain them. Much of their existing value, as far as their use is concerned, would be sacrificed, if they were ever procured very cheaply and abundantly. I am not referring to the effect which such a discovery would have on the existing stocks of the precious metals. Double this quantity: suppose that every person who had a sovereign found himself to-morrow in the possession of two, and the value of his money would be reduced to one-half, or, which is the same thing, the price of everything would in a brief space be doubled. But I am referring to the future use of these metals. In order that they should maintain their utility, as part of the machinery of exchange, they should be costly. If, however, they are costly, and everybody who possesses them for his own purposes wishes to get rid of them, it is clear that society in the aggregate will try to do with as small a stock as it possibly can. The wisdom of our Government in ancient times was believed to consist in collecting as great a hoard as possible of the precious metals. But every individual in his private capacity was irresistibly constrained to do his best to reverse this policy of the Government, and to make the circulation of the pieces of money which he might possess as rapid and as effective as possible. In brief, society wishes, as far as it can, to economise the use of this very costly mechanism of exchange. But except under a certain condition, it can effect this economy only to a very limited extent. This condition is credit, applied to the operations of banking. Credit is the trust or confidence which the possessor of property reposes in the integrity and solvency of another, and by which he is induced to trust his property in the hands of such a person, not that, in case he lends money, the very same pieces of money should be restored to him, but that he shall get their equivalents at his pleasure, sometimes with interest. Hence comes banking. Banking was known in its most rudimentary form to the ancients. Persons devoted themselves in Greece and Rome to the business of receiving deposits from customers, which they pledged themselves to pay on the production of a cheque or tally. There is no doubt that these ancient bankers made loans with the funds which were put into their hands, for, as we read in the Scriptures, they paid interest or usury on such deposits. But the modern system of banking had its beginnings in great commercial cities like Venice, Genoa, Amsterdam, and Hamburg. The merchants who carried on trade in these towns took in exchange for goods the moneys of all civilised nations. A Venetian trader might have in his possession the florins of the Papal treasury, the byzants of the Greek empire, the deniers of France, the silver pence of England, coins in different metals, and of different degrees of weight and fineness. Now such a currency would be very cumbrous and inconvenient, and a remedy for the hindrance and delay involved in such a system would be very gladly welcome. A bank was therefore established, which took all this money, and issued, in exchange for it, receipts. Such a bank, called a bank of deposit, pledged itself to issue no more receipts than it had money, and of course, as the expense of conducting such an institution must have been a considerable sum, it charged some slight premium on its notes or receipts. So useful, however, was this paper found to be, that merchants were often glad to pay a considerable premium for the convenience. In the long run all these banks failed, and for the same reason. The bank lent its metallic money, and so broke faith with its depositors. It might perhaps have avoided this consequence, if it had lent its money for short periods or on merchandise. But it unfortunately lent its funds to public companies, who failed, for in those days people did not understand the very rudiments of banking. Thus, for example, the Bank of Amsterdam, which for many generations had the highest character for integrity and solvency, was found, when the Low Countries were occupied by the revolutionary armies at the close of the last century, to have lent nearly all its silver to the Dutch East India Company, and to be totally insolvent. The Bank of England, which is the parent of the modern system of banking, was established at the conclusion of the seventeenth century. It did not profess to retain the money of its depositors or shareholders, for it lent the whole or nearly the whole of its funds to the Government, issuing notes upon this security, and giving interest on its notes. As a consequence, its notes were frequently inconvertible, that is, the parties possessing them could not obtain cash in exchange for them, and the notes were therefore at a discount, that is, they were sold or exchanged for less than their nominal value. Bank of England avoided the error of the old banks of deposit. It did not lend its notes for long periods, but only for short or mercantile bills. But it fell into an opposite error, which is The indeed remediable, but a great inconvenience. It did not back its notes by a sufficient amount of gold or silver. But the originators of the Bank of England were not acquainted with some of those conditions of banking which are now matters of scientific certainty. I have already stated that it is the object of individuals and of the community at large to make the precious metals, in the shape of coin, as useful as possible. The individual does so by getting rid of all the money which he knows by experience to be in excess of his current necessities. The community does so by diminishing the amount of the metals which it employs as a means of exchange, and this in two ways, by making them circulate as rapidly as possible, and by discovering and employing some substitute for them, which will act as an equivalent for them. Thus, though it may seem strange at first, the efficiency with which stocks of the precious metals may be made to perform the work of exchange, is frequently relative to the facility by which means are devised under which they will not move at all. Let me illustrate this fact in the simplest manner. Let us suppose that Smith of London owes Black of Edinburgh £100, and Black of Edinburgh owes Brown of London £100. If some expedient can be adopted-and it is adopted on a prodigious scale in practice-by which Smith's debt may be made to pay Black's debt, the circle will be completed, and the transactions liquidated, without making use of any single piece of gold or silver. And, again, even if notes were issued in exchange for gold only, the community would save something in the costly machinery of the precious metals, for it would prevent the loss which ensues from their wear, when they are transferred from hand to hand, in the ordinary process of buying and selling. Those operations, therefore, by which cheques or bills are exchanged against each other, and by which notes are employed instead of gold or silver, are economies-the one in saving the cost of carriage and risk, the other in saving wear. But this is by no means the only advantage which ensues from the issue of notes. It will be or should be plain to every one, that no one takes a note willingly, except on the distinct understanding that he can get what the note represents itself to be worth, at his pleasure. A five-pound note will not buy five pounds worth of food, unless the receiver is as certain as man can be, that he can get five pounds for his piece of paper. Neither State nor individual can create wealth or money, by putting valueless bits of paper into circulation, as has been found over and over again. The severest penalties, the most despotic authority, will not give such pieces of paper any value beyond that which their possessor finds the general public assigns to them. When in the early days of the French Revolution the Government of that country strove to put paper into circulation which was issued by its authority when it denounced as traitors those who would not take these notes at their nominal value, and actually punished persons with death who declined to part with their property in exchange for such notesthe Government was utterly baffled. It is not hard to see the reason. The whole community refused to submit to such a command, and it was impossible to execute the whole community. Again, when a person takes a note, he expects to get gold or silver, and not any other kind of property. Notes entitling the bearer to be paid in a certain amount of public stock, or of land, or of tea, sugar, corn, or any other kind of food, will not circulate. Such instruments-properly, warrants to receive goods are or may be very valuable kinds of property, but they are not paper money. They are securities which may be bought and sold, but they have to be reckoned in money, and turned into money before they can be of avail to the holder. It is impossible to circulate bank-notes on any other security than that of gold and silver, however valuable the security may be; and attempts to effect such a circulation, though they have been made over and over again, are invariably failures, and invariably end in bankruptcy. A bank-note to be capable of circulation must be and remain an order to receive so much money, and nothing but money will be its equivalent. It does not, however, follow that everybody who holds a note is determined to turn it as soon as he can into gold or silver. It is enough for him to know that he can do so when he pleases. Hence, not one note in ten thousand is presented to the Bank for change into gold or silver. If there were any doubt about the possibility of changing them at their full value, they would be presented soon enough. But as long as people believe that their notes could be and would be paid on demand, they do not present them for payment, and for the simple reason that to do so would be to abandon the advantages which a note possesses. These are, that it is easily portable; that it much facilitates the counting of money; and that if it be lost, the sum which it represents may be, under certain conditions, recovered. The holder of notes is in some particulars more safe from fraud, violence, or mischance than the holder of gold or silver. Long experience enables bankers to calculate the likelihood of persons presenting their notes for payment, and therefore instructs them as to the quantity of gold and silver which they must keep in order to cover the contingency of these notes being presented. Let us suppose that this is one-third of the amount of notes in circulation. It will be plain, then, that in case the banker has sufficient property to warrant him in cir culating the other two-thirds (and he is a swindler if he has not sufficient property), he can retain his property in his own hands, and get such profit as comes from it, and lend the other two-thirds to the public, and get a profit by them. Nor does this practice go against what I have stated above. It is true that there is no gold or silver to back up two-thirds of the notes, but there is property which can be, or ought to be, on an emergency turned into gold or silver, and the proceeds of which may be therefore devoted towards meeting these obligations, should the necessity arise. A banker is bound to keep his property in such a shape as to be able to obtain, as soon as he requires it, the funds which are necessary towards cancelling all the debts which he incurs by the issue of his notes. One source, then, of a banker's profit is derived from the issue of his notes. But it is by far the smallest part, and in the United Kingdom very few bankers are allowed by law to obtain this profit. The chief profit of a banker is derived from using the amounts left in his hands by his customers. There is a great convenience in keeping an account with a banker-i.e., in depositing a sum of money with him, and in drawing it out by cheques. The cheque is as convenient as notes are to the debtor and creditor, and it has other advantages. By a little precaution, the transmission of the cheque may be made absolutely safe. A payment by cheque is virtually a receipt. The aggregate of cheques drawn by a person in a year represent his expenditure, and form a record of it. Now the banker, who expects his customers always to have a certain sum with him, is able by experience to calculate what will be the average number of cheques drawn on him, and will keep such a sum by him as will enable him to meet these liabilities. In a great many cases, the person who draws and the person who receives the cheque keep an account with the same banker, and then the payment and receipt are only the writing down a set of figures on one side of one account, and the same figures on the other side of another account. Sometimes, however, the banker offers his customer a share of the profits which are made by lending money, and in this way some banks attract an enormous amount of capital in the form of deposits, and get their profit for the pains they are at in receiving and managing the sums left with them, and for the risks they run. By these means a small sum of metallic money is made the means for effecting a prodigious multitude of transactions. Every bill of exchange, bank-note, cheque, is expressed in money, and the person who undertakes the obligation stipulates to provide the money expressed in the security. In the great majority of instances no money is either used or needed. The cheques which pass from hand to hand on a great day of business in one room in London, are equal to all the gold in all the banks in London. But these cheques are exchanged without the use of a single piece of money. Nothing can exceed the perfection with which this mechanism is carried on. But I repeat, the presumption on the face of all these cheques is that they will be paid in gold. If any risk arose that they could not be, a demand would be made that they should be, and what is called a panic follows. That such a risk does not arise is due to the credit in which dealers stand, and this credit is a trust in the merchant or dealer's honesty. The motive force in the great engine of commerce is the virtues of integrity and good faith. LESSONS IN NAVIGATION.-III. MIDDLE LATITUDE SAILING-MERCATOR SAILING TO TAKE VI. Middle-Latitude Sailing.-The occasions on which a ship sails due east or west, and can take advantage of the easy means of finding her longitude just explained, are of course rare. But by the rule called mid-latitude sailing, the diff. long. can be deduced from the departure found by plane sailing. It will be remembered, in connection with Fig. 4, that the departure on the course A B was explained to be an imaginary line or value, greater than the arc of parallel c B, and less than A d, and therefore equal to some similar parallel lying between them. If we can find a parallel on which the departure can be taken as the true change of longitude, we can ascertain the latter by the last rule, the departure being taken as distance sailed due east or west. The difficulty is to say exactly where such parallel is to come, but the general practice is to put it half-way between "lat. left" and "lat. in," a practice never strictly correct, and sometimes glaringly wrong. In low latitudes, where meridians incline to each other but slightly, and in all cases where the difference of latitude is slight in comparison with the departure made, the rule may be trusted for practical purposes; but in high latitudes, where the meridians converge rapidly, or wherever the diff. lat. is considerable, it cannot be relied on. Suppose a ship, steering southerly, makes a difference of latitude of 4°, from 22° to 18° N., and a departure from long. 30° W. of 160 miles west; what is her difference of longitude according to the mid.-latitude rule? 160 The difference in the result would have been insignificant. But suppose the ship had made as much as 10° diff. lat. from the high latitude 65° N., then mid. lat., by rule, would have been taken as 60° N., whereas by the table it should be 60° 19′ N. With a departure of, say, 500 miles, this inaccuracy would cause dangerous error in the longitude, as may be seen by working (7) with the corrected and uncorrected values of mid. lat. Nevertheless, by using Workman's Table we may safely find the longitude by "mid-latitude sailing," which we may regard as an appendix by which the theory of plane sailing is rendered complete. Half-way between 22° and 18° N. is 20° N.; 160 miles on an are of a parallel in 20° N. lat. is therefore the measure of the difference of longitude. In other words, the ship's oblique course a curved line, the exact position of whose finial point is unknown to us-has been resolved into two straight courses, one pointing due south, the other due west. The length of both is known in miles, but this gives no direct clue to the measure of longitude contained in that pointing west, unless we can tell in what latitude it is to be measured. Knowing this under the rule, we apply formula (6)— VII. Mercator Sailing.-Besides mid.-latitude sailing, there is another plan by which the difference of longitude may be deduced from the departure, and as it is scientifically accurate if correct data be given, it is usually to be preferred. This is Mercator sailing, so named after a Flemish chart-maker, Gerard Mercator* (1512-1594), whose charts were based upon the prin ciple now to be explained. Formula (6) may be recast for middle-latitude sailing, as follows, as the last example shows: D Diff. Lat. Diff. Long. Mid. Lat. Departure. departure cos. mid. lat. As a nautical mile everywhere, except at the equator, exceeds a minute of longitude, it follows that the number of miles of departure is always less than the number of minutes of longitude traversed. Hence it is clear that some greater departure (expressed in miles) than that which properly belongs to the course and distance traversed will exactly express the real departure in minutes of longitude-in other words, will give us the differthe diff. of latitude in some known proportion, and take the ence of longitude. All we have to do, then, is to exaggerate corresponding exaggerated departure, in miles, as the diff. of longitude in minutes. Of course the exaggerated diff. lat. is merely a means to an end, and the ship's position, as to latitude, is not to be fixed by it, but by the true diff. lat. found by plane sailing. We will now prove the rule by which the requisite addition to diff. lat. is found, premising that, owing to the increasing convergence of meridians towards the poles, the proportionate addition to be made increases with increased distance from the equator. In Fig. 9, let Abc be one of the infinitely small triangles in These relations, like those in plane sailing, are involved in the construction of a right-angled triangle, and the two can be united in a single figure, which shows at a glance the relations subsisting between course, distance, diff. lat., and diff. long.-the four things which we alone care to know -and the intermediate expressions deB parture and mid. lat., which serve only to link the others together. See Fig. 8, where AB is the distance representing the rhumb line in Fig. 4, and where in fact the triangle ABC is a reproduction thus the diff. long. made. Therefore, by formula (6) or (7), c' b′ of Fig. 5. Here all the lines (except c D, which represents nothing) are measured in a like denomination, viz., minutes; for the nautical miles, in which CB and A B are measured, are identical with minutes of latitude, and DB of course is measured in minutes of longitude. A It has been said that the particular latitude in which the departure just equals the difference of longitude due to the ship's oblique course, is not quite truly found by the mid-latitude rule. There is, however, a table, called Workman's Table of Corrections to be added to Mid. Latitude (not commonly included in books of tables), by which the true latitude can be found. In the column headed by the number of degrees nearest to the diff. lat. found, and opposite to the degree of latitude given as "mid. lat." by the rule, will be found the number of minutes translation. * Properly Gerard Kauffman, of which name Mercator is the Latin known, cb can be computed in minutes (c'b' in miles) without difficulty. This reasoning only holds good where A c is so small that the diff. of latitude between c and the point midway between A and c can be neglected, and "mid. lat." treated as identical with "lat. in." If A c represent diff. lat. resulting from an ordinary day's sailing, we cannot fix c, and therefore c'B', in the same simple manner; we must split AC into minute parts, say l' each, and calculate the meridional diff. lat. for each; the sum will be the meridional diff. lat. A c', by which we can compute C' B'. The other sides of the triangle being measured in nautical miles, B'C' will of course be in miles, equal in number, as already explained, to the minutes of longitude in the departure. Or we may say that B'C' will work out in equatorial degrees of longitude. Tables have been calculated of the meridional latitude in minutes-called meridional parts--corresponding to every degree and minute of latitude, from 1' to 89° 59', whence the meridional diff. lat. can be found for any given diff. lat. Thus a ship sails from 30° N., on a course and distance which give 10° 600' as her diff. lat., by which her "lat. in" is found to be 40°. Her merid. diff. lat. is thus found: : or, C' B' .. C'B' AC' x tan. A; = diff. long. (in minutes) merid. diff. lat. X tan. course (9) If departure be given instead of course, we may sayDiff. lat. merid. diff. lat. :: departure: diff. long. The increased latitude in minutes, or the "meridional parts" corresponding to any given latitude, are thus arrived at:-For the first minute from the equator we multiply 1 by the natural secant of 1'; result is still 1, the nat. sec. not differing from unity in seven places of decimals. The increased equivalent of the second minute is 1 x nat. sec. of 2′ = 10000002; increased equivalent of 2' = sum of two increased lats., or 2.0000002. really equal only 30 nautical miles are made to occupy the same space as at the equator, where they equal 60 miles, the map is evidently distorted, but the distortion is made symmetrical by the device of increasing each minute of latitude in proportion to the increase given to the minutes of longitude on its parallel. Thus, though countries and distances lying towards the poles are enormously exaggerated in size, they are comparatively symmetrical in shape. The degrees of latitude are increased in accordance with the table of meridional parts. The benefit conferred upon mankind by Gerard Mercator, in the invention of his cylindrical projection of the earth's surface, can scarcely be overrated. To get rid of the curved meridians, substituting parallel straight lines for them, was a device which breathed a new life into Navigation. Scientific and satisfactory as is the theory of Mercator sailing, it is better to obtain the longitude by the mid.-latitude method, if the course exceeds, say, 60° from the meridian, because diff. long. is obtained by multiplying merid. diff. lat. by tangent of course. If the angle of course exceed 45°, then tan. of course exceeds unity, so any error in estimating the course may lead to a large error in longitude. Thus by either mid. lat. or Mercator sailing, we may consider the theory of plane sailing completed and extended. VIII. To take a Departure.-Although the mariner estimates and records his vessel's movements as well as he can, yet he loses no opportunity of fixing her position by observations of land or sky. The bearing of the last headland, lighthouse, or other prominent object expected to be seen, is taken from two positions, and noted, together with the course and distance run from the first point of observation to the second. Unless one of the observations is taken, by chance or design, when the object is exactly on the beam-i.e., at right angles to the course -or the course is of such length as to subtend 90° at the object, the fixing the ship's distance from the object at the last observation will involve the computation of an oblique triangle, the only one we have yet had to do with. Suppose a ship sailing down Channel observes Beachy Head bearing N.W. After running 10 (nautical) miles W.S.W., the Head bears N.N.E.AN. the last time of observation? How far is the ship from the Head at C second position. We have now to solve Distance. B Fig. 10. A and so on, up to 5399', which is only 1' short of the pole. position marked can be taken as lat. and long. left, and the day's The sea-charts used by navigators are always on what is termed Mercator's projection, which is intimately connected with the above theory of sailing. The earth, instead of being a globe, is assumed to be a cylinder, which is unrolled as a flat map. Hence every parallel of latitude is a circle, or rather line, equal to the equator; and hence degrees of longitude are equal in all latitudes. As 60′ of long. in a latitude where they IX. Great Circle Sailing.-The advantage of sailing on a rhumb line is that the successive meridians are cut at ore angle; in other words, one compass course may be steered throughout the whole voyage. The rhumb line between any two places, though a spiral on the globe, becomes a straight line on the chart. This makes it very easy to lay down the compass course to be steered to reach a given point on the chart; but it has been known for centuries that the rhumb line is not the shortest route between two places, though the fact that it is steered on a uniform compass course gives the idea that it must be a straight and therefore direct line. But the fact that it cnts at the same angle successive meridians which are not parallel is sufficient proof that its direction is constantly changing, and that the ship's head never points in the same direction, if properly steered, for two miles together. The real direct course is an are of the great circle passing through the two places (and the centre of the earth), as will be readily found by stretching a thread between them on the terrestrial globe. People who look only at maps often wonder why our ocean steamers should tako Cape Race, in Newfoundland, on their way to New York, instead of going, as they call it, straight across the Atlantic; and they wonder why their passengers should talk about icebergs encountered. Careful study of a good-sized globe will dispel these illusions, and show that the stretched thread between England and New York actually reaches latitudes so high that at all seasons of the year it cannot be followed. Apply the same length of thread in the line indicated by the (apparently) straight course on the chart, and it will fall short by some hundreds of miles. For all this, great circle sailing has been little practised until late years, for although it is easy enough to find the great circle course between two places, it is less easy to steer it, as the compass course is always changing, owing to the course-a truly straight line in its relation to the earth's surface-cutting the converging meridians at different angles. The calculation involved in ascertaining the incessant but varying change of compass course until lately barred the way against ordinary navigators. Now, however, tables drawn up by Mr. Towson, of Devonport, are published by the Admiralty, based on the principle of breaking up the arc into short rhumb-line stages equal to one degree of longitude. The tables show the change of course to be adopted at each stage; thus the sailor keeps up his familiar and, it must be owned, simple rules of sailing, and yet the great circle, though not exactly, is tolerably followed. Of course when a ship sails on the old system on a meridian, or on the equator, she is practising great circle sailing without probably intending it. We have now finished our exposition of the mathematical principles and processes by which the mariner traces his vessel's track upon the ocean, or keeps his "account by dead reckoning." It has been stated that the data from which he calcu lates are themselves greatly liable to error, and in view of this it may be thought we have bestowed unnecessary exactness upon results which can never be taken without question. In practice, too, there is no doubt that many things we have made subjects of calculation are simply guessed by the mariner with results usually accurate enough for the purpose, if he be up to his work. This is true, but nevertheless there is no ground work for practice so sure as a good knowledge of theory, and even the practical work of guessing results is best done by those who understand how to evolve them with scientific accuracy. The science of Nautical Astronomy-or rather the art of using the instruments by which longitude and latitude are found at sea by celestial observations-is too technical and limited in its character to find place in elementary lessons like these, which seek only to apply to navigation those mathematical principles of general utility which have been taught in CARBONATES. Carbonate of Lime.-This mineral is very widely distributed When it appears in a crystalline form, it is as a rhombohedron in Nature, since of it are composed all limestone and chalk rocks. of the hexagonal system-a figure which is the hemihedral form of the icosahedron (see page 176). When crystallised it is called Cale Spar, and if the crystals are transparent it is termed Iceland Spar. The fact that it possesses the property of double refraction has been alluded to and illustrated in page 49. Its constitution is, 1 atom of carbonic acid and 1 of lime. Some specimens contain a little carbonate of strontia; but Aragonite has precisely the same composition as calc spar. this is not an essential ingredient. It crystallises, however, in the trimetric system; hence carbonate of lime is said to be dimorphous. Aragonite is of a wine-yellow colour; its crystals with gypsum, is found in the fossil belemnite, and it is said are usually clustered and radiating. It is frequently associated that cold springs containing carbonate of lime deposit cale spar, while hot springs yield aragonite. It is named from Aragon, in Spain. Carbonates of lime effervesce when touched with acid. Several of the ores of the metals appear as carbonates. It is a carbonate of iron, and is the chief source of that useful Clay Ironstone is found in beds in the Carboniferous system. metal in England. and appears in the hexagonal system in rhombohedrons and six- to brown, and the scales are often translucent. It contains 1 atom of Malachite is carbonate of copper. of oxide of copper, and 1 of water. other sections of this work. But just as the mariner must covered in Australia that its value is much deteriorated. MINERALOGY.—VII. NITRATES. ONLY two important nitrates appear in Nature. Nitrate of Potash, or saltpetre, is peculiarly valuable as being the chief ingredient in gunpowder. It is frequently found in caves of limestone districts, and in India and other countries appears as an efflorescence on the soil in the hot weather which succeeds copious rains. In Norway and Sweden it is artificially prepared by exposing refuse heaps to the action of the air. At the end of three years the "nitre bed" is lixiviated —that is, washed with water. This water on evaporation yields crystals of saltpetre in long thin prisms. The salt is trimetric. PHOSPHATES. Apatite is phosphate of lime. It is never pure, but the fluoride and chloride of lime are invariably associated with it in small quantities. When found crystallised it is in short six-sided prisms of a greenish colour. It is found in many metamorphic rocks, and in some granites. When present in nodules in stratified rocks it is accounted to be of organic origin, for phosphate of lime is the chief constituent of bones. Wavellite is a hydrated phosphate of alumina; it is usually found as small hemispheres which have a finely radiated structure within, attached to the surface of the rock, generally of a greenish shade of colour. When forced off, they leave a stellate circle on the rock. |