examined numerous quartz crystals were observed, rhombohedra and six-sided prisms, many imperfect, and some adherent, so as to form crosses.

In another experiment made by the writer, a little mercury was put into a test tube, and above it about three inches of white of egg. The tube discharging the silicic fluoride gas was immersed in the mercury, and as the gas rose, it passed through the white of egg, speedily coagulating it to the consistence of thick clotted cream. This creamy mass was then dried, and ignited in a platina crucible over a Bünsen burner, until the organic matter was destroyed. The residue-very small in quantity-was found to consist of silica, in a vesicular state. In some cases the walls of the vesicles were so thin as to give rainbow colours when light was thrown on them, or slantingly through them. No crystals were formed in this experiment.

REGISTRATION OF COINS.

THOSE who are curious in regard to such comparatively small matters, and numismatists generally, will have observed that all the gold and silver coins above the value of the threepenny piece issued from the British Mint during the last two years have one or more small numerical figures imprinted upon their surfaces, immediately over the date. The object of this rather singular arrangement has given rise to a considerable amount of speculation, and many letters of inquiry have, we believe, been addressed to the Mint authorities as to the real motive of the innovation. Having ourselves obtained, from the best source, the true reason assigned for stamping numbers on the coins of the realm, we have this month the satisfaction of conveying a similar amount of enlightenment to the public.

It is done with a view to identifying the particular die by which each individual piece of money was struck, the particular press at which the latter received its impressions, the day on which it came into existence, and the name of the pressattendant under whose agency it was born. Thus, if a defective coin, which had escaped the vigilant and scrutinizing eye of the examiner at the Mint, and passed out into the channels of general circulation, were subsequently discovered -no matter how long after its issue-it would be possible, by a reference to Mint records, to establish its identity, and to fix the responsibility for its defect upon the unhappy wight

who managed the machine which stamped it into being. What the punishment attached to the offence may be we know not, for that, no doubt, would be made to depend upon the presence or absence of extenuating circumstances, and the leniency or otherwise of Mint officials.

The mode of effecting the registration we shall now endeavour to explain-one kind of coin being taken as an illustration of the rule applied to the whole of the denominations of money systematically numbered. Let it be supposed, for example, that the Mint is about to inaugurate a coinage of sovereigns for the year 1866, and that it has been determined to establish in perpetuity, and by official documentary evidence, the birthday of each individual coin-the date itself notifying the year of its production. Then the first step after the reception of the precious metal from the Bank of England, will be the preparation of dies. Each of these, after receiving its impression at the hands of the Die Multiplier, and while yet in its annealed and plastic state, is returned to the Engraver for numbering. On the first die which reaches him he imprints, by means of punch and hammer, the figure one; on the second, figure two; on the third, figure three; on the fourth, four; and so on up to twenty, thirty, or more, as it may happen. The whole are afterwards hardened, tempered, and polished, and then transmitted to the coin-stamping room. Here they are received by the officer in charge. Under his direction, die No. 1 will be affixed, with its companion obverse, say to press No. 4, at nine in the morning, on the 16th of July; die No. 2 to press No. 5; No. 3 to press No. 6, and so on-R. M.; J. S.; W. R.; T. H., etc., being the initials of the respective youths attending the presses. All these facts are recorded by the Superintendent of the Department on a tabular form provided for the purpose. The presses are then made to commence their money-making careers for the day; and, blanks of gold being supplied ad libitum, they continue, so long as the dies remain uninjured, to give forth numbered sovereigns in large quantities. It is possible, indeed, that the numbers on the dies may be reproduced to the extent of 25,000 or 30,000 on the surfaces of the coins

yielded by each press. When either of the dies fails by cracking or becoming otherwise defaced, it is at once removed, and its place is filled by another, the like particulars in reference to which are duly entered, as before, in the journal of registration.

As a general rule, no pair of dies lasts longer than a single day in a Mint coining-machine, and it may be fairly supposed that those of which we have spoken as engaged in stamping gold coins on the suppositious 16th instant, will not outlive

that period. They will therefore all be removed at its close, their vacant positions being occupied by others bearing consecutive numbers, and of which the same careful notes will be taken. Thus day after day throughout the remainder of the year the work of numbering, registering, and stamping may go on, and the birth, parentage, and education of each piece of money be accurately recorded. The coinage of every year is made distinctly identifiable by the date, and therefore the first die for each denomination of coin used in each year is marked as No. 1, and sequitur in due order.

Such are the arrangements for effecting the registration of the coinage of the realm as at present put in force at Her Majesty's Mint-whether usefully so or not is another question. To us there seems to be some practical objections to the process, however in theory it may seem ingenious and serviceable. Surely it must be very difficult for a press-boy-unless Arguseyed-to examine with any care the surfaces of pieces of money which are struck off from his machine at the rate of seventy per minute, and which accumulate with astounding rapidity in the catch-pan below, he having the press to feed and keep in order during its working. Equally puzzling must be the task of examining the dies between each stroke of the press! Would it be just to punish or blame a youth for the nonfulfilment of an impossible duty? Great care of course is necessary in the performance of the delicate task of creating sovereigns or shillings, but it can only be effectually exercised by a judicious division of labour. No one person can be justly made responsible for the commission of a fault, unless he have the power to prevent its occurrence. As in printing the sheets of periodicals and newspapers, a certain percentage of them is damaged and wasted, so is it in the rapid stamping of coins by steam machinery. The greatest diligence on the part of press attendants will not prevent this inevitable result, and the true remedy for it lies in the subsequent elimination of soiled or defective specimens by the aid of the searching eye and quick hand of a numismatic critic or inspector of coins-at least, that is our deliberate opinion. Herein lies one of the weak points pertaining to the system of coin numbering. Outside the walls of the Mint that system can be of no service whatever. It certainly is of little moment to the public whether the coins jingling merrily in their pockets, or glistening cheerfully in their porte-monnaies, were struck off in the month of May, June, or August, in the forenoon or afternoon of any particular day, or whether the machine which communicated their images and superscriptions were, for the time being, under the care of Master Tom Stiles or Master Jack Nokes.

That the coinage of the realm should be as near perfection

as it can be made, is true, but we fail to see that the addition of two or three figures to the devices which so richly ornament the various denominations, advances it one stage nearer that point, or answers any really useful end.

COTTA ON ROCKS.*

PROFESSOR Cotta tells us in a preface that the present English version of his work on rocks may be regarded as its third edition, the modifications and alterations having all been made with his concurrence.

It is a very valuable work, and although we could have wished that a more elaborate account had been given of chemical researches and synthetical experiments, elucidating the probable origin of many rocks, such a course would have greatly extended the bulk of the volume, which is now compact enough and cheap enough to answer the purpose of a general manual for students.

After some chapters on the minerals entering into the composition of rocks,† Professor Cotta enters upon the question of texture, and points out that all rocks may be divided into two principal classes, granular and compact. "The word granular is, however, usually only applied when the different mineral parts are all of a granular shape, of nearly the same size, and are crystallized into each other." Rocks composed of grains, pebbles, or fragments mechanically cemented together, may be arenaceous, as sandstones; conglomerates, in which more or less rounded masses much bigger than grains, are caused to adhere by some natural mortar or cement; or breccias, in which the fragments are angular. "The term compact is usually only applied to a rock when its particles adhere firmly and closely together (without being fused into one mass like glass)." A loose friable condition is termed earthy, and a fused one vitreous, or opalescent, as the properties designated by those terms may be exhibited. Every rock, according to Professor Cotta's nomenclature, must be either granular or compact, using both terms in a wide sense. The granular series may be any rocks, fine, or coarse, in which the individual component parts are recognizable, while in compact

Rocks Classified and Described: A Treatise on Lithology, by Bernhard von Cotta. An English Edition by Philip Henry Lawrence, with English, German, and French Synonymes. Revised by the author. Longmans.

+ It is necessary to remind readers unacquainted with geology that in technical use the word rock comprehends all sorts of natural formation.

rocks no such distinction of individual parts can be observed. The texture of a rock is termed porphyritic, when distinct crystals or crystalline particles are distributed through an otherwise compact principal mass or matrix. Schistose (foliated), slaty (cleaved), shaly (laminated), and fissile are terms expressive of different sorts of parallel structure, and will be easily understood. Mr. Jukes says, "When we wish to be precise we speak of the foliation of schist, the cleavage of slate, and the lamination of shale."

With reference to slaty cleavage Professor Cotta dissents from Poulet Scrope's conjecture that it may have arisen from friction of some kind, nor does he agree with those who ascribe it to a crystalline action, but he accepts the views of Sharpe, Haughton, Sorby, and Tyndall, that when not identical with stratification, it has, in most cases, been caused by pressure at right angles to the cleavage plane applied to the rock during or after its formation.

To give definite meanings to vesicular, scoriaceous, and pumiceous, the first will be restricted to cases in which the vesicles are isolated; the second to instances in which they occupy a space equal to the solid part of the rock; and the third to cases in which the hollow parts predominate over the solid. "A rock is said to be amygdaloidal when the vesicular cavities are filled either entirely or in part with new mineral substances. The filling in of these cavities is always a process subsequent to the formation of the rock." The Professor adds, "We infer from all the attendant circumstances that the formation of those amygdaloids must have been a very slow process.... Hence we may explain the fact that the most recent of all igneous rocks, the lavas, although they are very often vesicular, are never amygdaloid. Hence the frequency and completeness of the filling up of the cavities increases almost in direct ratio with the age of the rock." It is observed that igneous rocks rich in silica are not only less frequently vesicular, but that when the vesicles occur they are less frequently filled up, "which probably arises from their containing fewer soluble substances adapted to the formation of amygdaloids, in particular less lime and magnesia." Oolitic texture belongs to certain limestones and iron stones, formed by the conglomeration of rounded or egg shaped bodies which may be very small, or like peas (pisolitic) in pea stone. The term nodular needs no explanation, and Mr. Dana's term spherulitic is applied to some igneous rocks exhibiting pearl-like globules.

Rocks are placed in three great divisions-igneous, metamorphic crystalline schists, and sedimentary. It is of those in the first division only that we now propose to speak. The igneous rocks may be volcanic-that is, erupted rocks, which

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