in general, signifies the natural formation of any substance into a regular figure, resembling that of crystal. Hence the phrase of crystallized ores, crystallized salts, &c., and even the basaltic rocks are now generally reckoned to be effects of this operation: (See BASALTES AND VOLCANO). The term, however, is most commonly applied to bodies of the saline kind, and their separation in regular figures from the water, or other fluid in which they are dissolved, is called their crystallization*. The word crystallization is never applied to the freezing of water, or to the consolidation of metals after they have been melted; though it might certainly be applied with as much justice to these substances as to any others; for all of them concrete into a certain regular form, from which they never deviate, unless disturbed. When water freezes slowly, it always forms regular crystals of ice, which are constantly of the same form. They are long, needle-like masses, flattened on one side, and joined together in such a manner, that the smaller are inserted into the sides of the greater; and thus these compound crystals have the appearance of feathers, or branches of trees with leaves. The most remarkable circumstance attending this crystallization is, that the angle formed by the insertion of the smaller pieces into the larger is either 60 or 120 degrees. The figures assumed by metals of different kinds have not been so exactly investigated, except in the regulus of antimony, which is observed always to take a stellated form. Experience also shows, that all kinds of earths, or other mineral matters, are capable of assuming a crystalline form, and may easily be made to do so by taking away part of the water which dissolves them.

Different salts assume different figures in crystallization, and are thus most easily distinguished from one another. The methods of reducing them into this form, for sale, are mentioned under the article CHEMISTRY, no. 573. But besides the large crystals produced in this way, each salt is capable of assuming a very different appearance of the crystalline kind, when only a single drop of the saline solution is made use of, and the crystallization viewed through a microscope.

For our knowledge of this species of crystallization we are indebted to Mr Henry Baker, who was presented picrocrystal with a gold medal for the discovery, in the year 1744. These microscopical crystals he distinguishes from the large ones by the name of configurations; but this term seems inaccurate, and the distinction may well enough be preferred by calling the large ones the common, and the small ones the microscopical, crystals of the salt. His method of making these observations he gives in the following words:

"I dissolve the subject, to be examined, in no more than a quantity of rain or river water than I am certain it is sufficient to saturate. If it is a body easily dissolvable, I make use of cold water; otherwise I make the water warm, hot, or even boiling, according as I find it necessary. After it is perfectly dissolved, I let it rest for some hours, till, if overcharged, the redundant saline particles may be precipitated and settle to the bottom, or float into crystals; by which means I am most likely to have a solution of the same strength at one time as at another; that is, a solution fully charged with as much as it can hold up, and no more; and by these precautions the configurations appear alike, how often soever tried; whereas, if the water be less saturated, the proportions at different times will be subject to more uncertainty; and if it be examined before such separation and precipitation of the redundant salts, little more will be seen than a confused mass of crystals.

The solution being thus prepared, I take up a drop of it with a goose quill cut in fashion of a scoop, and place it on a flat slip of glass of about three quarters of an inch in width, and between three and four inches long, spreading it on the glass with the quill, in either a round or an oval figure, till it appears a quarter of an inch, or more, in diameter, and so shallow as to rise very little above the surface of the glass. When it is so disposed, I hold it as level as I can over the clear part of a fire that is not too fierce, or over the flame of a candle, at a distance proportionable to the heat it requires (which experience only can direct), and watch it very carefully till I discover the saline particles beginning to gather and look white, or of some other colour, at the extremities of the edges. Then (having adjusted the microscope before-hand for its reception, armed with the fourth glass, which is the fittest for most of those experiments), I place it under my eye, and bring it exactly to the focus of the magnifier; and, after running over the whole drop, I fix my attention on that side where I observe any increase or pushing forwards of crystalline matter from the circumference towards the centre.

This motion is extremely slow at the beginning, unless the drop has been overheated, but quickens as the water evaporates; and, in many kinds, towards the conclusion, produces configurations with a swiftness inconceivable, composed of an infinity of parts, which are adjusted to each other with an elegance, regularity, and order, beyond what the exactest pencil in the world, guided by the ruler and compasses, can ever equal, or the most luxuriant imagination fancy." When this action once begins, the eye cannot be taken off, even for a moment, without losing something worth observation; for the figures alter every instant till the whole process is over; and, in many sorts, after all seems at an end, new forms arise, different entirely from any that appeared before, and which probably are owing to some small quantity of salt of another kind, which the other separates from, and leaves to act after itself has done: and in some subjects, three or four different sorts are observable, few or none of them being simple and homogeneous.

When the configurations are fully formed, and all the water evaporated, most kinds of them are soon destroyed again by the moisture or action of the air upon them; their points and angles lose their sharpness, become uneven and defaced, and moulder, as it were, away. But some few are permanent, and being inclosed between glasses, may be preserved months, or even years, entertaining objects for the microscope.

It happens oftentimes that a drop of saline solution can hardly be spread on the slip of glass, by reason of the glass's smoothness, but breaks into little globules, as it would do if the surface were greasy: this was very troublesome, till I found a way of preventing it, by rubbing the broken drop with my finger over the glass, so as to leave the surface smeared with it; on which smeared place, when dry, another drop of the solution may be spread very easily in what form one pleases.

It likewise sometimes happens, that when a heated drop is placed properly enough for examination, the observer finds he can distinguish nothing: which is owing to saline steams that, rising from the drop, cover and obscure the object-glass, and therefore must immediately be wiped away with a soft cloth or leather.

In all examinations by the microscope of saline solutions, even though made in the day-time, I always employ the light of a candle, and advise every observer to do likewise: for the configurations being exceedingly transparent, are rendered much more distinguishable by the brown light a candle affords, than by the more white and transparent day-light; and besides, either by moving the candle or turning the microscope, such light may be varied or directed just as the object requires.

In this manner were produced the beautiful crystalizations represented Plate CLII. They are vastly different from such crystals of the same salts as are obtained by the common processes; but Mr Baker assures us they are no less constant and invariable than they, and that he has repeated the experiments a great number of times with the same success.

Fig. 1 shows the microscopical crystals of nitre or salt-petre. These shoot from the edges, with very little heat, into flattish figures of various lengths, exceedingly transparent, and with straight and parallel sides. They are shown in their different degrees of progression at the letters a, b, c, d, e; where a represents how they first begin. After numbers of these are formed, they will often dissolve under the eye, and disappear entirely; but if one waits a little, new shoots will push out, and the process go on afresh. These first figures sometimes enlarge only without altering their shapes, and sometimes form in such sort as the crystallization drop represents; but if the heat has been too great, they shoot hastily into ramifications very numerous and beautiful, but very difficult to be drawn; and which Mr Baker therefore did not attempt. There seems all the while a violent agitation in the fluid, and most commonly, towards the conclusion, a few octaedra (composed of eight triangular planes, or two quadrangular pyramids, joined base to base) make their appearance.

2. Blue vitriol produces crystals round the edges, very short at the beginning, but increasing gradually, as represented at the figures 1, 2, 3, which denote their difference of form, and the progress of their growth. These crystalline shoots are solid, regular, transparent, and reflect the light very beautifully from their polished sides and angles. As the watery part evaporates, numbers of long slender bodies like hairs are seen here and there, some lying side by side, or crossing each other as at 4, others forming star-like figures with many radiations (5, 5). This salt shoots but slowly, and therefore requires patience. At last the true crystals begin to appear commonly in the middle of the drop, and are very prettily branched, as at 6.

3. Distilled verdigris, dissolved as above directed, and immediately applied to the microscope, shows abundance of the regular figures, 1, 2, 3, 4, 5, 6, 7; but if the solution is suffered to stand for a few hours, and a drop of it is then heated over the fire on a slip of glass, till it begins to concrete about the sides, and then examined, sharp-pointed, solid, figures, bisected by a line cut through the middle, from which they are cut away towards the edges, begin to appear, and shooting forwards (1, 1, 1). These figures are often striated very prettily from the middle line to the edges obliquely (2, 2); and frequently they arise in clutters; and shooting from a centre (3, 3). These figures are a long time in growing; and whilst they are doing so, regular crystals appear forming in several parts of the drop, of the most lovely emerald colour, and reflecting the light from their sides and angles, which are most exactly disposed, and finely polished. No crystals are formed in the middle till the water is nearly evaporated; and then they begin to form hastily, for which reason they must be carefully attended. Their common figure resembles two long /s crosting each other in an angle of about 60°, and shooting branches every way; each of which again protrudes other branches from one, and sometimes from both, its sides; making together an appearance like four leaves of fern conjoined by their stalks (5, 5). Separate clutters of the same sharp pointed figures, as those at the edges of the drop, are also formed in the middle of it (6). Sometimes also they put on another form, like the leaves of dandelion (7). Very beautiful figures are likewise produced by a kind of combination of sharp points and branches (8, 8). All these crystals are of a most beautiful green colour, but deeper or lighter, according to the time of their production. The deepest are constantly produced first, and the paler ones afterwards. Towards the end of the process some circular figures are formed, extremely thin, and so slightly tinged, with green lines radiating from a centre, as to be almost colourless (9, 9). When all seems Crystallization seems in a manner over, bundles of hair-like bodies appear frequently scattered here and there throughout the drop, like those of blue vitriol already described.

4. Alum. The microscopic crystals of this salt prove more or less perfect according to the strength of the solution and the degree of heat employed in making the experiment. The solution of alum, however saturated with the salt, will not be found over strong after standing some days; for in that time many crystals will have formed in it. This separation will often leave the remainder too weak for the purpose; but by holding the vial over, or near the fire, the crystals will again dissolve. After it has stood about half an hour, it may then be used. The drop put on the glass, and properly heated, exhibits commonly at first a dark cloud which appears in motion somewhere near the edge, and runs pretty swiftly both to the right and left, until it is either stopped by the intervention of regular crystals, or else it proceeds both ways at once, till having surrounded the whole drop, the two ends rush together, and join into one (a, a). This cloudy part, which seems to be violently agitated while it is running round, appears on a strict examination to consist of salts, shot into long and very slender lines, much finer than the smallest hair, crossing each other at right angles. As they go along, rows of solid crystals are produced from their internal edges. These are composed of many oblique plain sides (b, b), and which have all a tendency towards the figures of the regular crystals to be described presently. But it frequently happens, that, in some parts of the drop, many minute and circular figures are seen, rising at some little distances from the edge, which enlarging themselves continually, appear at last of a star-like form (c, c). The crystals in the middle seldom appear till the fluid seems almost wholly evaporated; when, on a sudden, many straight lines appear pushing forwards, whose sides or edges are jagged, and from which other similar straight and jagged lines shoot out at right angles with the first. These again have other small ones of the same kind shooting out likewise from themselves, and compose altogether a most beautiful and elegant configuration (D). Each of these lines increasing in breadth towards its end, appears somewhat club-headed (e, e, e). Sometimes, instead of sending branches from their sides, many of these lines rise parallel to each other, resembling a kind of palisade, and having numberless minute transverse lines running between them (F'). But the most wonderful part of all, though not producible without an exact degree of heat and right management, is the dark ground work (G). It consists of an infinity of parallel lines, having others crossing them at right angles, and producing a variety scarcely conceivable from lines disposed in no other manner; the direction of the lines (which are exquisitely straight and delicate) being so frequently and differently changed, that one would think it the result of long study and contrivance. During the time this ground work is framing, certain lucid points present themselves to view most commonly on one side. These grow continually larger, with radiations from a centre, and become star-like figures as before mentioned. Some of them send out long tails, which give them the appearance of comets; and at the end of all, a dark lineation in various directions darts frequently through, and occupies all or most of the spaces between them, making thereby no ill representation, when viewed by candle-light, of a dark sky, illuminated with stars and comets. The regular crystals are often formed in the same drop with the others (/).

5. Borax. If a drop of solution of borax is held too long over the fire, it hardens on the lip of glass in such a manner that no crystals can appear. The best method is to give it a brisk heat for about a second, and then applying it to the microscope, the crystals will quickly form themselves as represented in the figure.

6. Sal ammoniac begins with shooting from the edges great numbers of sharp, but thick and broad, spicule; from whose sides are protruded, as they rise, many others of the same shape, but very short; parallel to each other, but perpendicular to their main stem (1). These spicules arrange themselves in all directions; but for the most part obliquely to the plane from whence they rise, and many are frequently seen parallel to one another (1, 1). As they continue to push forwards, which they do without increasing much in breadth, some shoot from them the small spicule only (2); others divide in a singular manner by the splitting of the stem (3); and others branch out into smaller ramifications (4). Before the middle of the drop begins to shoot, several exceedingly minute bodies may be discerned at the bottom of the fluid. These in a little while rise to the top, and soon distinguish their shape as at (5). Their growth is very quick, and for some time pretty equal; but at last some branch gets the better of the rest, and forms the figure (6). The other branches enlarge but little after this, all the attraction seeming to be lodged in that one that first began to lengthen; and from this, more branches being protruded, and they again protruding others, the whole appears as at (8). It is not uncommon to see in the middle of the drop some crystals, where, instead of the straight stems above described, there is formed a kind of zig-zag, with spicules like those in the other figures (7).

7. Salt of Lead, or salcarum saturni. A little of this salt dissolved in hot water, which it immediately renders milky, after standing a quarter of an hour to subside, is in a fit condition for an examination by the microscope. A drop of it then applied on a lip of glass, and held over the fire to put the particles in action, will be seen forming round the edge a pretty even and regular border of a clear and transparent film or glewy substance (aaaa); which if too sudden and violent a heat be given, runs over the whole area of the drop, and hardens so on the glass as not to be got off without great difficulty. But if a moderate warmth be made use of, which likewise must not be too long continued, this border proceeds only a little way into the drop, with a kind of radiated figure composed of fine lines, or rather bundles of lines, beginning from the centres in the interior edge of the border, and spreading out at nearly equal distances from each other every way, towards the exterior (bbbb). From the same centres are produced afterwards a radiation inwards, composed of parallelograms of different lengths and breadths; from one, and sometimes both the angles of these, are frequently seen shootings so exceedingly slender, that they are... perhaps the best possible representations of a mathematical line. The extremities of the parallelograms are generally cast off at right angles; but they are sometimes also seen oblique (ccc). Centres with the like radii issuing from them, and some of the glutinous matter for their root, are sometimes formed in the drop, entirely detached from the edges; and in these it is very frequent to find a kind of secondary radii proceeding from some of the primary ones; and others from them again to a great number of gradations, forming thereby a very pretty figure (D).

8. Salt of Tin, produces at the edges of the drop a number of octaedra, partly transparent, standing on long necks, at small distances from each other, with angular shoots between them (aa). At the same time, solid and regular opaque cubes will be seen forming themselves in other parts of the drop (bb). In the middle of the same drop, and in several other parts of it, very different figures will also be formed; particularly great numbers of flat, thin, transparent, hexagonal bodies (ccc); some among which are thicker (e), and a few appear more solid, and with fix sloping sides rising to a point, as if cut and polished (d). The figure (f) is composed of two high pyramids united at their base. Some in this kind of form are found truncated at one of their ends, and others at both. Several of the hexagonal bodies may be observed with sloping sides, forming a smooth, triangular, rising plane, whose angles point to three intermediate sides of the hexagon (g).

9. Epsom Salt, begins to shoot from the edge in jagged figures (a). From other parts differently figured crystals extend themselves towards the middle, some of which have fine lines proceeding from both sides of a main stem, in an oblique direction; those on one side shooting upwards in an angle of about 60°, and those on the other downwards in the same obliquity (c, f). Others produce jags from their sides nearly perpendicular to the main stem, thereby forming figures that resemble some species of the polypody (e); but in others the jags are shorter (d). Now and then one of the main stems continues shooting to a considerable length, without any branchings from the sides; but at last tends out two branches from its extremity (g). Sometimes a figure is produced having many fine and minute lines radiating from a centre (b). The last shootings in the middle of the drop (b) are not unlike the frame-work for the flooring or roofing of a house, but with the angles oblique; and sometimes a form of another kind presents itself (i).

10 Scarborough Salt, begins to shoot from the edges: first of all in portions of quadrilateral figures, much resembling those of common salt; but two of their angles, instead of 90°, are about 100°. They shoot in great numbers round the borders of the drop, having their sides as nearly parallel to one another as the figure of the drop will allow; some proceed but a little way, others farther, before they renew the shoot (aa). In some places they appear more pointed and longer (b); and sometimes, instead of the diagonal, one of the sides is seen towards the edge, and the other shooting into the middle (c). The middle crystals (d, e, f) seem to be of the vitriolic kind.

11. Glauber's Salt, produces ramifications from the side of the drop, like the growth of minute plants, but extremely transparent and elegant (c). Some of them, however, begin to shoot from a centre at some distance from the edge, and protrude branches from that centre in a contrary direction (b). Sometimes they shoot from one, and sometimes from more sides of the central point in different varieties (d). Other figures are produced from different parts of the edge of the drop (a, f, e); but the most remarkable and beautiful crystallization forms last of all near the middle of the drop. It is composed of a number of lines proceeding from one another at right angles with transparent spaces and divisions running between them, appearing altogether like streets, alleys, and squares, (gg). When this crystallization begins, it forms with great rapidity, affording the observer a very agreeable entertainment; but its beauty is of very short duration; in a few moments it dissolves and vanishes like melted ice, which renders the drawing of it very difficult.

12 Salt of Jesuits bark. The few shootings which this salt produces at the edge of the drop are of no regular figure (a). The whole area becomes quickly filled with great numbers of rhombi, of different sizes, extremely thin and transparent (b). Some of these enlarge greatly and acquire a considerable thickness, forming themselves into folds of many sides (cc). Near the conclusion some crystals of sea-salt are formed (d, d), and likewise a few odd triangular figures (e).

13. Salt of Liquorice, begins shooting from the edge with a sort of rhombic spicula (a). Some four-branched figures like those of vitriol commonly appear, but moulder away before their ramifications are completed, leaving only their stamina behind (bb). The middle of the drop is usually overgrown with great numbers of parallelograms, some exceedingly transparent, being mere planes; having sometimes one, sometimes more, of the angles canted in such a manner as to produce pentagonal, hexagonal, and other figures. Others have much thickness, and form parallelopipeds or prisms (e). Some of the plane figures now and then protrude an irregular kind of shooting which appears very pretty (d).

14. Salt of Wormwood. The first shootings of this salt from the edges of the drop appear of a considerable thickness in proportion to their length: their sides are deeply and sharply jagged or indented, being made up of many somewhat obtuse angles, and their ends pointed with angles of the same kind (a). But other shoots frequently branch out from these original ones, and they again send forth others, making altogether a very pretty appearance (bb). The crystals of this salt are very different from each other, consisting of squares, rhombi, parallelograms, &c. (c).

15. Salt of Tobacco. If a moderate degree of heat is given to a solution of this salt, its first shootings will be from the edges of the drop, in slender tapering figures, ending with very sharp points, but at considerable distances from one another. Along with these are formed other crystals, nearly of the same kind, but entirely detached, and farther within the drop, having the thicker ends towards the centre of the drop, and the sharp points turned towards its edge (a). When a little more heat has been given, other spicula are produced from the edge, whose ends spread on either side, and then terminate in a point; Crystallization and which have all along their sides triangular pointed crystals, placed alternately so as to represent a zig-zag, with a line drawn through its middle (b). The regular crystals are produced in the middle of the drop, and are either hexagons or rhombi (c). When the moisture is nearly exhaled, there are sometimes seen to shoot from, or rather under the spicule, upon the plane of the glass, a representation of leaves very small at their first appearance, but gradually increasing (d). A violent agitation may be discovered in the fluid by the first magnifier during the whole process; but especially at the beginning, and extremely minute crystals rising from the bottom.

16. Salt of Hartshorn. On the application of a very small degree of heat, salt of hartshorn shoots near the edges of the drop into solid figures somewhat resembling razors or lancets, where the blade turns into the handle by a clasp (d). The crystals of this salt are produced with great velocity, and are somewhat opaque, shooting from the edges of the drop, on both sides a main stem, and with a kind of regularity, rugged branches like those of some sorts of coral (a c). But sometimes, instead of these branches, sharp spicules, some plain, and others jagged, are protruded to a considerable depth on one side only (b). As the fluid exhales, some one of the branching figures generally extends to a great length, producing on one side shoots that are rugged and irregular, and on the other curious regular branches resembling those of some plant (c).

17. Salt of Urine, shoots from the edges of the drop in long parallelograms like nitre (a a). But in other places, along the sides of the drop solid angles are formed, that seem to be the rudiments of common salt (b). Some of the parallelograms increase much in size, and spread themselves in the middle, so as to change their first figure, and become three or four times bigger than the rest; and these have a dividing line that runs through their whole length from end to end, whence issue other short lines at small distances, opposite to one another; all pointing with the same degree of obliquity towards the base (c e). Among these enlarged figures, some few shoot still forward and tapering towards a point, but, before they form one, swell again, and begin as it were anew; and thus they proceed several times before their figure is quite finished (a a). The figures 1, 2, 3, 4, 5, 6, are the regular crystals of this salt when it is allowed to dissolve in the air, and no heat at all is given.

18. Rheum, or the clear liquor which distils from the nostrils when people catch cold, is strongly saturated with salt. A drop of it on a slip of glass will soon crystallize in a beautiful manner, either with or without heat; but if heated to about the warmth of the blood, and then viewed through the microscope, many lucid points will be seen rising and increasing gradually, till their form is known to be quadrangular, with two transparent diagonals crossing each other (d d). These diagonals shoot soon after far beyond the square, protruding other lines at right angles from their sides; and thus they go on to form the most elegant and beautiful crystals (b b, c c). When a drop of rheum is set to crystallize without any heat, instead of branched crystals over the whole area, such are formed only in the middle; but, about the edges, plant-like figures are produced shooting several stems from one point, and Crystallization resembles a kind of sea-moss (E).

19. Camphire, though insoluble in water, dissolves very readily in spirit of wine. A drop of this solution spread upon a slip of glass crystallizes instantly in the beautiful manner represented in the figure.

20. Manna easily dissolves in water, and a drop of the solution is a very pretty object. Its first shootings are radiations from points at the very edge of the drop; the radiating lines seem opaque, but are very slender (a a a). Amongst these arise many minute transparent columns, whose ends grow wider gradually as they extend in length, and terminate at last with some degree of obliquity (b). Some few figures radiating from a centre every way, and circumscribed by an outline, are produced within the drop (d d). But the most surprising and elegant configuration is composed of many clusters of radiations shooting one from another over great part of the drop, and making all together a figure not unlike a certain very beautiful sea-plant (C).

The phenomena of crystallization have much engaged the attention of modern chemists, and a vast number of experiments has been made with a view to determine exactly the different figures assumed by salts in passing from a fluid to a solid form. It does not, however, appear, from all that has yet been done, that any certain variety in rule can be laid down in these cases, as the figure of saline crystals may be varied by the slightest circumstances. Thus, sal ammoniac, when prepared by a mixture of pure volatile alkali with spirit of salt, shoots into crystals resembling feathers; but if, instead of a pure alkali, we make use of one just distilled from bones, and containing a great quantity of animal oil, we shall, after some crystallizations of the feathery kind, obtain the very same salt in the form of cubes.

Such salts as are sublimeable crystallize not only in the aqueous way by solution and evaporation, but also by sublimation; and the difference between the figures of these crystals is often very remarkable. Thus sal ammoniac by sublimation never exhibits any appearance of feathery crystals, but always forms cubes or parallelopipeds. This method of crystallizing salts by sublimation has not as yet been investigated by chemists; nor indeed does the subject seem capable of investigation without much trouble; as the least augmentation of the heat beyond the proper degree would make the crystals run into a solid cake, while a diminution of it would cause them fall into powder. In aqueous solutions, too, the circumstances which determine the shapes of the crystals are innumerable; and the degree of heat, the quantity of salt contained in the liquor, nay, the quantity of liquor itself, and the various constitutions of the atmosphere at the time of crystallization, often occasion such differences as seem quite unaccountable and surprising.

Mr Bergman has given a dissertation on the various forms of crystals; which, he observes, always resemble man's geometrical figures more or less regular. Their variety at first appears infinite; but by a careful examination it will be found, that a great number of crystals, seemingly very different from each other, may be produced by the combination of a small number of original figures, which therefore he thinks may be called primitive. On this principle he explains the formation of the crystalline gems as well as salts; and the results of his observations are as follow.

I. One of the primitive forms is that named by our author *spathaceous*; and these, he says, properly agglutinated, may form the great variety of dissimilar bodies found among crystals.

In the calcareous spar we find a combination of rhombi, whose obtuse angles contain $101\frac{1}{2}$ degrees, and the acute $78\frac{1}{2}$. By a combination of these is formed the calcareous spar, which appears in the form of a tesser or oblique parallelopiped; but by other combinations of the same planes, crystals apparently of the most opposite forms may be generated. Thus, for the formation of an hexadral prism, consisting of six equal and similar parallelograms terminating at both ends in three rhombi which form a solid angle, we have only to suppose a continual addition of rhombi equal, similar, and parallel to the oblique parallelopiped or crystal of the calcareous spar. Thus, suppose the figure ABCDE (fig. 1.) to represent a nucleus of the kind just mentioned, the axis of which passes through the two opposite angles B E; it is evident, that by a continual application of rhombi, such as FG, HI, &c. to both sides of the axis, we shall at last produce the figure A B, fig. 2. and which represents the hexadral prism required. This kind of crystal, our author tells us, belongs chiefly to the stones called *schoerl*, and is therefore called the *schoerlaceous* form. It belongs likewise to some others of the calcareous tribe.

From the schoerlaceous crystal that of the garnet is easily produced by a flopping of the accumulation of the planes as soon as the sides of the prism have acquired a rhomboidal figure. Thus a complete dodecahedron is formed, which is always the figure of the garnet when perfect.

The figure of the garnet is easily changed into another, frequently assumed by the hyacinth, by the regular application of equal and similar rhombi to each of the solid angles, which angles are formed by four planes. The garnet, when complete, has six angles composed of four planes, and eight with three. The formation of this kind of crystal will be understood from an inspection of fig. 3. In this operation the four rhombi are changed into an equal number of oblong hexagons; LHAB into LHhabb; and so of the other rhombi represented by the different letters of the figure.

In some cases the original planes decrease according to a certain law; and this, from whatever cause it may arise, must necessarily change the appearance of the terminating planes, and occasionally either augment or diminish their number. Thus, instead of a prism, we shall have a double pyramid, one tending upwards and the other downwards, as will be easily understood from what has been already said. This is the form assumed by the calcareous crystals commonly called *pig-tooth* by the miners.

If the decreasing series of rhombi is stopped before they vanish ultimately in a point, the formation of truncated pyramids, of which many examples are to be met with in the mineral kingdom, must necessarily take place. In cases of this kind, it is easy to see why the pyramids, if struck in one direction, will break over smoothly and easily, but not in another.

It is not uncommon to find the original crystals themselves imperfect; in which case the large crystals, formed by combining them together, must deviate more or less from the perfect form. Thus, let ABCDEFG (fig. 4) represent the three rhombi which constitute the apex of a perfect schoerlaceous crystal; and let us next suppose the rhombus AG transposed in the direction of the line ab, CG along cd, and EG along ef. Thus, the regular hexagonal figure of the prism ABCDEF will be changed into an irregular one abcdDefG, consisting of nine unequal sides, whose apex is composed of three irregular pentagons, abBGf, cdDcB, and efFGD. The rough tourmalins of Tyrol and Ceylon particularly assume this form, though it sometimes belongs to bodies both of the calcareous and schoerlaceous kind.

Triangular crystals may be supposed to arise from those of the pentagonal kind; it being obvious, that crystals, the periphery of a pentagon, as abBGf, approaches more nearly to a triangle in proportion as the distance between ab and BF grows less; and when these distances vanish entirely, a trigonal prism is formed, terminated by three triangles: if the cutting line ab approach still nearer to the centre G, the form still remains the same.

Let us now suppose, that the garnet crystal, whose shape is represented fig. 5, instead of complete rhombi, the garnet has others accumulated about its axis, whose three external angles are truncated; or, which is the same thing, if the longitudinal margins of the prism be cut by planes parallel to the axis, crystals will be formed, whose shape is represented by the small letters in the figure. Calcareous crystals are sometimes found of this shape; but generally too low, that e nearly coincides with a, c with d, &c. and hence the pentagon abcd becomes almost of a triangular figure, which has been attributed to these crystals by some authors who did not understand their true origin. The pyritaceous crystals sometimes afford instances of this kind complete. Sometimes the garnet consists of 24 sides, by having all the margins truncated; a change which may easily be understood from what has been already mentioned. If the intersection cd of the planes ec and cr fall without the plane BG, a figure of a very different kind will be generated.

Sometimes the hyacinthine crystal assumes the cruciform appearance ABCDEFGHIJKLMNOP, fig. 6. Here the apex is at C, the figure ABCba being all crystal, in the same inclined plane, which is the case with the other three homologous figures. Now, in order to investigate the formation of these crystals, let us suppose the rhombi CO, CP, and CQ, to be completed, which to an eye placed in the high axis C will appear like as many squares situated in the subjacent plane. Thus we may understand the formation of the crystals of granite as well as of the hyacinth. The former may be supposed a quadrangular prism composed of four rhombi, touching one another only at their apices, and terminated at each end by four rhombi meeting at the apex. When this form is a little protracted, or augmented by applying to the apices similar and equal planes, it becomes that of the hyacinth; whence the granite crystal may be called the rudiment of the hyacinth also. The variety here mentioned... mentioned, of hyacinthine crystals, is met with in the Hartz mines. Mr Ehrhart says, that they are of a siliceous, and not of a calcareous, nature.

If planes similar to one another, but dissimilar to the fundamental ones, be added, a vast variety of shapes may be produced, of which it is needful to give more examples at present. Our author appeals to experience for the truth of it; and asserts, that the loose texture of calcareous crystals will clearly show their construction, if carefully and completely broken. The harder crystals can scarcely be broken in such a manner as to show their structure; but the scherds discover it very plainly, and even the garnets show themselves to be composed of laminae.

"Finally (says Mr Bergman), we may add one particular observation concerning prismatic and hexagonal calcareous crystals truncated perpendicularly; such sometimes occur, and they cannot derive their origin, in the manner above described, from the spathaceous particles, and by no other way can hexagonal prisms be generated. What, then, is the cause which destroys their apices? I confess this to be a question which I am wholly unable to answer, unless we may assume an accumulation of planes more and more deficient around the axis. We may hence conclude, that something unusual occurs; as the truncated extremity is opaque, while the rest of the prism is transparent; but the upper hexagonal section is smooth and polished."

On the whole, our author observes, that the greatest varieties may occur in the figures of crystals, though all of them may be generated from those of the spathaceous form, and the substance of all may be ultimately the same; whence we should be induced to put but little confidence in the figure. "If, then, (says he), this test, which undoubtedly is the most remarkable so far as externals reach, is of so little use, of what value can the others be? and with what success can we hope to form a system of mineralogy upon such distinctions? External criteria should certainly not be neglected, but who who trusts implicitly to them deceives himself."

II. From a consideration of the larger lamellae of which crystals are composed, our author naturally proceeds to an investigation of their smaller constituent parts. Here he is of opinion, that the different external appearance of all crystals is owing to varieties in their mechanical elements. A question, however, occurs, Whether those very minute molecules, which may, as it were, be called the laminae of crystals, be naturally possessed of a determinate angular figure, or whether they acquire it by crystallization? In answer to this, he mentions the following facts, which he has had an opportunity of observing himself.

1. "If the small particles which separate from lime-water, when exposed to the air, be inspected with a microscope, they will be found spathaceous.

2. "The greater spathous terrea, when accurately examined, are frequently found with striae running diagonally, such as often appear in saline crystals, by which their internal structure is discovered.

3. "The cubes of common salt not only exhibit diagonal striae, but frequently, upon each side, show squares parallel to the external surface, and gradually decreasing inwards (fig. 7.), by which we discover their internal structure: for every cube is composed of fix quadrangular hollow pyramids, joined by their apices and external surfaces; each of these filled up by others similar, but gradually decreasing, completes the form. By a due degree of evaporation, it is no difficult matter to obtain these pyramids separate and distinct, as in fig. 8. or fix of such, either hollow, or more or less solid, joined round a centre. This is the whole course of the operation from beginning to end. This takes place in the salting vegetable alkali, or sal diglucosus Sylvius; in the crystallized luna cornua; the galena or sulphurated lead; and quadrangular nitre, which is of the spathaceous form, produces a similar congeries of pyramids, and these almost equally distinct with the preceding cubic crystals. A solution of alum, upon evaporation, generally produces solid octaedra; but sometimes also it exhibits hollow pyramids, and upon such of them as are complete, the junctures are very distinctly marked by conspicuous lines.

4. "Sometimes, too, other salts indicate the same construction by visible diagonals. Fig. 9. represents a section of the hexagonal prism formed by Rochelle salt. The arrangement of the internal particles of this salt cannot be known when the crystal is complete; but when it is formed on the bottom of the vessel, as represented fig. 10., the lower side cannot be perfect; and this parallelogram exhibits two diagonals distinctly, as represented fig. 11. This is likewise the case with the salt extracted from human urine, called microcystic salt. Besides, we should observe of the vertical triangles, that they are alternately transparent and opaque in pairs; which plainly points out a difference in the situation of their elements. Some crystals of nitre are also marked with diagonals; a circumstance which in others is generally concealed by the close connection of the particles.

5. "If we examine the hollow pyramid of common salt farther, we shall find it composed of four triangles, and each of these formed of threads parallel to the base; which threads, upon accurate examination, are found to be nothing else than a series of small cubes: Therefore, although the above circumstances seem plainly to point out the formation of all crystals from the union and cohesion of pyramids, whose sides, being different in form and magnitude, occasion the differences of forms; it yet remains uncertain whether the same internal structure takes place in those whose minuteness renders them totally invisible; and whether the primary laminae possess a determinate figure, or are composed by the union of many shapeless particles. We have long known, that the smallest concretions which are visible by a microscope possess a determined figure; but these are compounds. In the mean time, until this veil be removed in some measure at least, we cannot avoid comparing the process of crystallization with the congelation of water.

"While the watery particles are concreting, they exert a double tendency; by one of which they are formed into spiculae, by the other these spiculae are ranged in such a manner with respect to one another as to form angles of 60 degrees: from hence the varieties observed in the particles of snow may be easily explained. The most simple figure is that where fix equal radii diverge from a centre in the angle above mentioned, as in fig. 12. The same angle will be preserved if the extremities of these be joined by right lines; which will also be the case, if each of the tri- angles thus formed be filled with right lines parallel to the base, as in fig. 13.

Let us now suppose the particles which are employed in crystallization endowed with a tendency to form spicules, and these spicules with a tendency to arrange themselves at equal angles of inclination, and we shall have both the triangles and the pyramids composed of them, even although the primary stamina had not a determined figure. As the angles of inclination vary, the triangles and pyramids will also vary; and hence the different forms of crystals will be produced, which may to a certain degree be investigated geometrically, the angles being given.

III. Mr Bergman now considers the various ways in which crystals may be produced; which are, 1. By water; 2. By liquefying heat; 3. By a volatilizing heat.

1. The most common method of obtaining crystals is by means of water; as by this medium saline substances are very readily taken up, and appear again in a solid form when the liquid is properly diminished by evaporation. It is not only when dissolved in water that they acquire determinate forms; this happens also when they are sufficiently attenuated and mixed with it; for substances not soluble in water will remain suspended in it, when, by sufficient division, they have acquired as much surface as makes them approach the specific gravity of the fluid; and it seems very probable, that many of the earths met with in the mineral kingdom, which have a regular form, have coalesced in this way. We must, however, carefully distinguish between mechanical mixture and true solution, even though both should agree in weight. When solid bodies are mechanically mixed with water, they will remain at the bottom of the vessel if laid there in powder, unless diffused by agitation; but soluble substances totally and spontaneously distribute themselves through the menstruum even without any agitation, though this certainly accelerates the solution.

2. Another method of obtaining crystals is by fusion and slow cooling. Thus sulphur, when melted and cooled, shoots into long straws, acquiring at the same time an electrical property: bismuth, zinc, and regulus of antimony, acquire a telescopically appearance; nay, the last of these, when set to cool in a conical mould, becomes stellated, not only on the upper surface or basis of the mass, but along the whole axis. Glases also, when melted and slowly cooled, will sometimes shoot into beautiful crystals. Our author mentions his having sometimes seen the icoria of furnaces, where iron had been melted with the addition of calcareous earth, of a regular prismatic figure; and when crude iron has been melted with lime, he has sometimes also found complete octaedra in the icoria. In large metallic masses, however, the under parts are generally so much pressed by the weight of those above, that they show no signs of crystallization, though beautiful crystals are often formed on the surface of gold, silver, iron, &c.

3. The particles of bodies volatilized by heat, if during cooling they are sufficiently at liberty, often obey the laws of attraction, and form crystals. To this class we may refer those which are condensed from the vapours of regulus of antimony, called the flores argentinus. The galena which is frequently interpered among the copper-ore at Fahlun tends forth a vapour which condenses on the upper strata, forming hollow pyramids, which are the bases of the cubes of galena, entirely similar to those which compose common salt. In the heaps of arsenical ore exposed to the fire at Loes, our author has collected very beautiful crystals, of white, yellow, and red colours, partly tetraedral and partly octahedral. Some of these exhibit hollow pyramids, whose sides consist of threads parallel to the base, and exactly similar to those formed in the moist way. These crystals, when complete, frequently show the junctures of the pyramids very distinctly by straight lines; and by careful examination, we may be able to trace the whole process through its various steps, from the very beginning to the end of the operation.

Prisms of any kind may be formed by the apices of proper pyramids meeting together in a certain number of prisms round the same point. The apex may also be formed by a single pyramid having its vertical angle turned outward. Thus, by adding to the cube ABCD the quadrangular pyramids ABE DCF, we shall have a four-sided prism (fig. 16); and thus, though very seldom, common salt sometimes acquires an apex. If we apply to one or both of the apices of the octahedron ACBD, fig. 17, a hollow pyramid adb, similar and equal to the fundamental figure, we will have a prism of the same kind: alum, however, has never been observed of a prismatic form by our author, though sometimes consisting of octaedra imperfectly joined together, as in fig. 18. Four-sided pyramids may be composed of four tetraedra, and consequently 24 of the same may make up a cube; "and (says our author) it has also a double apex of 32. Thus we have a new construction, which undoubtedly sometimes takes place; for, as I have already said, arsenical crystals sometimes take the tetraedral, sometimes the octahedral, form, which may therefore easily be mutually exchanged.

It is with less facility that hexagonal prisms are formed of such pyramids as have the same number of sides, unless tetraedra be admitted. In fig. 19, four hexagonal and six tetragonal pyramids meet; the former are easily resolved into six and the latter into four tetraedra (fig. 20); 48 of which consequently make up the whole mass, supposing this to be the method followed by nature. I have no doubt that this construction is probable on many accounts; for it requires only the most simple elements, and such as are conformable to the figures of all crystals. That tetraedra adapted to this purpose have sometimes dissimilar and unequal sides, makes not against the supposition: but what is most to the purpose is, that sometimes such tetraedra are employed without the smallest doubt. All these circumstances are of no small weight; but as long as no traces of tetraedra are to be found among the pyramids of common salt, the laws of sound reasoning forbid us to draw any general conclusion. I am perfectly certain that nature does frequently employ pyramids in this operation; it remains for future experiments to determine whether this be always the case."

IV. We come now to consider the ultimate cause of crystallization, concerning which there have been many different theories. Some have been of opinion that there cannot be any crystallization without a saline principle in some degree existing in the crystallizing substance. This opinion, however, is opposed by Mr Bergman on the following grounds:

1. He supposes crystallization to be an effect of attraction. Crystallization; consequently, as all other matters as well as salts are subject to the laws of that attraction, we cannot consider the regular and symmetrical form in which they arrange themselves as peculiar to saline bodies; and hence crystals are also produced by such methods as will sufficiently attenuate and disengage the integrant parts from each other.

2. The more simple that any saline body is, and the more free from any kind of heterogeneous matter, the more difficult it is to reduce it into a crystalline form. Thus the pure acids and caustic alkali cannot be made to assume the form of crystals without the greatest difficulty.

3. The similarity of forms in crystals, Mr Bergman observes, "does not depend upon the acid; as the prismatic and quadrangular nitre are formed from the same acid, though joined indeed to different alkalies. Neither is the basis sufficient to determine the figure; for the vegetable, as well as the mineral alkali, when saturated with marine acid, will produce cubical crystals. The external appearance, therefore, depends on the menstruum and the base jointly. We are not, however, to imagine from thence that there is present a neutral or middle salt whenever the figure of such a one is discoverable; for not the smallest particle of alum is found in nickel or lead when united with nitrous acid, though both these compounds yield octahedral crystals." Here we may again remark, that the figure of crystals depends upon circumstances altogether unknown, of which Dr Eafon, in a paper on this subject in the Manchester Transactions, gives a remarkable instance in gypsum, which is known to be a combination of the vitriolic acid with a calcareous basis; yet this compound is found naturally crystallized in five ways, so very different from each other, that mineralogists have distinguished them by five distinct names, viz. 1. Lapis specularis. 2. Striated gypsum. 3. Gypseous alabaster. 4. Selenites properly called. 5. A gypseous spar frequently adhering to the veins of ore in mountains. All of these, when chemically examined, exhibit precisely the same phenomena, and are really nothing but different crystallizations of the same compound salt.

4. Mr Bergman likewise observes, that there is a great variety in the forms of crystals, though the matter remains the same; of which examples have been given in the calcareous crystals, and in the different kinds of gypsum just mentioned. Among the pyrites also we meet with cubes striated in a very singular manner; the lines of one side being perpendicular to those which distinguish the different sides, as represented fig. 14.; but among these there are likewise tetraedra, octaedra, dodecaedra, and icosaedra, to be met with.

5. A great number of crystals are either totally destitute of any saline matter, or possess it in such a small degree that no experiments hitherto tried have been able to discover the smallest sensible traces of it. Thus mica sometimes shoots into hexangular prisms composed of parallel lamellae, the elementary spiculae of which are disposed as in fig. 15.; gems, schorls, granites, and other earthy bodies, are frequently found figured, though no saline matter can be discovered by analysis; and the same holds good of gold, silver, lead, tin, bitum, and zinc, united with mercury, all of which regular forms, according to the quantity of the mercury.

"If we have recourse (concludes Mr Bergman) to the supposition of an hidden saline substance which cannot be discovered by art, it must surely be unreasonable to attribute to such a principle so great a power as that of arranging the particles in the order necessary for crystallization; a cause, beyond question, unequal to the magnitude of the effect; for how is it possible that a saline matter, the presence of the smallest atom of which cannot be discovered by the most delicate tests, shall in pure water have yet power to effect the icy crystallization with such force as to overcome the strongest obstacles? How can a saline matter, which by no test can be discovered, have power, in an amalgam of gold, to arrange the ponderous particles of both metals in a particular manner? What salt is able to form the stellated regulus of antimony? What the hexagonal lamelle of mica?"

On this subject, we may remark, that whether we affirm or deny a saline principle to be the cause of crystallization, the ultimate power by which it is effected must be equally unknown. A saline principle can make other bodies crystallize along with it only by virtue of the disposition it has of itself to assume a crystalline appearance; and we must therefore seek for the cause of this crystallization of the salt, as well as of the substance with which it is mixed. Mr Bergman, as well as others, have endeavoured to account for this on the principle of attraction; but with little success. Sir Isaac Newton supposes the particles of salt to be diffused through the solvent fluid at equal distances from each other; on which account he concludes that they must come together in regular figures. Mr Bergman considers the particles which form saline substances as endowed with a twofold tendency; one to arrange themselves in spiculae, the other for the spiculae to arrange themselves at certain angles of inclination; and as these angles vary, different forms of crystals must be produced. Both these effects, he thinks, may be owing to the same cause, viz. a mutual attraction between the particles; which, according to the various shapes and particular figures of the atoms, at one time arranges them in the form of spicular, and again connects the spiculae already formed under different angles of inclination.

This seems to be much the same with what other chemists understand by the polarity of the saline particles, by which they are arranged in certain directions. All this, however, is totally insufficient to explain the phenomenon. If, according to Sir Isaac Newton's supposition, the particles were brought together by a general attraction, after being placed at equal distances by the solvent for some time, we must expect to find all kinds of salts crystallized in the same manner, or rather running into one solid lump. The arrangement of the particles, or their tendency to arrangement, assigned by Mr Bergman as a cause, is only explaining the phenomenon by itself; for it is the cause of this tendency which is the point in question. Now, that the attraction of the saline particles to each other cannot be the cause of crystalline arrangements, is evident from the following considerations: 1. The crystals of every kind of salt contain water as an essential part of their composition; and if deprived of this, they lose their crystalline form entirely, and fall into powder. It is plain, therefore, that the saline particles attract not only one another, but some part of Crystallization

the water which dissolves them; whence it seems probable that the processes of crystallization and vegetation are analogous to each other. This is likewise confirmed by the many curious vegetations of salts known by the name of efflorescences. These cannot be owing merely to attraction; because they frequently protrude from a large saline mass, in which they ought rather to be detained by the attraction of the rest. Thus, if a quantity of the residuum of Glauber's spirit of nitre distilled with a large proportion of vitriolic acid, be exposed to a moist air, beautiful ramifications somewhat resembling shrubs will sometimes shoot out to the length of more than an inch. This surely cannot be the effect of attraction; but rather of some repulsive power by which the particles of the large mass at first tend to separate from one another. 2. Attraction, in such a manner as would dispose the particles into certain determinate forms, cannot take place where they are all homogeneous, which must be the case with metals; all of which are capable of forming crystals when slowly cooled; such crystallizations, therefore, must be produced by some other power.

Mr Bergman considers the congelation of water as a species of crystallization; and in order to prove the similitude, he takes notice, that it is by means of the matter of heat that this element becomes fluid. He observes likewise, that salts, in the act of crystallizing, part with heat as water does in the act of being converted into ice. It would seem, therefore, that the particles were arranged in certain forms by the action of the heat when passing from a latent to a sensible state. From a late experiment, it would seem that the electric fluid was principally concerned. This was first discovered by Lichtenberg, and consists only in sprinkling powdered rosin upon an electrophorus, which in certain circumstances arranges itself into stars with radii similar to those of the crystals of snow. See Electricity.