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much esteemed as articles of food. Those caught in the waters of Chesapeake bay are preferred in their soft state, and are regarded as great luxuries. In Europe, where they are also a favorite article of food, they are preferred after the new shell has become hard.

CRUVEILHIER, JEAN, a French physician, born at Limoges, Feb. 9, 1791. He studied under Boyer and Dupuytren, was a professor at Montpellier, and became attached to the faculty of Paris in 1825. In 1826 he reorganized the anatomical society, which in 1769 had been founded by Dupuytren. The first part of his great work on pathological anatomy (completed in 1842) appeared in 1829. He has also written on descriptive anatomy, and on the anatomy of the nervous system.

brittle texture. Its hardness is 2.5; sp. gr. 3. It cleaves in 3 directions, 2 of which are rectangular. It occurs in veins in gneiss with pyrites and galena, and has been found at Arksut, in West Greenland, and at Miask, in the Urals. At the former place it constitutes a mass 80 feet thick and 300 feet long, included between layers of gneiss, and associated with argentiferous galena and copper and iron pyrites.-See a paper communicated to the geological society by Mr. J. W. Taylor, 1856.

CRYPTO-CALVINISTS, a name given in the latter half of the 16th century to the favorers of Calvinism in Saxony, on account of their secret adhesion to the doctrines of Geneva.

CRYSTAL PALACE, the name of the structure in which the great exhibition of works of CRUVELLI, SOPHIE (BARONESS VIGIER), a industry of all nations was held in London, in German vocalist, born in Bielefeld, Prussia, Aug. 1851. It was erected after a design of Mr. (af29, 1830. Her family name is Cruwell, which terward Sir) Joseph Paxton, on the S. side of she Italianized into Cruvelli. Her musical edu- Hyde park, opposite Prince's gate, and composed cation was acquired in Paris, but she made her mainly of glass and iron, with its floors of wood. début upon the German stage, to which her rep- Its length was 1,851 feet; width in its broadutation was for several years confined. She est part, 456 feet; area, 21 acres. It contained afterward sang in Milan, Venice, and other Ital- illustrations of modern industry from about ian cities. In 1852 she made her first appear- 17,000 exhibitors, was opened May 1, 1851, ance in London at the queen's theatre, then visited by over 6,000,000 people, closed Oct. 11, under the direction of Lumley, and was success- 1851, and the building taken down shortly afterful. Her voice, a soprano of great strength and ward. A new and permanent crystal palace has purity, her dramatic powers, youth, beauty, and since been erected (opened June 10, 1854), at a commanding person, created an extraordinary cost of about £1,450,000, 8 m. from London, enthusiasm in her favor, and both in London on Penge hill, near Sydenham, with splendid and in Paris, which she visited in the same year, gardens and waterworks, and arrangements for she became perhaps the most popular singer of musical and other public entertainments, and the day. The constant demands upon her voice containing, beside industrial exhibitions, an exwere beginning to impair its quality, when in the tensive museum of ancient and medieval art latter part of 1856 she was married to the baron and of minerals, representations of antediluvian Vigier, since which time she has not appeared animals, specimens in all branches of zoology upon the stage. Ahmed Pasha, son of Mehemet and botany, and other departments of science.Ali, lately left her a fortune of 1,000,000 francs, Crystal palaces, in imitation of that of London, and an almost equal sum in diamonds. and for the same purpose of universal industrial exhibition, were opened in New York, July 14, 1853, in Munich in 1854, and in Paris, May 1, 1855. The New York crystal palace was situated in Reservoir square, and designed by Messrs. Carstensen and Gildemeister. The main building covered 173,000 square feet, galleries included, with an additional building of 33,000 square feet. It was composed of 45,000 square feet of glass, 1,200 tons of cast and 300 tons of wrought iron, and surmounted by a dome. This beautiful structure was destroyed by fire, Oct. 5, 1858.

CRUZ, JUANA INEZ DE LA, a Mexican poetess, born near the city of Mexico in 1651, died April 17, 1695. She was very quick at acquiring knowledge, and was able to speak and write Latin with fluency. She was only about 17 when she resolved to become a nun, and entered the convent of St. Jerome at Mexico, where she remained until her death. During her life she was called the "tenth_muse," and in Spain, where she is known as the "nun of Mexico," her poems have been very popular. Her writings have been collected in 3 vols. 4to.

CRYOLITE (Gr. «pvos, ice, and Xoos, stone), a mineral so named from its fusibility in the flame of a candle. It is a compound of sodium, fluorine, and aluminum, and is used for the preparation of the new metal aluminum. Large quantities are imported into England for this purpose from Greenland, where it was discovered by a missionary and carried many years ago to Copenhagen. It was supposed to be sulphate of barytes, until examined by Abilgard, who found it to contain fluoric acid. Klaproth afterward detected soda. It is a snow-white mineral, partially transparent, of vitreous lustre and

CRYSTALLINE LENS, a lenticular transparent body, placed between the aqueous and vitreous humors of the vertebrate eye, at about its anterior third; it is about 4 lines in diame ter and 2 in thickness in man, and its axis corresponds to the centre of the pupil. The lens is flat in proportion to the density of the medium in which the eye is habitually placed, being very flat in birds of the highest flight, and very convex in aquatic mammals and diving birds; in fishes it is almost spherical. This most important refracting structure of the eye is imbedded in the anterior portion of the vitreous

humor, and is enclosed in a membranous capsule, to which it is prevented from adhering by the "liquid of Morgagni." Its structure is complicated, but it consists, when fully formed, of fibres arranged side by side, and united into lamina by serrations of their edges; the fibres originate in cells; the vessels are confined to the capsule, and are derived from the central artery of the retina; when hardened in spirit, it may be split into 3 sections, composed of concentric laminæ; it is made up of 58 parts of water, and 42 per cent. of soluble albumen; the central parts are the densest, and this property increases with age. Beside its refractive power, necessary for distinct vision, it is generally believed that a change in its place, by means of the ciliary muscle and the erectile tissue of the surrounding ciliary processes, is the mechanism by which the eye is adapted to distinct vision at varying distances; beside the anatomical arrangement of the parts, this view is rendered more probable by the development of this muscle in predaceous birds which have a great range of vision, and by the loss of this power of adaptation when the lens of the human eye is removed or displaced in the operation for cataract. For the diseases of the lens and its capsule and their treatment, the reader is referred to the article CATARACT.

CRYSTALLOGRAPHY, the science of form and structure in the inorganic kingdom of nature. In the organic kingdoms, the animal and vegetable, each species has a specific form and structure evolved from the germ according to a law of development or growth. In the inorganic kingdom also, which includes all inorganic substances, whether natural or artificial, a specific form and structure belong to each species, and the facts and principles involved therein constitute the science of crystallography. The forms are called crystals; so that animals, plants, and crystals are the 3 kinds of structures char acterizing species in nature. As the qualities of crystals depend directly on the forces of the ultimate molecules or particles of matter, crystallography is one of the fundamental departments of molecular physics, and that particular branch which includes cohesive attraction. Cohesive attraction in solidification is nothing but crystallogenic attraction, for all solidification in inorganic nature is crystallization. The solidification of water, making ice, is a turning it into a mass of crystals; and the word crystal is appropriately derived from the Greek κpuσTalλos, ice. The solidification of the vapors of the atmosphere fills the air with snow-flakes, which are congeries of crystals or crystalline grains. Solid lava, granite, marble, iron, spermaceti, and indeed all the solid materials of the inorganic globe, are crystalline in grain; so that there is no exaggeration in the statement that the earth has crystal foundations. The elements and their inorganic compounds are, in their perfection, crystals. Carbon crystallized is the diamond. Boron is little less brilliant or hard; and could we reduce oxygen to the solid

state, it would probably (as we may infer from its compounds) have no rival among nature's gems. Alumina is the constituent of the sapphire and ruby, and silica of quartz crystals. Magnesia also has its lustrous forms. The metals all crystallize. Silica and alumina combined, along with one or more of the alkalies or earths, make a large part of the mineral ingredients of the globe, its tourmaline, garnet, feldspar, and many other species, all splendent in their finer crystallizations; and limestone, one of the homeliest of all the earth's materials, as we ordinarily see it, occurs in a multitude of brilliant forms, exceeding in variety every other mineral species. The general principles in the science of crystallography are the following: I. A crystal is bounded by plane surfaces, symmetrically arranged about certain imaginary lines, called axes. II. A crystal has an internal structure which is directly related to the external form, and the axial lines or directions. This internal structure is most obviously exhibited in the property called cleavage. Črystals having this property split or cleave in certain directions, either parallel to one or more of the axial planes, or to diagonals to them; and these directions are fixed in each species. In some cases, cleavage may be effected by the fingers, as with mica and gypsum; in others, by means of a hammer with or without the aid of a knifeblade, as in galena, calcite, fluor spar; in others, it is indistinguishable, as in quartz and ice. In all species, whether there be cleavage or not, crystals often show a regular internal structure through the arrangement of impurities, or by internal lines, striations, or imperfections; and, when there has been a partial solution or erosion of the crystal, there is often a development of new lines and planes, indicating that the general symmetry of the exterior belongs to the whole interior. III. The various forms of crystals belong mathematically to 6 systems of crystallization: the monometric, dimetric, trimetric, monoclinic, triclinic, and hexagonal. The greater part of the crystalline forms may be regarded as based on 4-sided prisms, square, rectangular, rhombic, or rhomboidal in base; and the rest, on the regular 6-sided prism. The 4-sided prisms are either right prisms (erect) or oblique (inclined). Any such 4-sided prism may have 3 fundamental axes crossing at the centre, 1 vertical axis connecting the centres of the opposite bases, and 2 lateral, connecting the centres of either the opposite lateral faces, or the opposite lateral edges. The 6-sided prism is right, and has 4 axes, 1 vertical and 3 lateral. In the right 4-sided prisms, the intersections of the axes are all at right angles; in the oblique, one or all of them are oblique angles. A. Right or orthometric systems. 1. Monometric system: the 3 axes equal, and thus of one kind. The system is named from the Greek povos, one, and μerpov, measure. cube, for example, has 3 equal axes with rectangular intersections; the axes connect the centres of the opposite faces. The regular oc

The

tahedron, rhombic dodecahedron, and tetrahedron, are other solids of this system. The octahedron is contained under 8 equal equilateral triangles, and is like two 4-sided pyramids placed base to base. The lines connecting the apices of the solid angles are the axes; as in the cube, they are 3 in number, equal in length, and rectangular in their intersections. The rhombic dodecahedron is contained under 12 equal rhombic faces, and is an equilateral solid like the cube and octahedron. All the forms of the monometric system are thus equilateral, and every way symmetrical. No one of the axes is distinguished as the vertical. Examples: garnet, diamond, gold, lead, alum. 2. Dimetric system: the vertical axis unequal to the lateral, and the lateral equal; the axes thus of 2 kinds. The dimetric system is named from the Greek dus, twice, and μerpov, measure. The square prism is an example. As the base is a square, the lateral axes, whether connecting the centres of opposite lateral faces or edges, are equal; while the vertical may be of any length, longer or shorter than the lateral. Under this system, there are square octahedrons, equilateral 8-sided prisms, and 8-sided double pyramids, beside other forms. Examples: idocrase, zircon, tin. 3. Trimetric system: the vertical axis unequal to the lateral, and the lateral also unequal, or, in other words, the 3 unequal. The trimetric system is named from the Greek rpis, 3 times, and μerpov, measure. In the rectangular prism (a right prism with a rectangular base), the 3 axes are lines connecting the centres of opposite faces, and are unequal. In the right rhombic prism the vertical axis connects the centres of the bases, and the lateral, the centres of the opposite lateral edges. They have the same relations as in the rectangular prism; that is, they are rectangular in their intersections and unequal. Of the 2 lateral axes in this system, the longer is called the macrodiagonal, and the shorter the brachydiagonal. Examples: sulphur, heavy spar, epsom salt, topaz. B. Oblique or clinometric systems. 4. Monoclinic system: one only of the intersections oblique. This system is named from the Greek povos, one, and kλvw, to incline. If we take a model with 3 unequal axes arranged as in the trimetric system, and then make the vertical axis oblique to one of the lateral, we change the system into the monoclinic. While the right rhombic prism belongs to the former, the oblique rhombic prism, and other related forms, belong to the latter. Examples: borax, glauber salt, sugar, pyroxene. 5. Triclinic system: all the 3 intersections oblique. The system is named from rpis, 3 times, and kλw, to incline. The forms are oblique prisms contained under rhomboidal faces. The axes, whether connecting the centres of opposite faces or of opposite edges, are unequal, and all the intersections are oblique. Examples: blue vitriol, axinite. C. The axes 4 in number. 6. Hexagonal system. In the regular hexagonal prism, the vertical axis connects the centres of the bases, and the 3 lateral, the centres of the

opposite lateral faces or edges. Examples: bery! or emerald, apatite. Beside the hexagonal prism, the system includes the rhombohedron and its derivative forms, inasmuch as the symmetry of these forms is hexagonal. The rhombohedron is a solid, bounded like the cube by 6 equal faces equally inclined to one another, but those faces are rhombic, and the inclinations are oblique. The relations of the rhombohedron may be explained by comparison with a cube. If a cube be placed on one solid angle, with the diagonal from that angle to the opposite solid angle vertical, it will have 3 edges and 3 faces meeting at the top angle, and as many edges and faces, alternate in position, meeting at the opposite angle below; while the remaining 6 edges will form a zigzag around the vertical diagonal; these 6 edges might be called the lateral edges, and the others the terminal. The cube, in this position, is in fact a rhombohedron of 90°. If the cube were elastic, so that the angles could be varied, a little pressure would make it a rhombohedron of an angle greater than 90°, that is, an obtuse rhombohedron; or by drawing it out, it would become a rhombohedron of an angle less than 90°, or an acute rhombohedron. The diagonal here taken as the vertical axis, is the true vertical axis of the rhombohedron; and as there are 6 lateral edges situated symmetrically around it, there are 3 lateral axes crossing at angles of 60°, as in the regular hexagonal prism. Examples: calcite, sapphire, quartz. IV. The relative values of the axes in any species are constant. In the monometric system, the axes are equal, and the axial ratio is, therefore, that of unity. Calling the 3 axes a, b, c, it is in all monometric species a:b:c: 1:1:1. In the dimetric system the vertical axis (a) is unequal to the 2 lateral (b, c). Calling the lateral 1, the vertical may be of any length greater or less than 1; and whatever the value, it is constant for the species. Thus in zircon, the value of a is 0.6407, and the axial ratio is a b c = 0.6407: 1: 1. In calomel, the ratio is 1.232: 1: 1. In the trimetric system, the 3 axes are unequal, but the ratio is constant for each species, as in the dimetric. Taking the shorter lateral axis (b) as unity, the ratio for sulphur is a : b: c = 2.344: 1: 1.23; for heavy spar, 1.6107: 1: 1.2276. In the monoclinic system, the obliquity of the prism is a constant, as well as the relative values of the axes. In glauber salt, this inclination is 72° 15', and the ratio of the axes is a: b: c = 1.1089: 1:0.8962. In the hexagonal system, as in the dimetric, the vertical (a) is the varying axis; but its value is constant for each species. In quartz, a: b:c: d = 1.0999:1:1:1; in calcite, 0.8543:1:1:1. In other words, taking the lateral axes at unity, the vertical (a) in calcite is 0.8543. Crystallography owes its mathematical basis to this law. The constancy of angle for each species, stated in § II., is here involved. V. Each species, while having a constant axial ratio, may still crystallize in a variety of forms. Thus the diamond, which is mono

metric, occurs in octahedrons, in dodecahedrons, and in solids like octahedrons, but having low pyramids of 3 or 6 faces in place of each octahedral face (called tris-octahedrons and hex-octahedrons), and in various combinations of these forms. So dimetric species, as idocrase, may occur in simple square prisms, or in square prisms with the lateral edges truncated or bevelled, or with different planes on the basal edges or angles, or in 8-sided prisms, or in square octahedrons, &c. In the species calcite, the number of derivative forms amounts to several hundreds. This simple fact shows that while cohesive attraction in calcite, for example, sometimes produces the fundamental rhombohedron, it may undergo changes of condition so as to produce other forms, and as many such changes as are necessary to give rise to all the various occurring forms of the species, with only this limitation, that they are all based on the fundamental axial ratio, 0.8543: 1. VI. In all cases of derivative or secondary forms, either (1) all similar parts (parts similarly placed with reference to the axes) are modified alike, or (2) only half, alternate in position, are modified alike. This law may be explained by reference to a square prism. In this prism there are 2 sets of edges, the basal and lateral; the 2 sets are unlike, that is, are unequal, and included by different planes. One set may therefore be modified by planes when the other is not; moreover, when one basal edge has a plane on it, all the others will have the same plane, that is, a plane inclined at the same angle to the base; or if one has a dozen different planes, all the others will have the same dozen. Again, if a lateral edge is replaced by one plane, that plane will be equally inclined to the lateral planes, because those planes (or, what is equivalent, the lateral axes) are equal; and in addition, all the lateral edges will have the same plane. In a cube, the 12 edges are all equal and similar; and hence, if one of them has a plane on it, there will be a similar plane on each of the 12. Hence, we may distinguish a cube, modified on the edges, however much it may be distorted, by finding the same planes on all the 12 edges of the solid. The 8 angles of a cube are similar, and hence they will all have similar modifica tions. This remark applies also to the 8 angles of a square prism. The square prism and cube differ in this, that in the cube, when there is one plane on each angle, that plane will incline equally to each of the 3 faces adjoining, because these faces are equal; while in the square prism, the plane will incline equally to the 2 lateral planes and at a different angle to the base. This general law, "similar parts similarly modified," is in accordance with what complete symmetry would require. The exception mentioned, of half the parts modified without the other half, is exemplified in boracite, in which half of the 8 solid angles of the cube have planes unlike those of the other half-a mode of modification that gives rise to the tetrahedron and related forms; in tourmaline, in which the planes at

one end of the crystal differ from those at the other; and in pyrites, in which on each edge there is only one plane out of a pair of bevelling planes. All such forms are said to be hemihedral (Gr. ov, half, and dpa, face), while the former are said to be holohedral (from oλos, all, and édpa). Many hemihedral crystals, when undergoing a change of temperature, have opposite electric poles developed in the parts dissimilarly modified. VII. The derivative forms, under any species, are related to one another by simple multiples of the axial ratios. In calcite, the fundamental rhombohedron has the axial ratio just mentioned, 0.8543: 1, that is, a = 0.8543. There are a number of derivative rhombohedrons among the crystalline forms of this species; one has the vertical axis a; another a; others a, a, 2a, 3a, 4a, and so on, by simple multiples of the vertical axis of the fundamental form. So in zircon, of the dimetric system, while a (vertical axis)=0.6407, the lateral being unity, there is one derivative octahedron with the axes a 1: 1; another, 2a: 1:1; another, 3a: 1:1; and there are 3 other forms (8-sided pyramids) whose axes are severally 3a 3:1; 4a: 4: 1; 5a : 5: 1; or writing out the value of a, they are 1.9221: 3: 1; 2.5628: 4:1; 3.2035:51. It is obvious that if an octahedron of zircon have the vertical axis 2a (or the whole ratio, 2a: 1:1), its interfacial angles may be calculated, the value of a being known = 0.6407. The calculation is simpler still, provided the basal angle of the pyramid, a : 1:1, be known; for the tangents of half the basal angles will vary as the vertical axes, or, in this case, will be as 1: 2. Moreover, if the angles of the octahedron, a : 1: 1, be known from measurement, the value of the axis a may be thence calculated. The derivative forms thus enable us to ascertain the dimensions of the axes of crystals. Crystals are often much distorted, and cubes are thus changed to square prisms, rectangular prisms, and other forms; and prismatic and octahedral crystals are liable to similar distortions. But the distortions seldom affect the angles. These facts still further illustrate the mathematical basis of crystallography. They also show that the modifications which cohesive attraction (or, what is the same, crystallogenic attraction) undergoes in order to produce the various derivative forms of any substance, take place according to a law of simple ratios. VIII. The physical characters of crystals have a direct relation to the forms and axes. Cleavage, hardness, color, elasticity, expansibility, and conduction of heat, differ in the direction of different axial lines, and are alike in the direction of like axes. The difference of color between light transmitted along the vertical and lateral axes of a prism is often very marked, and the name dichroism (Gr. dis, twice, xpoa, color), or the more general term pleochroism, is applied to the property. The hardness often differs sensibly on the terminal and lateral planes of a prism, and also, though less sensibly, in other different directions.

IX. The angles of the crystals of a species, though essentially constant, are subject to small variations. The unequal expansion of inequiaxial crystals along different axial directions, alluded to under the last head, occasions a change of angle with a change of temperature; other small variations arise from impurities, or isomorphous substitutions, or irregularities of crystallization. There are also many instances of curved crystallizations which are exceptions to the general rule. A familiar example of curving forms is afforded by ice or frost as it covers windows and pavements. Diamonds have usually convex instead of plane faces. Rhombohedrons of dolomite and spathic iron often have a curving twist; half the faces are concave and those opposite convex. Other imperfections arise from an oscillating tendency to the formation of 2 planes, ending in making a striated curving surface. Thus 9-sided prisms of tourmaline are reduced to 3-sided prisms with the faces convex. X. While simple crystals are the normal result in crystallization, twins or compound crystals are sometimes formed. The 6-rayed stars of snow and the arrow-head forms of gypsum are examples of compound crystals. In the stars of snow there are 3 crystals crossing at middle; in the arrow-shaped crystal of gypsum, 2 crystals are united so as to form a regular twin. Many of these twin crystals may be imitated by cutting a model of an oblique prism in two vertically through the middle, and then inverting one part on the other and uniting again the cut surfaces. In such a twin, the top of one half of the crystal is really at the bottom, and the bottom of the same half at the top. To explain its formation, it is necessary to suppose that the nucleal or first particle of the crystal was a double molecule made up of 2 molecules, in which one was thus inverted on the other. Such twins, as well as other facts, prove that molecules have a top and bottom, or, in more correct language, polarity, one end being positive and the other negative, this being the only kind of distinction of top and bottom which we can suppose. Axial lines or directions of attraction are in fact necessarily polar, if it be true, as is supposed, that molecular force of whatever kind is polar. In the case of the compound crystal of snow, the nucleal particle must have consisted of 3 or 6 molecules combined. Those prismatic substances are compounded in this way which have the angles of the prism near 60° and 120°, and for the reason that 3 times 120°, or 6 times 60°, equal 360°, or the complete circle. In a case where this angle is nearly of 360° (as in marcasite), the twins consist of 5 united crystals. In compound crystals of another kind, the composition is produced after the crystal has begun to form, instead of in the first or nucleal particle. A prism, as in rutile, after elongating for a while, takes a sudden bend at each extremity at a particular angle, depending on the values of the

axes.

In another case, as albite, which is triclinic, a flat prism begins as a thin plate; then

a reversed layer is added to either surface; then another like the first plate; then another reversed; and so on, until the crystal consists of a large number of lamellæ, the alternate of them reversed in position, yet all as solidly united as if a simple crystal. Such a kind of composition may be indicated on the surface in a series of fine striations or furrows, each due to a new plane of composition; and they are frequently so fine as to be detected only by means of a magnifying glass. This mode of twin is additional proof of the polarity of the crystallogenic molecule. If there were not some inherent difference in the extremities or opposite sides of the molecules or their axes, which is equivalent to polarity, there could not be this series of reversions during the formation of the crystal. External electric or other influence may be the cause of the reversion. XI. While simple and twin crystals form when circumstances are favorable, in other cases the solidifying material becomes an aggregate of crystalline particles. Regular crystals often require for their formation the nicest adjustment of circumstances as to supply of material, temperature, rate of cooling or evaporation, &c.; and hence imperfect crystallizations are far the most common in nature. A weak solution spread over a surface may produce a deposit of minute crystals, which, if the solution continues to be gradually supplied, will slowly lengthen, and produce a fibrous or columnar structure. In other cases, whether crystallization take place from solution or fusion or otherwise, the result is only a confused aggregate of grains, or the granular structure. Under these circumstances, the tendency in force to exert influence radially from any centre where it is developed or begins action, often leads to concentric or radiated aggregations, or concretions. The point which first commences to solidify, or else a foreign body, as a fragment of wood or a shell, becomes such a centre; and aggregation goes on around it until the concretion has reached its limits. Basalt and trap rocks which have been formed from fusion are often divided into columns, and the columns have concave and convex surfaces at the joints or cross fractures, proving that they are concretionary in origin. The centre or axis of each column is the centre of the concretionary structure, and therefore it was the position of the first solidifying points in the cooling mass. The distance therefore between the initial solidifying points determines in any case the size of the columns; and as the columns are larger, the thicker the cooling mass, the distance is greater, the slower the cooling. The cracks separating the columns are supposed to be owing to contraction on cooling. XII. The system of crystallization of a given substance sometimes undergoes a total change, owing to external causes. Carbonate of lime ordinarily crystallizes in rhombohedrons, and is then called calcite; but in certain cases it crystallizes in trimetric prisms, and it is then called aragonite. The aragonite appears to

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