on a filter, washed and dried, but allowed to remain in the liquid, they gradually change after the liquid has cooled, and in about a week are completely converted into calcspar; in pure water this transformation goes on much more slowly. When carbonate of lime is fused under strong pressure, as in Hall's method, it invariably crystallizes on cooling in the form of calcspar. A tolerably large crystal of arragonite falls to pieces at a low red heat without losing weight, and forms a white opaque coarse powder, having a sp. gr. of only 2.706. Hence it follows that carbonate of lime crystallizes at about 100° C. in the form of arragonite, but at a lower temperature, or at a red heat, in the form of calcspar. The arragonite which occurs in the caverns of volcanic districts must have been formed by infiltration while the mass was yet warm. According to these experiments, carbonate of strontia is not necessary to the formation of arragonite; indeed, many specimens of natural arragonite are free from it. Since, however, arragonite and carbonate of strontia crystallize in the same form, the latter may often become mixed with crystals of the former. If chloride of strontium be decomposed by carbonate of ammonia in the cold, carbonate of strontia precipitates in an indeterminate form, but assumes the form of arragonite on being heated. Chloride of barium and chloride of lead, treated with carbonate of ammonia in the cold, yield precipitates of carbonate of baryta and carbonate of lead in the form of arragonite. The carbonates of baryta, strontia, and lead, cannot be made to assume the form of calcspar. (H. Rose, Pogg., 47, 353.)

Nitrate of potash usually crystallizes in prisms of the form of arragonite: but if a drop of the aqueous solution of this salt be left to evaporate on a glass plate and the crystallization observed under the microscope, it will be found that side by side with the prismatic crystals at the edges of the drop, a number of obtuse rhomboids of the calcspar form are produced just like those in which nitrate of soda crystallizes. As the two kinds of crystals increase in size and approach one another, the rhomboids become rounded off and dissolve, because they are more easily soluble than the others, while the arragonite-shaped prisms go on increasing in size. When the two kinds of crystals come into immediate contact, the rhomboidal ones instantly become turbid, acquire an uneven surface, and after a short time throw out prisms from all parts of their surfaces. Contact with foreign bodies also brings about the transformation of the rhomboids while they are wet. If the drops are so shallow that the liquid dries round the rhomboids before they are disturbed, they will remain for weeks without disintegrating, and bear gentle pressure with foreign bodies without alteration; but stronger pressure, or scratching, or the mere contact of a prismatic crystal of saltpetre causes them to change, a delicate film. proceeding, as it were, from the point of contact and spreading itself over their surfaces; they then behave towards foreign bodies like a heap of fine dust, but retain their transparency. The rhombohedrons are also transformed without alteration of external appearance when heated considerably above 100° C.: they then become much harder, because the fine powder first produced bakes together into prismatic crystals. A hot solution of saltpetre yields when slightly cooled nothing but prismatic crystals; but at 10° C., (+ 14° Fah.) prismatic and rhombic crystals appear together; if alcohol be added, the latter are formed most abundantly; the addition of potash, nitric acid or nitrate of soda produces no alteration. (Frankenheim, Pogg. 40, 447; also J. pr. Chem. 16, 1.)

Sal-ammoniac which commonly crystallizes in regular octohedrons

appears at higher temperatures to assume forms belonging to the right prismatic system. (Frankenheim, J. pr. Chem. 16, 3.)

Iodide of potassium, which usually crystallizes in cubes, likewise forms square prisms with truncated summits (fig. 32) which cannot be regarded as cubo-octohedrons because their e-faces make an angle of 120' with p and of about 150° with q. (Kane, Phil. Mag. J. 16, 222.)

Chromate of lead occurs in red lead spar in the form of oblique rhombic prisms: but in chromate of lead from the Bannat the same substance presents forms belonging to the square prismatic system, having the same angles as molybdate of lead. (Johnston, Phil. Mag. J. 12, 387.)

Sulphate of nickel (Ni O, SO3, 7Aq) crystallizes (a) below 15° C. (59° Fah.) in right rhombic prisms (fig. 73); (b) between 15° and 20° C. (59° and 68° Fah.) in acute octohedrons with square bases (fig. 36, 37); and (c) above 30° C. (86° Fah.) in oblique rhombic prisms, also in forms belonging to the right, square, and oblique prismatic systems: it is therefore trimorphous. The right rhombic prisms (a) when exposed to sunlight for a few days, neither liquefy nor lose their form or water of crystallization, but when broken are found to be made up of square-based octohedrons often several lines in length.

The following salts isomorphous with sulphate of nickel have hitherto been obtained in only two out of the three forms just mentioned. Sulphate of zinc (Zn O, SO3, 7Aq) crystallizes below 52° C. (125-6° Fah.); in form a, below 52° C., as observed by Haidingen in less transparent crystals like c; if crystal a be heated in oil or in a glass tube above 52° C, it becomes soft at certain points without losing water excepting any that may be adhering to it mechanically, and from these points bundles of milk-white crystals c shoot out towards the inside of the transparent crystal until the whole is completely transformed. If the crystals obtained above 52° be slowly cooled after drying they remain tolerably clear; but when cooled quickly before drying they become opaque, and when broken are often found to consist of an aggregate of crystals a, these having been first formed in the adhering mother-liquid and subsequently extended through the crystals already formed. Sulphate of magnesia (Mg O, SO3, 7Aq) like sulphate of zinc, yields right rhombic prisms a below 52°, and oblique rhombic prisms c above 52°; and the crystals a when heated above 52 are immediately converted into an opaque aggregate of crystals c, which proceed from the surface of the crystals and meet in the middle. niate of zinc (Zn O, Se 03, 7Aq) crystallizes at a lower temperature like a, at a higher temperature like b, and the crystals a undergo an alteration of internal structure when exposed to sunshine. (Mitscherlich, Pogg. 6, 19 and 12, 144.)

Sele

Acid phosphate of soda (Nu O, PO3, 4Aq) crystallizes in two series of forms (fig. 61-64) both of which belong to the right prismatic system but have incompatible angles. (Mitscherlich.)

Vesuvian (fig 39) and garnet (fig. 3) consist of the same chemical compound, crystallized in forms which belong to the square prismatic and regular systems respectively (Comp. page 106).

Karsten likewise regards as dimorphous compounds: Augite and tabular spar; felspar and albite; sodalite and scapolite.

B. Amorphism.

Every solid body is either crystalline or amorphous. In the latter state it is destitute not only of crystalline form but of all traces of crys

talline structure even in its smallest particles. It has no power of double refraction as many crystals have, no planes of cleavage, being equally easy or equally difficult of separation in all directions, and exhibits when broken not a granular but a conchoidal fracture. Marble and even common limestone are not amorphous bodies but aggregates of small, imperfectly developed crystals: glass is amorphous.

We often find the same body assuming the crystalline or the amorphous condition according to the circumstances under which it passes from the liquid to the solid state: some bodies again are more inclined to the crystalline, others to the amorphous state: some are known to exist in one only of these conditions. The same body is generally speaking specifically heavier, harder, and less soluble in the crystalline than it is in the amorphous state: the atoms seem to be more closely packed in the former condition than in the latter. According to Graham, an amorphous body also contains a larger quantity of combined heat than one which has a crystalline structure. The passage of a body from the amorphous to the crystalline state is called by Fuchs Transformation, and the change from the crystalline to the amorphous state Deformation. The amorphous state is particularly apt to occur when the atoms, either from viscosity in the liquid or a too rapid passage from the liquid to the solid state, are not able to arrange themselves in that peculiar manner which constitutes crystalline structure.

An amorphous body may be produced:

1. By fusion-the process is then called Vitrification.-Common glass, many slags, obsidian, pumice-stone, pearl-stone, vitrified borax, phosphoric acid, arsenious acid, arsenic acid, boracic acid, &c. All bodies which solidify in the amorphous state form viscid liquids when melted. If a body appears transparent immediately after cooling from a state of fusion it may generally be regarded as amorphous; but if it becomes opaque upon cooling, although it was transparent while melted, it is then most probably crystalline, (e. g. hydrate of potash and carbonate of lime): because the numerous small crystals interlacing each other in all directions refract light in a confused and irregular manner. According to Graham, acid phosphate of soda gives out less heat at the moment of solidification than acid arseniate of soda; the former solidifies in the form of a transparent glass; the latter in that of an opaque fibrous mass.

2. By evaporation of a solution.-A solution of gum, glue, white of egg, soluble glass, &c. in water, and of most resins in alcohol, leaves the dissolved bodies in an amorphous state when the liquid evaporates. All these bodies require but a very small quantity of the solvent to retain them in the liquid state: consequently, after the greater portion of it has evaporated, they still remain dissolved and form very thick solutions, the viscosity of which appears to prevent the particular mode of approximation of the atoms required for crystallization.

3. By precipitation. Most if not all voluminous, gelatinous, and viscid precipitates must be regarded as amorphous. Some of them retain this condition even after remaining for a long time in the liquid, and form, after washing and drying, earthy or transparent masses having a conchoidal fracture like alumina or phosphate of lime; others sink to the bottom of the liquid in which they are produced, and with various degrees of rapidity collect themselves into an aggregate of small crystals, e.g. uric acid and carbonate of lime. [Comp. Link (Pogg. 46, 258); Mitscherlich (Ann. Chim. Phys. 73, 389); Erdmann (J. pr. Chem. 19, 343, 345 and 353.)]

The following is a list of bodies, elementary substances included, in which both conditions have been observed:

Carbon is crystallized in the diamond, amorphous in charcoal and lamp-black, and according to Fuchs's view in graphite also.

Phosphorus kept under water in the dark becomes covered with a white opaque crust which contains no water but consists of pure phosphorus, and when heated above 104° Fah. melts again without loss of weight to the state of ordinary phosphorus. (H. Rose, Pogg. 27, 563.) One of these two conditions is probably amorphous.

When sulphur is heated considerably above its melting point to about 180° or 200° C. (356° or 390° Fah.) till it becomes quite viscid and then poured into water, it solidifies to a soft hyacinth-red vitreous mass, which however in a few days resumes its crystalline structure and becomes yellow and opaque. (Fuchs.) When this soft vitreous sulphur is placed in an oven heated to about 98° C. (208° Fah.) its temperature as soon as it reaches 93° C. (199-4° Fah.) rises suddenly to 110° C. or 230° Fah. and it then becomes hard: consequently, at a temperature near its melting point the sulphur passes quickly to its ordinary crystalline state, the change being accompanied by a disengagement of heat. (Regnault, Ann. Chem Phys. 76, 206.) At different temperatures melted sulphur may exist in three different states, colourless, yellow and red. By warming a small quantity of flowers of sulphur on a glass plate, or by sublimation, we may obtain colourless drops which often remain for weeks without crystallizing: they appear to correspond to the rhombo-octohedral sulphur. At a higher temperature these drops become yellow, then by a quick transition green, and lastly dark red. A drop of sulphur unequally heated exhibits a sharp demarcation of the red and yellow portions; the crystals first formed by cooling do not extend into the red part. The yellow melted sulphur may perhaps correspond to the oblique rhombic, the red to some other (the amorphous) variety. (Frankenheim, J. pr. Chem. 16, 5.)

Silicium which has not been ignited burns when heated in the air: but that which has been previously ignited in hydrogen gas does not: the former is probably amorphous, the latter crystalline and therefore more coherent.

The remarkable properties of platinum-black are in all probability due to an amorphous condition of the metal.

The grey native sulphuret of antimony and the brown-red substance called Kermes mineral, must be regarded as the same compound Sb S3 in its crystalline and amorphous states. Kermes mineral fused out of contact of air suffers no alteration in weight, but crystallizes on cooling into a mass resembling the native sulphuret: on the other hand, Fuchs has shown that when the latter is kept in the fused state for a considerable time in a narrow glass tube (if fused for a short time only the alteration is not complete) and then thrown into cold water, a shining dark grey mass is obtained, which appears of a dark hyacinth-red colour by transmitted light when in thin films, has a conchoidal fracture and a sp. gr. of 4.15, while that of the native sulphuret is 4.752; when rubbed it yields a red-brown powder similar to Kermes mineral, but somewhat darker; whereas the native sulphuret, however finely it may be powdered, retains its grey colour. When this quickly cooled mass is again melted and suffered to cool slowly, it returns to the crystalline state of the native sulphuret. Rapid cooling prevents the crystalline arrangement, and the body remains for the most part amorphous.

The black sulphuret of mercury obtained by treating a protosalt of mercury with excess of sulphuretted hydrogen has exactly the same composition as cinnabar, viz., Hg S, and when sublimed passes without alteration of weight to the condition of that substance: on the contrary, according to Fuchs, when finely pounded cinnabar is heated till it begins to sublime, and then thrown into cold water, it is converted into the black sulphuret. In this case, contrary to that of sulphuret of antimony, the crystalline sulphuret is red and transparent, whilst the amorphous variety is grey and opaque.

Chromate of lead, when slowly cooled from a state of fusion, is brown and yields a yellowish brown powder, but when suddenly cooled by throwing it into cold water, it is red, and yields a red powder. (Marchand, J. pr. Chem. 19, 65.)

Quartz has a specific gravity of 2.652, refracts light doubly, is but very slightly soluble in boiling solution of potash, and does not harden, however finely it may be divided, in contact with lime and water. Opal has a sp. gr. of 2.09, refracts singly, dissolves easily in boiling caustic potash, slowly in the same when cold, and hardens with lime and water into a mortar. Both minerals consist of silica. Opal, however, contains from 3 to 12 per cent. of water, and the difference between them has been ascribed to this circumstance, opal being regarded as a hydrate of silica. But for this the quantity of water in opal is too small and too variable. Fuchs, therefore, regards opal as amorphous silica, and his view is supported by the fact that opal, after all its water has beeu driven off by ignition, presents almost the same appearance and is nearly as soluble in potash as before. Silica, artificially prepared (by fusing any siliceous mineral with potash, treating the fused mass with dilute hydrochloric acid in excess, evaporating to dryness, and digesting in water,) has likewise, after ignition, the same action with caustic potash that opal has, and must therefore in like manner be regarded as amorphous. Chalcedony is a mixture of quartz and opal; boiling caustic potash dissolves out the latter and leaves the former in the state of cachelong.

Arsenious acid, when sublimed on the large scale, solidifies from the effect of the high temperature into a perfectly transparent glass. This white glass, when kept at ordinary temperature for several months, becomes turbid and subsequently white and opaque. In this case there is probably a transition from the amorphous to the crystalline state; but it is remarkable that (according to Guibourt) the specific gravity diminishes from 3.785 to 3.695, and that the opaque acid dissolves rather more abundantly both in hot and in cold water than the transparent, whereas in other cases the change from the amorphous to the crystalline state is accompanied by increased density and diminished solubility. When the transparent acid is dissolved in boiling dilute hydrochloric acid and the solution left to cool slowly, every crystal as it separates emits a vivid light. The opaque acid, when similarly treated, exhibits no phosphorescence, unless it contains some of the transparent acid mixed with it. It appears then that the transition from the amorphous to the crystalline state, which takes place when the acid crystallizes from its solution in hydrochloric acid, is accompanied by an emission of light. (H. Rose, Pogg. 35, 481.) This phenomenon seems also to show that even in a state of solution the atoms of a solid may be arranged, sometimes in the amorphous, sometimes in the crystalline order; in the present instance, since the solution of the transparent acid emits light on crystallizing and