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circumstances likewise have an influence on this matter, and that compounds may exist composed of the same substances in the same proportions, and yet possessing very different properties. To this part of the subject belong Mitscherlich's theory of Dimorphism, Fuchs's theory of Amorphism, and Berzelius's theory of Isomerism, Polymerism, and Metamerism.

a. Differences in the Properties of Compounds, which may be explained on

the Hypothesis of different Modes of Arrangement of their Compound Atoms.

2. Dimorphism and Trimorphism. The same substances, whether simple or compound, may crystallize in forms which belong to two or three different systems of crystallization, or which, even if they belong to the same system, yet exhibit such differences in their corresponding angles as to render it quite impossible to reduce them to the same form: this was first shown by Mitscherlich. This difference of crystalline fornı is associated with difference of specific gravity, hardness, colour, and other properties. Whether a body shall crystallize in one system or another seems to depend chiefly ou temperature. Crystals formed at one particular temperature, and then exposed to that temperature at which crystals of a different kind are produced, often lose their transparency, and, without alteration of external form, become changed into an aggregate of small crystals of the latter kind. We

may therefore imagine that the atoms of the solid crystal displace one another in such a manner as to bring about that particular arrangement which they are disposed to assume at the altered temperature, the new arrangement belonging to a different crystalline system.

The cases of Dimorphism hitherto observed, including those relating to simple substances, are as follows:

Carbon in the diamond forms crystals belonging to the regular system, in graphite to the rhombohedral system,-unless the latter are to be regarded as pseudomorphous crystals.

Sulphur crystallizes, on cooling from a state of solution in sulphuret of carbon, in rhombic octohedrons belonging to the right prismatic system (fig. 41-44), exactly like those of native sulphur; if, on the other hand, melted sulphur be allowed to cool slowly till a portion of it has become solid, and the still liquid portion be then poured out, the solidified portion exhibits oblique rhombic prisms belonging to the oblique prismatic system. These are at first perfectly transparent, of a deep yellow colour, and somewhat harder and denser than those of sulphur crystallized in the cold; but after being kept for a few days at ordinary temperatures, they become opaque, and of a straw-yellow colour. At the lower temperature, therefore, the atoms of sulphur arrange themselves in such a manner as to form a rhombic octohedron, at the higher temperature just below the melting point (about 107° C., or 224° Fah.), the mode of arrangement is such as to produce an oblique rhombic prism. When these last-mentioned crystals are brought to a lower temperature, a general displacement of the atoms appears to take place, whereby they are brought into the particular relative position which belongs to the rhombic octohedron; and this change destroys their transparency, because in place of one crystal an aggregate of crystalline particles is produced which refract light in different directions (Mitscherlich). According to Frankenheim (J. pr. Chem. 16, 5), sulphur assumes the form of the oblique rhombic prism when precipitated from solutions or snblimed at temperatures near its melting point.

Native copper generally occurs in cubes and other forms belonging to the regular system; but Hauy once found it in double six-sided pyramids with truncated edges (fig. 138). Seebeck likewise obtained copper after fusion in crystals belonging to the rhombohedral system. According to Haidinger and H. Rose (Pogg. 23, 197), however, these crystals, which appear to belong to the rhombohedral, are really macle crystals of the eube with pyramidal summits (fig. 9), and therefore belong likewise to the regular system.

Suboxide of copper occurs in ordinary red copper ore in regular octohedrons and other forms belonging to the regular system, but in copperbloom it exhibits a regular six-sided prism, whose planes of cleavage are parallel to the faces of an obtuse rhombohedron. (Succow, Pogg. 34, 528.) This may be regarded as a case of dimorphism similar to that of copper, insofar as the latter is really dimorphous.

Protoxide of lead crystallizes after fusion, as well as from a saturated solution in hot concentrated caustic potash, in yellow rhombic octohedrons. If, however, the solution is not fully saturated with oxide of lead, so that crystallization does not take place till after complete cooling, red crystalline scales are deposited on the yellow rhombic octohedrons just formed: if the red crystals are heated they turn yellow on cooling, in consequence of passing into the drst form. (Mitscherlich, J. pr. Chem., 19, 451.)

Oxide of titanium, Ti Oʻ, occurs in nature in the two forms of anatase and rutile. Although both these crystals belong to the square prismatic system, their angles are incompatible; they cannot be reduced to the same primitive form; the specific gravity also of anatase is 3.826, that of rutile 4.249.

Arsenious acid, As 0, generally crystallizes in regular octohedrons; but Wöhler (Pogg. 26, 177) found it also in the form of native oxide of antimony, sb'03 (Weissspiessglanzerz), which belongs to the right prismatic system. Wöhler also obtained artificially crystallized oxide of antimony in regular octohedrons. Consequently As 0% and Sb 03 are iso-dimorphous ; i.e., they are capable of crystallizing in two different forms which are similar each to each.

Disnlphuret of copper, Cu’S, appears in copper glance in crystals of the rhombohedral system (fig. 131, 132, 135, 137); but Mitscherlich (Pogg. 28, 157), by melting together large quantities of copper and sulphur, obtained it in regular octohedrons. These two forms are the same as those of copper and its red oxide.

Bisulphuret of iron occurs in nature as iron pyrites in crystals belonging to the regular system, (fig. 18, 19, 20,) and as white iron pyrites in those of the right prismatic system, the latter being of a paler yellow and much softer. Breithaupt imagines that the oblique rhombic sulphur which may be supposed to exist in common iron pyrites, bas imparted the hemihedral character to the iron which has retained its original system, —and that the white pyrites, which in form resembles the rhombo-octohedral sulphur, may contain this kind of sulphur; and, accordingly, that the white pyrites has been formed at a lower temperature than the common variety.

Protiodide of mercury separates from solution, and likewise sublimes at a very gentle heat in scarlet tables belonging to the square prismatic system, but when sublimed at a higher temperature, in sulphur-yellow rhombíc tables of the oblique prismatic system. The red crystals turn

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yellow as often as they are heated, and resume their red tint on cooling. The yellow crystals obtained by sublimation retain their colour when cooled; but on the slightest rubbing or stirring with a pointed instrument, the part which is touched turns scarlet, and this change of colour extends, with a slight motion, as if the mass were alive, throughout the whole group of crystals as far as they adhere together. In this case the yellow crystals retain their external form unchanged, while the compound atoms must have taken up the relative position which belongs to the red crystals; the yellow crystals are therefore pseudomorphous. The same crystals turn yellow every time they are heated, and red again on cooling. (Hayes, Sill. Am. J., 16, 174; also Schw., 57, 199.) The original red crystals also turn yellow when heated, and retain this colour after cooling for several days, even when touched with foreign bodies, and at length spontaneously, but very slowly, resume their red colour. When the red crystals are sublimed at a very gentle heat, red and yellow crystals sublime together. If a glass plate, having both red and yellow crystals on it, be warmed so gently that the red ones do not change colour, but sublimation nevertheless goes on, both red and yellow crystals collect on a plate held above the former. Now, since the upper plate is cooler than the lower, and the latter is not hot enough to change the colour of the red crystals, the yellow crystals on the upper plate can bave come only from those of the same colour on the lower; they must, therefore, have sublimed as yellow crystals, and the vapour of the yellow crystals must be different from that of the red ones. (Frankenheim, J. pr. Chem., 16, 4.)

Carbonate of lime, Ca 0, CO, in the form of calcspar, whose sp. gr. = 2.721, belongs to the rhombohedral, in arragonite, whose sp. gr. is 2.931, to the right prismatic system. (An explanation of this difference was formerly sought in the fact discovered by Stromeyer, viz., that arragonite usually contains small quantities of carbonate of strontia.) The same peculiarity is presented by carbonate of iron, FeO, CO?, which in sparry iron ore of 3.872 sp. gr.) has the form of calcspar, in junkerite (of 3.815 sp. gr.) that of arragonite. Hence Ca, Coʻ, and FeO, CO’ are isodimorphous. If a solution of carbonate of lime in water containing carbonic acid be left to evaporate at the ordinary temperature, nothing is obtained but calcspar, in microscopical and for the most part truncated primitive rhomboids (fig. 142); if, on the contrary, the solution be evaporated over the water-bath, arragonite is obtained in small 6-sided prisms, mixed with a few crystals of calcspar, because the temperature of the solution is lower at first than it afterwards becomes, and the sp. gr. of the liquid is not higher than 2.803. When an aqueous solution of chloride of calcium is mixed at ordinary temperatures with an aqueous solution of carbonate of ammonia, a voluminous flocculent precipitate of chalky (amorphous?) carbonate of lime is first produced, which if immediately collected on a filter, washed and dried, remains unaltered, possessing a sp. gr. of 2.716, and appearing under the microscope to consist of small opaque granules; but if this same precipitate be left for some time in the saline liquid from which it has been precipitated, it collects into microscopical crystals of calcspar, of 2.719 sp. gr. If the same saline solutions be mixed boiling, the carbonate of ammonia being added to the chloride of calcium, arragonite is obtained, mixed with a small portion of calcspar. If, on the contrary, the chloride of calcium be added to the carbonate of ammonia, arragonite is obtained alone, in exceedingly small crystals of 2.949 sp. gr. If, however, these crystals are not immediately collected

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 line 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° Fal.) 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 conimonly 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, SO, 79q) crystallizes (a) below 15° C. (59° Fah.) in right rhombic prisms (fig. 53); (6) between 15° and 20° C. (59° and 68° Fah.) in acute octobedrons 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 0, SO, 7A9) crystallizes below 52° C. (125.6° Fah.); in forma, 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 52o 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 baving been first formed in the adhering mother-liquid and subsequently extended through the crystals already formed. Sulphate of magnesia (MgO, SO', 7A9) 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, wbich proceed from the surface of the crystals and meet in the middle. Seleniate of zinc (Zn 0, Se 09, 7A9) 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.)

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

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

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