Colour.-Colourless substances generally produce colourless compounds; but colourless nitrogen combined with colourless oxygen forms blue nitrous acid and red hyponitric acid; and in the organic kingdom we see a great variety of coloured compounds formed by the union of carbon (which is colourless, at least in the diamond), hydrogen, oxygen, and sometimes also nitrogen. Coloured bodies, such as sulphur, selenium, iodine, and the metals generally form coloured compounds by combination amongst themselves; nevertheless, iodide of potassium, chloride of lead, and chloride of silver, &c. are colourless. The compounds of coloured with colourless bodies are sometimes coloured, sometimes not; thus the compounds of oxygen with sulphur, selenium, iodine, bromine, chlorine, and most of the lighter metals, are white, those with the heavier metals coloured. In the present state of chemical knowledge the colour of a compound cannot be determined beforehand from those of its constituents; it often differs greatly from them. The red metal copper combined with colourless oxygen forms a brown-black oxide, and this combined with colourless sulphuric acid forms a white salt, which again in combination with water produces the blue crystals of hydrated sulphate of copper or blue vitriol. Grey chromium with a certain quantity of oxygen forms a green oxide, which, combined with various colourless acids, forms salts of which some are green, some violet; with a larger quantity of oxygen chromium forms the yellowish red chromic acid, whose compounds with bases are sometimes yellow sometimes red.

The law of Persoz, (Ann. Chim. Phys. 60, 127; also Ann. Pharm. 18, 256), viz. that when the higher oxide of a metal is white or slightly coloured, the lower is blue or dark coloured, and when the higher oxide has a dark colour, the lower is white or faintly coloured, is true as regards cerium, titanium, tantalum, tungsten, molybdenum, and manganese, but not with regard to arsenic, antimony, and tellurium, both whose oxides are white or light-coloured, nor with regard to copper, silver, gold, platinum, and others, both whose oxides are dark-coloured.

f. Chemical and Physiological Relations.

A chemical compound generally differs altogether from its elements, both in its affinities and in its action on living animal bodies. In some cases, combination develops active chemical and physiological properties, in others it destroys those which previously belonged to the elements.

Neither sulphur nor oxygen exhibits any affinity for the greater number of salifiable bases; but the affinity of sulphuric acid for these bases is very strong again, neither of these elements reddens the blue colour of litmus, but this effect is readily produced by sulphuric acid; the same elements are also tasteless and destitute of corrosive action, whereas sulphuric acid has a sour taste and is highly corrosive. Nitrogen, again, which by itself is one of the most indifferent of the elements, produces in combination with oxygen the corrosive substance nitric acid, with hydrogen the powerful alkali ammonia, and with carbon and hydrogen the highly narcotic hydrocyanic acid. The poisonous action of many metals is not developed till they are combined with oxygen, chlorine, or other bodies of like nature. Are these properties actually produced by the act of combination, or are they previously latent in the elements and brought into active operation when those elements are combined with others?

Persoz (Ann. Chim. Phys. 60, 127, also Ann. Pharm. 18, 255) has laid down the two following laws:-1. All bodies which in combination with chlorine form compounds volatile below the boiling point of mercury,

que rhombic prism when precipitated from solutions or sublimed at peratures near its melting point.

Native copper generally occurs in cubes and other forms belonging to he 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 cube 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 first form. (Mitscherlich, J. pr. Chem., 19, 451.)

Oxide of titanium, Ti O2, 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 O3, 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 03 and Sb 03 are iso-dimorphous; i. e., they are capable of crystallizing in two different forms which are similar each to each.

Disulphuret of copper, Cu2S, 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, has 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 rhombic tables of the oblique prismatic system. The red crystals turn

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.

a. 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 form 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 on 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 sublimed 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 cube 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 O2, 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 03, 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 03 and Sb 03 are iso-dimorphous; i. e., they are capable of crystallizing in two different forms which are similar each to each.

Disulphuret of copper, Cu2 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, has 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 rhombic tables of the oblique prismatic system. The red crystals turn

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 have 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 O, C O2, in the form of calespar, 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, Fe O, 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, Co2, and Fe O, CO2 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 calespar, 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 calespar. 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