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refractive power which the compound gases should exhibit according to calculation, taking the mean between the refractive powers of the component gases ;-D, their specific gravities ;-E, the specific refractive power, obtained by dividing the observed refractive power by the specific gravity.

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From these numbers Dulong concludes that when a compound gas is of an acid nature its refractive power is below the calculated mean, but above the mean when the gas is alkaline or neutral; hydrochloric ether, however, forms an exception to this rule. If it be admitted that the refractive power of a substance is directly proportional to its density and inflammability, the latter will be found by dividing the refractive power by the specific gravity. The quotients in column E agree very well with this view; hydrogen has the greatest refractive power, and oxygen, the substance most opposed in its properties to combustible bodies, the smallest. The other numbers also agree, excepting that the refractive power of sulphuret of carbon should be smaller than that of sulphuretted hydrogen, since the former contains 2 atoms of the less combustible substance sulphur to 1 atom of carbon, the latter equal numbers of atoms of sulphur and hydrogen; the refractive power of nitrous oxide, again, ought to be smaller than that of nitrogen, since nitrogen must lose some of its refracting power by combining with oxygen.

Some compounds, in passing from the gaseous to the liquid state, increase in refracting power more than in density, as was first noticed by Arago and Petit. The absolute refractive power of liquid sulphurous acid ought, according to calculation from that of the gas to be 0 661: its actual value, however, is, according to De la Rive, 0.78 (Ann. Chim. Phys. 40, 410, extr. Pogg. 15, 528); that of liquid ammonia should by calculation from that of ammoniacal gas be 0.725, and that of sulphuretted hydrogen 0.767; but according to Faraday, the refractive powers of both these liquids exceed that of water, which is 0.784.

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 comipounds 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 blne 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 chronic 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, aud 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,

produce acids when combined with oxygen; viz. hydrogen [?], carbon, boron, phosphorus, sulphur, selenium, bromine, iodine, nitrogen, silicium, titanium, tungsten, vanadium, chromium, uranium, manganese, arsenic, antimony, tin, and osmium. This law appears to fail in the case of mercury itself, which produces no acid, but is notwithstanding less volatile than corrosive sublimate. 2. All compounds containing 1, 3, 5 or 7 atoms of oxygen are either acid or basic ; those on the contrary which contain 2 or 4 atoms of oxygen are, with a few exceptions which disappear on a particular hypothesis, neither acid nor basic. Exceptions to this Jaw are-C0, which is neither acid nor basic, Mo 0%, V Ò?, and Pt 0?, which are bases, and CO, SO?, Se 0?, Ti Oo, Sn O’, and Te O’, which are acids. The manner in which Persoz endeavours to get rid of these exceptions looks like a gratuitous assumption. It is nevertheless true that by far the greater number of acids and bases contain an uneven number of atoms of

oxygen. The case in which marked chemical and physiological properties existing in elements are caused to disappear by combination is most strikingly exhibited in the combination of acids with salifiable bases: the effect is then called Neutralization. When an acid and a base combine in certain proportions, their opposite properties are mutually destroyed, and a more or less indifferent compound is the result. Hydrochloric acid, for example, tastes and smells strongly acid and reddens litmus; ammonia has a powerful alkaline smell and taste, restores the blue colour of litmus which has been reddened by an acid, reddens turmeric, and givesa green colour to violet juice—these changes of colour being again removable by acids; both these substances in the concentrated state exert a powerful caustic action on the animal body though in different ways, and cannot therefore be swallowed without injury, except in very small quantities and in a state of dilution. If now the aqueous solution of hydrochloric acid and aqueous solution of ammonia, be mixed in certain proportions (the required proportions may be ascertained by the use of litmus or turmeric paper), a perfectly neutral compound will be obtained, which reddens neither litmus nor turmeric, tastes and smells neither acid nor alkaline, but is devoid of smell and has a saline taste, has no corrosive action, and may be swallowed in much larger quantities. The two substances have therefore neutralized each other both chemically and physiologically; a neutral compound has been formed; neutrality, chemical equilibrium, chemical indifference has been produced. The proportion in which this mutual destruction of chemical properties is most completely effected is called the point of neutralization. Exactly one atom of hydrochloric acid is required to neutralize one atom of ammonia. If to this neutral compound a fresh quantity of hydrochloric acid be added, the character of the acid will again become evident by its sour taste and its effect on litmus; it will prevail, preponderate, or be in excess, or the ammonia will be supersaturated with hydrochloric acid: similar results, only of the opposite character, would be obtained by adding more ammonia to the neutral compound.

ADDENDA TO THE THEORY OF THE QUALITATIVE ALTERATION OF

ELEMENTS IN COMBINATION. Although the properties of a compound are mainly dependent on those of its elements, and on the proportion in which these elements are combined, it has nevertheless been shown by recent experiments that other

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The cases of Dimorphism hitherto observed, including those relating

Carbon in the dis moad forms erystals belonging to the regular system,
in graphite to the shoubohedral system,-unless the latter are to be
manku produtoszterphous crystals.
PerOut in rombie octohedrons belonging to the right prismatic sys-

Suipaar crystal ilmy on cooling from a state of solution in sulphuret of en el air. 11–++) ersetly like those of native sulphur; if, on the other natuh, merelted sulphur be allowed to cool slowly till a portion of it has bewerbe said, and the still liquid portion be then poured out, the solidified puiciat exàibits oblique rhombic prisms belonging to the oblique prisracie system. These are at first perfectly transparent, of a deep yellow Juliae, ime somewhat harder and denser than those of sulphur crystallized

er the cold; but after being kept for a few days at ordinary temperatures, shey become opaque, and of a straw-yellow colour. At the lower temperature

, therefore, the atoms of sulphur arrange themselves in such a duaer as to form a rhombic octobedron, 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. 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 rhombie oetohedron; and this change destroys their transparency, because in place of one erystal an aggregate of crystalline particles is produced hich refract light in different directions (Mitscherlich). According to Frankenheim (I. pr. Chem. 16, 5), sulph. nssumes the form of the

ing to a different crystalline system. to simple substances are as follows:

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que rhombic prism when precipitated from solutions or snblimed at iperatures 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 crystalJine 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 irst form. (Mitscherlich, J. pr. Chem., 19, 451.)

Oxide of titanium, Ti 0°, 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 03 and Sb 03 are 180-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 rhombobedral 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

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