leaden tube filled with the diluted acid; in the course of a few minutes the scale is permanently engraved.

Fluorides.-The compounds of the metals with fluorine for the most part fuse easily on the application of heat, and hence the origin of the terms fluor-spar and fluorine (from fluo, to flow). When ignited in a current of steam many of them are converted into the corresponding oxide, whilst hydrofluoric acid is formed. A large number of the fluorides are insoluble, or only sparingly soluble, in water. They are all decomposed when heated with oil of vitriol, and evolve hydrofluoric acid; but they are not so readily attacked by nitric acid. If heated with chlorine, many of the fluorides are decomposed, whilst chlorides of the metal are produced. The solutions of the soluble fluorides corrode the glass vessels in which they are contained: they give no precipitate with nitrate of silver, since fluoride of silver is soluble; but with salts of lead, barium, magnesium, and calcium, insoluble precipitates, consisting of the fluorides of these metals, are produced. The fluoride of calcium is so transparent as to be perceived with difficulty; but on heating the liquid, or on the addition of ammonia, it is rendered more opaque.

Many metallic fluorides combine with an additional atom of hydrofluoric acid, and form compounds which may often be obtained in crystals that are soluble in water. The double fluoride of potassium and hydrogen (KF,HF) has been already mentioned as a convenient source of concentrated hydrofluoric acid. Double fluorides of the alkaline metals, with the fluorides of the electronegative metals which form acids with oxygen may likewise be obtained with facility. Many insoluble metallic anhydrides, such as the tantalic, titanic, molybdic, and tungstic anhydrides, are thus dissolved by hydrofluoric acid, fluorides of the metals being formed, whilst the oxygen of these compounds produces water with the hydrogen of the hydrofluoric acid: the metallic fluorides so formed are dissolved by the excess of hydrofluoric acid, and give rise to new compound acids. Titanic acid, for instance, is thus converted into fluotitanic acid; Fi2+6 HF becoming (2 HF,FiF) + 2 H2O. Silica yields a similar compound (2 HF,SiF1).

Hydrofluoric acid, when mixed with nitric acid, readily dissolves silicon which has not been strongly ignited; but it is remarkable that the mixture does not dissolve either gold or platinum.

Numerous other compounds of fluorine have been prepared, but they are not of sufficient practical importance to require

COMBINING PROPORTION OF FLUORINE.

163

notice here: the compounds which it forms with silicon and with boron will be described hereafter (478, 483).

(404) Determination of the Combining Proportion of Fluorine.Although the chemist has hitherto been unable to isolate fluorine in a state of purity, yet its combining proportion has been determined with precision; and the mode of proceeding offers an instructive illustration of the resources of chemical analysis in such

a case.

:

The method of operating is as follows:-Pure fluor-spar is reduced to an impalpable powder, and dried; 100 grains of this powder are accurately weighed into a counterpoised platinum crucible, and concentrated sulphuric acid, also perfectly pure, is added in quantity sufficient to reduce the whole to the consistence of cream after standing for some hours, the excess of acid is expelled by the heat of a lamp: the temperature is raised very cautiously, and the crucible and its contents are finally heated to bright redness. In this operation the whole of the fluorine is expelled in the form of hydrofluoric acid, the calcium combining with the radicle of the sulphuric acid, and forming sulphate of calcium which remains behind, whilst the fluorine unites with the hydrogen. On weighing the crucible after the experiment is completed, the sulphate of calcium will be found to amount to 174'36 grains.

Now it is known that 68 grains of sulphate of calcium contain 20 of calcium and 48 of sulphuric acid radicle, 20 being the combining number or equivalent of calcium, though its atomic weight is 40 (13) :—

but 68: 20: 174′36 : x(=51′25);

17436 grains of sulphate of calcium must consequently contain 5125 of calcium; 100 parts, therefore, of fluor spar, if it consist only of fluorine and calcium, must be composed of 51.25 of calcium and 48.75 of fluorine. The combining proportion of fluorine is then found directly by the following calculation :—

CHAPTER VII.

SULPHUR-SELENIUM-TELLURIUM.

(405) Natural Relations of the Sulphur Group.—Between sulphur, selenium, and tellurium, a marked analogy in chemical character is observable. They are all characterized by a powerful attraction for oxygen. The properties of selenium are intermediate between those of sulphur and tellurium, which latter presents so much of the external characters and appearance of a metal, that it is usually described with the metals. The specific gravity and fusing-point of these elements increase as the atomic weight increases, as will be seen by comparing the numbers in the different cases.:

Amongst the compounds of each of these bodies with oxygen are two anhydrides; one with 2 atoms of oxygen corresponding with sulphurous anhydride Se, and another with 3 atoms of oxygen corresponding with sulphuric anhydride SO. One volume of the vapour of each of these three elements unites with 2 volumes of hydrogen to form 2 volumes of a sparingly soluble gaseous compound possessed of a disgusting odour and feebly acid character. Oxygen also presents a certain analogy with the members of this group, I volume of oxygen uniting with 2 volumes of hydrogen to form 2 volumes of steam; and the oxides and sulphides, generally, exhibit many points of resemblance.

The atomic volume of solid sulphur is 101, and that of selenium 103, or nearly identical; but that of tellurium, 128, is one-fourth higher. It may be further remarked that the corresponding compounds of sulphur, selenium, and tellurium are isomorphous.

The singular numerical relations which Dumas and others have pointed out between the atomic weights of the members composing these groups and those of several other elements equally closely allied, will be discussed at a future point (1034).

RELATIONS OF THE SULPHUR GROUP.

165

It will be sufficient here to remark, that in groups of electronegative elements of similar properties it is usually observable that the chemical activity of each element of the group is usually greater, the smaller is its combining number; sulphur, for example, being more active in its chemical relations than selenium, and selenium than tellurium: so again, fluorine is more energetic in its chemical actions than chlorine, chlorine than bromine, and bromine than iodine. In the metals, or basylous elements, the order of their activity is exactly the reverse, potassium being more active than sodium, and sodium than lithium.

The following table exhibits some of the corresponding compounds which the elements of this group form with oxygen and hydrogen :

§ I. SULPHUR: S=32, or S=16.*

Combining Volume below 1500°, ; above 1900°; Specific Gravity of Vapour, 6·617 at 900° F.; Theoretic Sp. Gr. at 1900°, 22168; Observed, at 1900°, 2:23; Melting-pt. 239°; Boiling-pt. 836°.

(406) Most of the sulphur used in England is obtained from Sicily, where it occurs in the native or uncombined state in beds of a blue clay formation, stretching from the southern coast of the island towards the base of Mount Etna. It is also found abundantly in volcanic districts generally, and particularly in those which border the Mediterranean. Many of the compounds of sulphur with the metals occur in great abundance as natural productions, especially the sulphides of iron, copper, lead, and zinc. Bisulphide of iron (iron pyrites) furnishes a large proportion of the sulphur consumed in the manufacture of oil of vitriol. Sulphur is still more extensively distributed in the oxidized condition

166

PROPERTIES OF SULPHUR.

as sulphuric acid, in combination with various earths; the sulphates of calcium, magnesium, barium, and strontium being abundant natural productions. Sulphur is likewise an essential constituent of many bodies of organic origin; it enters into the composition of several fœtid volatile oils; it is a necessary ingredient in the muscular tissue of animals, and is, indeed, always contained in the albuminoid or proteic compounds.

Properties.-Native sulphur is found either in amorphous masses, or in transparent yellow crystals, the form of which is derived from the octohedron with a rhombic base. The sulphur of commerce is presented either in the form of a harsh, yellow, gritty powder, known as flowers of sulphur, or in round sticks, constituting roll sulphur or common brimstone. In the latter condition it is a solid, nearly opaque, brittle substance, of a characteristic yellow colour, with a slight, peculiar odour. It is insoluble in water, and is consequently tasteless; it is a bad conductor of heat, and when grasped with a warm hand frequently crackles and falls to pieces from the unequal expansion; it is an insulator of electricity, and becomes negatively electric by friction.

Sulphur is highly inflammable, and when heated in the air it takes fire at between 450° and 500°, burning with a blue flame, and emitting pungent suffocating fumes of sulphurous anhydride. At 239° it melts, forming a yellow liquid which is less dense than the unmelted sulphur. In closed vessels it may, by a further heat, be distilled, the boiling-point being about 836° (Regnault); at this temperature sulphur yields a deep yellow vapour of sp. gr. 6·617: I volume of this vapour contains 3 atoms of sulphur. Bineau found that when sulphur is heated to about 1800° F. the vapour becomes dilated to three times the bulk that an equal weight of the vapour occupies at 900°, and that at this high tempe rature the volume occupied by an equivalent of sulphur vapour corresponds with that of an equivalent of oxygen; this observation has recently been confirmed by Deville and Debray.

Sulphur combines readily with chlorine, with bromine, and with iodine, especially when the action is favoured by heat. It also enters rapidly into combination with most of the metals, many of which, like copper, iron, and silver, if in a state of fine division, burn vividly when heated in its vapour. The compounds of sulphur with the metals are termed sulphides, or sulphurets. Generally for each sulphide a corresponding oxide exists, each atom of oxygen in the molecule of the oxide being represented in the sulphide by an atom of sulphur; and sulphur often displaces oxygen by double decomposition; I atom of sulphur is therefore