with clay and reburned, it does not require more than a full red heat to produce a good cement.

Hydraulic limes do not slake with any considerable emission of heat when moistened; they absorb the water without increasing much in bulk, and form a paste of small plasticity. In order that hydraulic lime may harden properly, it must not be submerged till it begins to set; it should then be kept moist until it is quite hard, otherwise it will always remain porous.

The rapidity with which these different kinds of hydraulic limes set, varies considerably with their composition. If the clay do not exceed 10 or 12 per cent. of the weight of the original limestone, the mortar requires several weeks to harden. If the clay amount to from 15 to 25 per cent., it sets in two or three days, and if from 25 to 35 per cent. of clay be present, the solidification occurs in a few hours. The substance to which the term Roman cement is now applied is a lime of this latter description. Roman cement is extensively prepared from nodules of septaria which occur in the valley of the Thames. It sets in a few hours after the mixture with water has been effected, and it soon rivals stone in hardness. According to Meyer, the composition of the nodules employed in the preparation of the cement is the following:

The cement obtained from the neighbourhood of Boulogne is almost identical in composition with the foregoing; and similar materials have been obtained in other countries, particularly in the beds of the Jurassic formation.

Concrete is a mixture of hydraulic mortar with small pebbles coarsely broken.

(647) Other Uses of Lime.-Lime is also largely employed as a manure, and it is particularly valuable upon very rich vegetable soils, such as those formed over peat bogs: its effects in these cases are partially due to the decomposition of the organic matter, which it renders soluble and capable of assimilation, while the lime itself is converted into carbonate. It has been found that limestone containing much carbonate of magnesium yields a lime unsuited to agricultural purposes; this has been attributed

APPLICATIONS OF LIME-SULPHIDES OF CALCIUM.

483

to the fact that magnesia absorbs carbonic acid much more slowly than lime, and remains caustic for a longer period, in which state it appears to be injurious to the tender shoots of young plants.

The strong attraction existing between lime and carbonic acid renders it a valuable material for separating this acid from the carbonates of potassium and sodium, when these alkalies are required in a caustic form. The attraction of lime for water furnishes a means of removing this liquid from many substances, which retain it with considerable force, such as alcohol; the finely powdered lime is mixed with the alcohol, and the mixture after being allowed to stand for a few days, with occasional agitation, is subjected to distillation: the anhydrous alcohol passes over, leaving the water combined with the lime. Slaked lime is employed as a direct chemical agent in the purification of coal-gas, and as a means of loosening the epidermis, and facilitating the removal of the hair from hides, as a preliminary to the process of tanning.

Peroxide of hydrogen forms an insoluble compound with lime, which is precipitated in crystalline scales when the peroxide is poured into lime-water: it is very unstable, and undergoes spontaneous decomposition at the temperature of the air. This substance has been described as binoxide of calcium.

(648) SULPHIDES OF CALCIUM.-Calcium forms several compounds with sulphur, some of which are soluble.

The protosulphide of calcium (EaS=72) is procured by decomposing a mixture of sulphate of calcium and charcoal by heat, as directed for sulphide of barium. It is insoluble in cold water, but when treated with boiling water in small proportion is converted into hydrate of lime, and a soluble bisulphide of calcium. Protosulphide of calcium is phosphorescent when newly prepared. This property was first observed by Canton, in an impure sulphide of calcium, which he obtained by calcining oyster-shells in an open fire for half an hour, then selecting the whitest and largest portions, and packing them with one-third of their weight of flowers of sulphur in a crucible with a luted cover; this was heated strongly for an hour: when cold, the crucible was broken, and the whitest pieces were placed in well-closed bottles.

By boiling slaked lime with excess of sulphur, a pentasulphide of calcium is obtained, and hyposulphite of calcium is formed at the same time: 3 ¤a✪+12 S+H2O=2 €aS ̧ +¤a§,H2

(649) PHOSPHIDE OF CALCIUM (CaP?=71).—This compound presents some interest, from its affording the most convenient

484

PHOSPHIDE OF CALCIUM.

source of some of the phosphides of hydrogen (455). It is It is prepared by distilling phosphorus over lime heated to low redness: a mixture of phosphide and pyrophosphate of calcium is the result, 7 P+7 Єa0=Єa,P,O,+5 EaP (P. Thénard). The most convenient method of conducting the operation

FIG. 337

27

is shown in fig. 337. In the lower part of a narrow deep crucible, a, a hole is drilled for the reception of the neck of a flask, B, which is luted into the aperture; a quantity of dry phosphorus is placed in the flask, and the crucible is filled with quicklime, broken into fragments of about the size of a hazel-nut; a lid is then luted upon the top of the crucible. Time having been given for the luting to become dry, the upper part of the crucible is raised to a red heat as quickly as possible, by surrounding it with ignited charcoal, the lower part of the furnace having been filled with cold charcoal, to prevent the heat from reaching the phosphorus too rapidly; the phosphorus becomes gradually volatilized as the heat reaches it. If the heat be too high, the phosphorus distils over without combining with the calcium.

Phosphide of calcium when procured in this manner forms an anhydrous mass of a dull red colour, hard enough to strike fire with steel it experiences no change in dry air or in oxygen at the ordinary temperature. At a high temperature it becomes partially decomposed by oxygen, chlorine, or hydrochloric acid; in a moist atmosphere it slakes, emits phosphuretted hydrogen, and crumbles to a brown powder. This powder, when thrown into water, or heated to 212°, evolves phosphuretted hydrogen, which is not self-lighting, and is mixed with free hydrogen.

Phosphide of calcium, in its unslaked form, is decomposed when thrown into water; phosphuretted hydrogen gas is evolved, and takes fire with the phenomenon already described (454): diluted acids produce its decomposition still more rapidly.

(649 a) Silicide of Calcium (Si,Ca).-Wöhler (Liebig's Annal. cxxvii. 257), in order to prepare this singular compound, directs 300 grains of graphitoid silicon to be finely powdered and intimately mixed with 3000 grains of chloride of calcium in a hot mortar, and to be rapidly shaken up in a wide-mouthed bottle with 350 grains of sodium cut into small pieces: meantime a Hessian crucible is to be brought to a full red heat in a good wind-furnace; a little fused common salt is to be thrown into the crucible, and upon this a mass of sodium of 350 grains; then

SILICIDE AND CHLORIDE OF CALCIUM.

485

the mixture of silicon and sodium and chloride of calcium, and the whole is covered with a layer of pulverized fused chloride of sodium: after this the cover is put on; the fire is then gradually raised, and maintained for half an hour at a temperature sufficient to melt cast iron. On breaking the crucible after it has cooled, the silicide of calcium ought to be found in the form of a well-fused button, which must be preserved in well-closed vessels.

Silicide of calcium has a leaden-grey metallic lustre and a scaly crystalline structure, with an indistinct indication of hexagonal plates. When exposed to the air it slowly crumbles down into a mass of graphite-like plates. If thrown into water a similar change occurs, attended with a very gradual but prolonged disengagement of hydrogen. This disintegration is due to the hydration and oxidation of part of the calcium and silicon, the new products remaining mixed with some unaltered silicide. Fuming nitric acid does not attack the silicide of calcium. Hydrochloric acid, as well as dilute sulphuric and acetic acids, converts it into the yellow substance already described (471), whilst hydrogen escapes.

Silicide of calcium has, in the hands of Wöhler, proved a source from which he has been enabled to procure various compounds of silicon, hydrogen, and oxygen, presenting some analogy with the compounds of carbon with the same elements, and will no doubt give rise to further researches of importance.

(650) CHLORIDE OF CALCIUM (CaCl„, 6 H„Ð=111+108, or CaCl, 6 Aq=55'5+54); Sp. Gr. fused, 2485, cryst. 1680; Composition in 100 parts, ea, 36.03; Cl, 63'97.-This salt is obtained as a secondary product in the manufacture of carbonate from chloride of ammonium, but it may be prepared by dissolving chalk in hydrochloric acid, evaporating to dryness, and fusing the residue at a red heat. Under these circumstances, a small portion of the chlorine is displaced by the oxygen of the air, so that the mass has an alkaline reaction, owing to the presence of lime. By evaporation of its solution it may be obtained in striated prismatic six-sided crystals with 6 H,, which fuse at 84°. In this form it produces great depression of temperature when dissolved in water, and if mixed with snow it furnishes a powerful freezing mixture. If the hydrated salt be exposed to a prolonged heat of 300° it forms a porous mass which still retains 2 H‚¤; in this state it is well adapted for the desiccation of gases. Chloride of calcium is extremely deliquescent; a saturated solution of the salt boils at 355°, and is sometimes employed where a steady temperature, not exceeding this point, is required. It

2

486

FLUORIDE OF CALCIUM, OR FLUOR-SPAR.

2

is soluble in alcohol, and may be obtained from its alcoholic solution crystallized in rectangular plates (CaCl,, 4 €,H) containing 4 atoms of alcohol. Chloride of calcium absorbs ammonia rapidly, and forms a compound with 8 atoms of the gas. A solution of the chloride, if boiled with quicklime and filtered while hot, deposits long, flat, thin crystals of a hydrated oxychloride, consisting of (EaCl, 3 EaÐ.15 H2O), which is decomposed both by water and by alcohol.

(651) FLUORIDE OF CALCIUM (ЄaF,=78, or CaF=39); Sp. Gr. 3'14; Composition in 100 parts, Ca, 5128; F, 48-72.-This is an abundant mineral, well known as fluor-spar, which occurs either massive, or crystallized in forms allied to the cube. It is found accompanying the lead veins in Cumberland, Derbyshire, and Cornwall, and is met with in a variety of other localities, of various colours, most frequently blue, green, or white. Fluor-spar is the principal source from which the compounds of fluorine are obtained. Fluoride of calcium, in minute quantity, is found in sea water (Dr. G. Wilson), and in many springs: it is a never-failing companion of phosphate of calcium in the bones and teeth of animals, and indeed is always found to accompany phosphate of calcium in the mineral kingdom also, in small but variable quantities. Most varieties of fluor-spar, when gently heated, become phosphorescent, emitting a pale green or violet light; if heated more strongly, the crystals decrepitate, and each fragment becomes enveloped for a few seconds in a beautiful halo of light. It loses this property after having been once heated; a phosphorescent fluor, dissolved in hydrochloric acid and precipitated by ammonia, retains its power of emitting light when heated, but if it had been previously heated sufficiently to destroy the phosphorescence, this property is not restored by solution and reprecipitation.

Powdered fluoride of calcium absorbs sulphuric acid if mixed with it at a low temperature, and forms a transparent, viscous mass, from which fumes of hydrofluoric acid are evolved by heating it to 100° F. Fluor-spar undergoes no change when heated with sulphuric anhydride, but with boracic anhydride it yields borate of calcium and fluoride of boron ; 3 ЄaF +4 В2O3=3(¤a 2 BO2) +2 BF3. Hydrochloric acid dissolves it in small quantity. If heated in a current of chlorine, a gas which corrodes glass is expelled. It is not known whether this is fluorine or chloride of fluorine. When fluor-spar is fused with the hydrated alkalies it undergoes no change: with the carbonates of the alkali-metals, fluoride of the alkaline metal and carbonate of calcium are formed.