in water; the alkaline earths dissolve with greater difficulty in that menstruum; and magnesia, the last member of the class, is only very sparingly soluble in it. The earths proper, and the oxides of the heavy metals are insoluble in water, or nearly so. The solutions of the alkalies and alkaline earths are caustic when sufficiently concentrated; they have an alkaline taste, change the yellow color of turmeric paper to brown, and restore the blue tint of reddened litmus paper; they saturate acids completely, so that even the salts which they form with strong acids do not change vegetable colors, whilst those with weak acids generally have an alkaline reaction. The earths proper and the oxides of the heavy metals combine likewise with acids to form salts, but, as a rule, they do not entirely take away the acid reaction of the latter.

The sulphur bases resulting from the combination of the metals of the alkalies and alkaline earths with sulphur, are soluble in water. The solutions have a strong alkaline reaction. The other sulphur bases do not dissolve in water. All sulphur bases form with sulphur acids sulphur salts.

a. OXYGEN BASES.

a. ALKALIES.

§ 30.

1. POTASSA (K O) AND SODA (Na O).

The preparation of perfectly pure potassa or soda is a difficult operation. It is advisable, therefore, to prepare, besides perfectly pure caustic alkali, also some which is not quite pure, and some which, being free from certain impurities, may in many cases be safely substituted for the pure substance.

a. Common solution of soda.

Preparation. Put into a clean cast-iron pan provided with a lid, 51⁄2 parts of crystallized carbonate of soda of commerce and 20 parts of water, heat to boiling, and add, in small portions at a time, thick milk of lime prepared by pouring 4 parts of warm water upon 1 parts of quicklime, and letting the mixture stand in a covered vessel until the lime is reduced to a uniform pulpy mass. Keep the liquid in the pan boiling whilst adding the milk of lime, and for a quarter of an hour longer, then filter off a small portion, and try whether the filtrate still causes effervescence in hydrochloric acid. If this is the case, the boiling must be continued, and, if necessary, some more milk of lime added to the fluid. When the solution is perfectly free from carbonic acid, cover the pan, allow the fluid to cool a little, and then draw off the clear solution from the residuary sediment, by means of a syphon filled with water, and transfer it to a glass flask. Boil the residue a second and a third time with water, and draw off the fluid in the same way. Cover the flask close with a glass plate, and allow the lime suspended in the fluid to subside completely. Scour the iron pan clean, pour the clear solution back into it, and evaporate it to 11 or 12 parts. The solution so prepared contains from 9 to 10 per cent. of soda. It must be clear, colorless, and as free as possible from carbonic acid; sulphide of ammonium must not impart a black color to it. Traces of silicic acid, alumina, and phosphoric acid, are usually found in a solution of soda

prepared in this manner; on which account it is unfit for use in accurate experiments.

Solution of soda is kept best in bottles closed with ground glass caps. In default of capped bottles, common ones with ground stoppers may be used; but in that case, the neck must be wiped perfectly dry and clean inside, and a slip of writing paper rolled round the stopper; since, if these precautions are neglected, it will be found impossible, after a time, to remove the stopper, particularly if the bottle is only rarely opened.

b. Hydrate of potassa purified with alcohol.

Preparation.-Dissolve some sticks of caustic potassa of commerce in rectified spirit of wine in a stoppered bottle, by digestion and shaking; let the fluid stand, decant or filter if necessary, and evaporate the clear fluid in a covered silver dish over the spirit-lamp, until no more aqueous vapors escape; adding from time to time, during the evaporation, some water, to prevent blackening of the mass. Place the silver dish in cold water until it has sufficiently cooled; remove the cake of caustic potassa from the dish, break it into coarse lumps in a hot mortar, and keep in a well-closed glass bottle. When required for use, dissolve some of it

in water.

The hydrate of potassa so prepared is sufficiently pure for most purposes; it contains, indeed, a minute trace of alumina, but is usually free from phosphoric acid, sulphuric acid, and silicic acid. The solution must remain clear upon addition of sulphide of ammonium; hydrochloric acid must only produce a barely perceptible effervescence in it. The solution, acidified with hydrochloric acid, must, upon evaporation to dryness, leave a residue which dissolves in water to a clear fluid; when boiled with molybdate of ammonia, it must exhibit no yellow colour; when treated with ammonia, it ought not to deposit slight flakes of alumina immediately; but only after standing several hours in a warm place.

c. Hydrate of potassa prepared with baryta.

Preparation. Dissolve pure crystals of baryta (§ 32) by heating with water, and add to the solution pure sulphate of potassa, until a portion of the filtered fluid, acidified with hydrochloric acid and diluted, no longer gives a precipitate on addition of a further quantity of the sulphate (16 parts of crystals of baryta require 9 parts of sulphate of potassa). Let the turbid fluid clear, decant, and evaporate in a silver dish as in b. The hydrate of potassa so prepared is perfectly pure, except that it contains a trifling admixture of sulphate of potassa, which is left behind upon dissolving the hydrate in a little water. This hydrate is but rarely required, its use being in fact exclusively confined to the detection of minute traces of alumina.

Uses.-The great affinity which the fixed alkalies possess for acids renders these substances powerful agents to effect the decomposition of the salts of most bases, and consequently the precipitation of those bases which are insoluble in water. Many of the so precipitated oxides redissolve in an excess of the precipitant, as, for instance, alumina, sesquioxide of chromium, and oxide of lead; whilst others remain undissolved, e. g., sesquioxide of iron, teroxide of bismuth, &c. The fixed alkalies serve therefore also as a means to separate the former from the latter. Potassa and soda dissolve also many salts (e. g., chromate of lead), sulphur compounds, &c., and serve thus to separate and distinguish them from other substances. Many of the oxides precipitated by the action

of potassa or soda exhibit a peculiar color, or possess other characteristic properties that may serve to lead to the detection of the individual metal which they respectively contain; such are, for instance, the precipitate of protoxide of manganese, hydrate of protoxide of iron, suboxide of mercury, &c. The fixed alkalies expel ammonia from its salts, and enable us thus to detect that body by its smell, its reaction on vegetable colors, &c.

§ 31.

2. AMMONIA-Oxide of Ammonium-(N H, O).

Preparation. The apparatus illustrated by Fig. 18 (§ 28) may also serve for the preparation of solution of ammouia, with this modification however, that no funnel tube being required in the process, the cork upon the flask a has only one perforation for the reception of the tube which serves to conduct the evolved ammonia into the washing bottle. Introduce into a 4 parts of chloride of ammonium in pieces about the size of a pea, and dry hydrate of lime prepared from 5 parts of lime; mix by shaking the flask, and add cautiously a sufficient quantity of water to make the powder into lumps. Put a small quantity of water only into the washing bottle (which should be rather capacious); but have 10 parts of water in the flask which is intended for the final reception of the washed gas. Set the flask a now in a sand bath, connect it with the rest of the apparatus, place the flask d in a vessel of cold water, and apply heat. The evolution of gas speedily commences. Continue to heat until no more bubbles appear. Open the cork of the flask a, to prevent the receding of the fluid. The solution of ammonia contained in the washing bottle is impure, but that contained in the receiver d is perfectly pure; dilute it with water until the specific gravity is about 0.96 10 per cent. of ammonia. Keep the fluid in bottles closed with ground stoppers. This is the best way of preparing solution of ammonia in small quantities. That prepared on a large scale in cast-iron vessels is of course cheaper.

Tests. Solution of ammonia must be colorless, and ought not to leave the least residue when evaporated on a watch-glass, nor should it cause the slightest turbidity in lime water (carbonic acid). When supersaturated with nitric acid, neither solution of nitrate of baryta, nor of nitrate of silver, must render it turbid, nor must sulphuretted hydrogen impart to it the slightest color.

Uses.-Solution of ammonia, although formed by conducting ammoniacal gas (N H,) into water, and letting that gas escape upon exposure to the air, and much quicker when heated, is conveniently regarded as a solution of oxide of ammonium (N H, O) in water, the equivalent of water (HO) existing with NH being assumed to form with the latter N HO. Solution of ammonia is accordingly regarded as corresponding to the solution of potassa and soda, which greatly serves to facilitate the explanation of many chemical transformations into which the ammoniacal compounds enter, the oxygen salts resulting from the neutralization of oxygen acids by solution of ammonia being also assumed to contain oxide of ammonium N H, O, instead of N H. In the present edition, therefore, ammonia signifies oxide of ammonium (N H, O), and corresponds to the oxides of potassium and sodium, or potassa and soda. Ammonia is one of the most frequently used reagents. It is especially applied for the saturation of acid fluids, and also to effect the

4

precipitation of a great many metallic oxides and earths; many of these precipitates redissolve in an excess of ammonia, as, for instance, the oxides of zinc, cadmium, silver, copper, &c., whilst others are insoluble in free ammonia. This reagent may serve, therefore, to separate and distinguish the former from the latter. Some of these precipitates, as well as their solutions in ammonia, exhibit peculiar colors, which may at once lead to the detection of the individual metal which they respectively contain.

Many of the oxides which are precipitated by ammonia from neutral solutions, are not precipitated by this reagent from acid solutions, their precipitation from the latter being prevented by the formation of a salt of ammonia. Compare § 51, choride of ammonium.

B. ALKALINE EARTHS.

§ 32.

1. BARYTA (Ba O).

Preparation.-a. From sulphate of baryta.

Mix together 6 parts

of finely pulverized sulphate of baryta, 1 part of powdered charcoal, and 1 part of flour; or 8 parts of sulphate of baryta, 2 parts of charcoal, and 1 part of common resin. Put the mixture in a crucible, and expose it in a wind furnace to a long-continued red heat; or put the mixture in an earthen pot, lute the lid on with clay, and expose the pot to the heat of a brick-kiln. Boil the crude sulphide of barium obtained by this process for some time with water, and, when the crystallization point is attained, filter off hot let the filtrate cool in a well-covered vessel. Pour off the mother liquor, which, together with the residue left from the boiling of the sulphide of barium, may be used for the preparation of chloride of barium; boil the crystals with just sufficient water to dissolve them; keep boiling (taking care to replace the evaporated water); add finely triturated and sifted copper scales in small portions at a time, until a filtered sample of the fluid gives a pure white precipitate when mixed with a little acetate of lead. Filter boiling. As solution of baryta eagerly absorbs carbonic acid from the air, great care must be taken to exclude the air in the processes of filtration and crystallization. To effect this it is necessary to place the beaker intended to receive the filtrate on a flat dish or flat iron pot containing a little milk of lime, invert a bell-glass over it with an opening in the top, insert the funnel through this, filter, then remove the funnel, close the opening in the top of the bell-glass with an india-rubber cap, and let the fluid thus protected from access of air stand for several days in a cool place. After this decant the fluid (baryta water), let the crystals (Ba O, HO+ 8 aq.) drain in a wellcovered funnel, dry them quickly between sheets of blotting-paper, and keep them in well-stoppered bottles. For use dissolve 1 part in 20 parts of water, and filter. The baryta water so obtained is preferable to that decanted from the crystals.

b. From nitrate of baryta. Project a mixture of 8 parts of finely pulverized nitrate of baryta and 3 parts of clean iron filings, a spoonful at a time, but not in too quick succession, into a ret-hot Hessian, or, better still, into a red-hot iron crucible. When the mass has left off

frothing, and become pasty and in the end friable, take it out with an iron spatula, let it cool, and when cool powder it. Boil the crucible now for half an hour with 64 parts of water in an iron pot, take the crucible out, put the powdered mass into the boiling fluid, add 16 parts more of water, boil another half hour, filter, and proceed with the filtrate as in a (Wittstein).

c. From Witherite. Mix intimately 100 parts of finely pulverized Witherite, 10 parts of powdered charcoal, and 5 parts of resin; or 100 parts of Witherite and 15 parts of finely powdered caking coal. Put the mixture in a pot or crucible, lute on the lid with clay, and expose to the heat of a brick-kiln. Triturate the mass, and boil the powder with water; filter, and proceed with the filtrate as in a. The undissolved residue, which consists of undecomposed Witherite and coal, may be used in the preparation of chloride of barium. This is a most excellent method, both as regards cheapness and purity of product.

Tests. Solution of baryta, or baryta water, must, after precipitation of the baryta by sulphuric acid, give a filtrate remaining clear when mixed with spirit of wine, and leaving no fixed residue upon evaporation in a platinum crucible.

In

Uses. Caustic baryta, being a strong base, precipitates the earths and metallic oxides insoluble in water from the solutions of their salts. the course of analysis we use it simply to precipitate magnesia. Baryta water may also be used to precipitate those acids which form insoluble compounds with this base; it is applied with this view to effect the detection of carbonic acid, the removal of sulphuric acid, phosphoric acid, &c.

The former is obtained by slacking pure calcined lime in lumps, in a porcelain dish, with half its weight of water. Hydrate of lime must be kept in a well-stoppered bottle.

To prepare lime water, digest hydrate of lime for some time with cold distilled water, shaking the mixture occasionally; let the undissolved portion of lime subside, decant, and keep the clear fluid in a well-stoppered flask.

Tests.-Lime water must impart a strongly-marked brown tint to turmeric paper, and give a considerable precipitate with carbonate of soda. It speedily loses these properties upon exposure to air, and is thereby rendered totally unfit for analytical purposes.

Uses.-Lime forms with many acids insoluble, with others soluble salts. Lime water may therefore serve to distinguish the former acids, which it precipitates from their solutions, from the latter, which it will of course fail to precipitate. Many of the precipitable acids are thrown down only under certain conditions, e. g., on boiling (citric acid), which affords a ready means of distinguishing between them by altering these conditions. We use lime water in analysis principally to effect the detection of carbonic acid, and also to distinguish between citric acid and tartaric acid. Hydrate of lime is chiefly used to liberate ammonia from the salts of ammonia.