generators, like those commonly employed in the Döbereiner hydrogen lamp. To construct such a generator, procure a glass cylinder 8 or 9 inches in diameter, and 12 to 15 inches high, and a stout tubu lated bell-glass 4 to 5 inches wide and 2 or 3 inches shorter than the cylinder. Procure also a basket of sheet-lead 3 inches deep, and an inch narrower than the bell-glass, whose sides and bottom are perforated with a number of small holes. Cast a circular plate of lead, of diameter 14 inches larger than that of the cylinder and of an inch thick. On what is intended for its under side solder three equidistant leaden strips to keep the plate in proper position as a cover for the cylinder. Obtain a well-made brass gas-cock, fit to each end of it either by its screws or by soldering, a piece of brass or leaden tube 3 inches long, to inches wide and stout in metal. Perforate the centre of the leaden plate, so that one of these tubes will snugly pass the orifice and secure it by solder, leaving 2 inches of the tube projecting below the plate. Attach a stout copper hook to the lower end of the tube by which to hang the leaden basket. By means of a good tightly fitting cork and cement, secure this tube into the neck of the bell-glass air-tight and so firmly that a weight of several pounds will not break the joining. Hang the basket, filled with lumps of fused sulphide of iron, within the bell and two inches above the bottom of the latter. To the tube, which extends above the stop-cock, attach, by a good cork, the neck of a 4 or 6 oz. tubulated globular receiver, which is to be loosely stuffed with cotton. To the tubulure of the receiver fit a tube bent at a right angle. On the depending arm of the latter tube is adapted, by a rubber connector, the wide tube which is to convey the gas into any solution. The cylinder is now filled with dilute sulphuric acid (1 vol. to 14 water) to within 1 inches of the top, the bell-glass, full of air, being in its place. By opening the cock the air is expelled, and by the action of the acid upon the sulphide of iron the bell-glass is speedily filled with hydrosulphuric acid which replaces itself without any waste until either the acid is saturated or the sulphide of iron is dissolved. In cold weather this apparatus operates best when slightly warmed, especially if the acid be not fresh.]

Sulphuretted hydrogen water (solution of hydrosulphuric acid) is prepared by conducting the gas into very cold water, which has been previously freed from the air by boiling. The operation is continued until the water is completely saturated with the gas, which may be readily ascertained by closing the mouth of the flask

the following modifications. Instead of a glass rod, as above prescribed, a stout tube of the same diameter may be used. The basket is simply made of sheet-lead or of a bit of wide lead tubing, or even a linen bag will suffice. A good cork will take the place of the caoutchouc stopper, particularly if it be soaked for some time in melted tallow to fill up its pores.]

with the thumb, and shaking it a little: if, upon this, a pressure is felt from within, tending to push the thumb off the aperture of the flask, the operation may be considered at an end; but if, on the contrary, the thumb feels sucked into the mouth of the flask, this is a sure sign that the water is still capable of absorbing more gas Sulphuretted hydrogen water must be kept in well-closed vessels, otherwise it will soon suffer complete decomposition, the hydrogen being oxidized to water, and a small portion of the sulphur to sulphuric acid, the rest of the sulphur separating. The best way of preserving it unaltered for a very long time is to pour the freshlyprepared solution immediately into small phials, and to place the latter, carefully corked, in an inverted position, into bottles filled with water.

Tests.-Pure sulphuretted hydrogen water must be perfectly clear, and strongly emit the peculiar odor of the gas; when treated with sesquichloride of iron, it must yield a copious precipitate of sulphur. Addition of ammonia must not impart a blackish appearance to it. It must leave no residue upon evaporation on platinum.

Uses.-Hydrosulphuric acid has a strong tendency to undergo decomposition with metallic oxides, forming water and metallic sulphides; and the latter, being mostly insoluble in water, are usually precipitated in the process. The conditions under which the precipitation of certain sulphides ensues differ materially; by altering or modifying these conditions, we may therefore divide the whole of the precipitable metals into groups, as will be found explained below. Hydrosulphuric acid is therefore an invaluable agent to effect the separation of metals into principal groups. Some of the precipitated sulphides exhibit a characteristic color indicative of the individual metals which they respectively contain. Hydrosulphuric acid serves thus more particularly for the special detection of tin, antimony, arsenic, cadmium, manganese, and zinc. For more ample information upon this point, I refer to the third section. The great facility with which hydrosulphuric acid is decomposed, renders this substance also a useful reducing agent for many compounds; thus it serves, for instance, to reduce salts of sesquioxide of iron to salts of protoxide, chromic acid to the state of sesquioxide of chromium, &c. In these processes of reduction, the sulphur separates in the form of a fine white powder. Whether the hydrosulphuric acid had better be applied in the gaseous form, or in aqueous solution, depends always upon the special circumstances of the case.

III. BASES AND METALS.
§ 31.

Bases are divided into oxygen bases and sulphur bases. The former result from the combination of metals or of compound radi

cals of similar character with oxygen, the latter from the combina. tion of the same bodies with sulphur.

The oxygen bases are classified into alkalies, alkaline earths, earths proper, and oxides of the heavy metals. The alkalies are readily soluble 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.

$ 32.

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, 15 parts of crystallized carbonate of soda of commerce and 3 parts of water, heat to boiling, and add, in small portions at a time, milk of lime prepared by pouring 3 parts of warm water upon 1 part 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 closely 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 6 or 7 parts. The solution so prepared has a sp. gr. of 1.13 to 1.15, and 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; or, better, the stopper is smeared with a little paraffine before putting it into its place. 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 spiritlamp, 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 diso. 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 11 water to a clear fluid; when boiled with molybdate of ammonia, it must exhibit no yellow color; 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 (§ 34) by heating with water, and add to the solution pure sulphate of potassa, until a portion of the filtered fluid, acidified by hydrochloric acid and diluted, o 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.

§ 33.

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

Preparation.-The apparatus illustrated by Fig. 22 (§ 30) may also serve for the preparation of solution of ammonia, 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