56 CONVERSION OF THE UNDISSOLVED SUBSTANCES, &c. [§ 21.

Y. FLUXION WITH ACID SULPHATE OF POTASS.

One part of finely pulverized and elutriated mineral is mixed with from five to six parts of powdered acid sulphate of potass in a platinum crucible, which ought to be covered in order to avoid the too rapid evaporation of the excess of acid; the flame of a spirit-lamp is then applied until the contents of the crucible begin to fuse, and the same temperature is maintained until the whole mixture is fused into a transparent mass. This mass, after cooling, is mixed with water and heated to boiling. According to the nature of the mineral operated upon, either the whole mass will be dissolved by this process, or only a part of it; the rest remaining undissolved.

$ 21.

4. CONVERSION OF THE DISSOLVED SUBSTANCES INTO A

PONDERABLE FORM.

The conversion of a substance in solution, into a form adapted for weighing, may be effected either by evaporation, or by precipitation. The former of these operations is applicable only in cases where the substance, the weight of which we are desirous to ascertain, either exists already in the solution in the form suitable for the determination of its weight, or may be converted into such form by evaporation with some re-agent. The substance must, moreover, be in solution, either by itself, or, at least, mixed only with such bodies as are expelled by evaporation, or at a red heat. Thus, for instance, the amount of sulphate of soda present in an aqueous solution of that substance may be ascertained by simple evaporation; whilst, if we wish to determine the amount of carbonate of potass present in a solution of this substance, it will be more advisable to convert this carbonate of potass into chloride of potassium, by mixing its solution with solution of sal ammoniac, and evaporating this mixture.

Precipitation may always be resorted to whenever the substance in solution admits of being transformed into a state or combination in which it is insoluble in the menstruum.

§ 22.

a. EVAPORATION.

In evaporation for pharmaceutical, or technico-chemical purposes, we have principally to consider the saving of time and fuel; but when this operation is undertaken for the purpose of quantitative analysis, these points are but subordinate; our chief care is to prevent loss, or contamination of the substance operated upon.

Fluids in a state of actual ebullition lose continually and unavoidably minute drops by bubbling and spirting. It is, therefore, advisable to use the water-bath at the boiling point for the evaporation of aqueous solutions; but for the evaporation of alcoholic or etherial solutions, the temperature of the water-bath may be more or less high, according to circumstances; of course always falling short of the boiling point. This arrangement will protect the solution from too high a temperature.

Should the operator possess a Beindorf's steam apparatus, or one similarly constructed, this may be advantageously substituted for the water-bath; otherwise we recommend a water-bath, constructed as follows.

PLATE XIV.

a

The engraving represents basin made of strong sheet copper. For use, this basin is half filled with water, which is kept boiling by the flame of a spirit or oillamp. Several rings, adapted to the different size of the evaporating dishes or saucepans, serve to

receive the latter The contents of the vessel from a to bought

to be from four to six inches.

Larger quantities of fluids may be evaporated also in the sandbath, or in a common stove, or even over a free fire, if we guard carefully against ebullition. Evaporation over a free fire, when properly and cautiously conducted, is a very convenient, expeditious, and clean process, which may be adopted in all cases where the fluids operated upon are clear, and not apt to deposit a precipitate during their evaporation. The operation is conducted most advantageously over a spirit or oil-lamp, the flame of which may easily be regulated, so as to maintain the temperature between 158° and 194°.

This species of evaporation ought, however, to be continued only as long as the fluid remains pretty dilute. The evaporation to dryness is invariably conducted best in a water-bath, or other bath of sufficiently low temperature.

The evaporation of fluids containing an insoluble sediment cannot well be conducted in the common way, in a dish over the free flame of a spirit or oil-lamp, even although but a very moderate degree of heat be maintained, since it would be hardly possible to prevent slight explosions of steam bubbles, arising from the sediment impeding the equal distribution of the heat. This may, however, be remedied by conducting the evaporation in a crucible, placed obliquely on the flame, as illustrated in Plate XV.

PLATE XV.

In this process the flame is guided in such a manner as to play upon the crucible above the level of the fluid.

Many salts have a tendency upon the evaporation of their solutions, to ascend efflorescing along the walls of the evaporating vessel; and, finally, to extend this efflorescence over the border of the latter; this tendency, of course, may cause a loss of the substance; however, if the solu

tions of such salts be evaporated in crucibles in the manner illustrated in Plate XV., there will be no danger of any loss arising. from this tendency, since the upper parts of the vessel will be so hot as to cause the ascending liquid to evaporate immediately, leaving the salt behind. When the operation is conducted in dishes in the common way, the disadvantage just alluded to may generally be remedied, simply by covering the border and uppermost part of the internal surface of the dish with an extremely thin coating of tallow.

Fluids evolving gas bubbles upon evaporation require, in the first place, to be heated in an obliquely-placed flask until all the gas is expelled. If this necessary preliminary be neglected, it will be found impossible to evaporate such fluids without loss.

If the operator can manage to conduct his processes of evaporation in a private room, where he may easily guard against any occurrence tending to suspend minute particles of dust or ashes in the air, he will find it a very easy task to keep the evaporating fluid properly pure, even without covering the dishes or saucepans in which the operation is conducted.

But in a large laboratory, or in a room exposed to draughts of air, or in which a coal fire is burning, the greatest caution is required to shield the evaporating fluids from the contamination of dust, dirt, and ashes.

PLATE XVI.

For this purpose the evaporating vessel is either covered with a sheet of blotting-paper, turned closely down over its borders, or a glass rod bent into a triangular shape, (Plate XVI.,) is placed upon the borders of the vessel, and a sheet of blotting-paper spread over it; and the position of the latter maintained by laying a glass rod across it, which is prevented from rolling down by the slightly turned-up extremities (a and b) of the triangle.

a

The following method, however, may be recommended as the

most advantageous.

Order of any sieve-maker two small and thin wooden hoops,

PLATE XVII.

(Plate XVII.,) one of which must be made to fit loosely in the other; spread a sheet of blotting-paper over the smaller of the two, and push the larger

over it. This will form a cover admirably adapted to the purpose for which it is intended. It will protect the evaporating fluid against any particles of dust floating in the air; it may be readily removed from the evaporating vessel; the paper does not come in contact with the fluid; and this kind of cover does not, in the slightest degree, impede or retard the process of evaporation. It is, moreover, very durable, and may at any time be readily renovated.

It is necessary sometimes to heat the salt residue remaining, upon evaporation in the water bath above 212°. For this purpose we may either use a sand-bath or an air-bath; the apparatus represented by Plate XIV is perfectly well adapted for the latter, (although it gets spoiled in course of time when put to this use ;) during the process, the salt mass ought to be continually stirred with a glass rod, or platinum spatula.

Evaporation is usually conducted in platinum, silver, porcelain, or glass dishes; sometimes in beaker-glasses, flasks, or retorts; and also in crucibles, as I have already had occasion to mention. In cases where a fluid is to be evaporated with exclusion of the air, a tubulated retort is used, and, according to circumstances, either hydrogen gas, or carbonic acid gas, is introduced through a tube inserted into the tubular aperture of the retort, and reaching into the latter within a very short distance of the surface of the liquid. The question, which kind of vessels are best suited to the various substances? may, after the remarks which we have had occasion to make in the course of this paragraph, be left to the operator's judgment.

With respect to the material of the evaporating vessels, the first and most essential point for consideration is that the evapo