closed either within its lamellæ, or adhering to it from the mode of its preparation, or absorbed by it from the atmosphere. It is perfectly obvious that to estimate correctly the quantity of a substance, we must, in the first place, remove the water which it may happen to hold in admixture. Most solid bodies, therefore, require to be dried before they can be quantitatively analysed.

The operation of drying is of the very highest importance for the correctness of the results; indeed it may safely be averred that many of the differences observed in analytical researches proceed entirely from the fact that substances are analysed in different states of moisture.

It must be borne in mind, of course, that many substances contain water among their essential constituents (constitutional, or basic, water, and water of crystallization). With this water we must not interfere; the operation of drying, which we have here in view, is intended only to remove the water accidentally admixed, or mechanically adhering to the substance, and which we will term here "moisture," the better to distinguish it from the water essentially inherent in a substance. Accordingly, in the drying of substances for quantitative analysis, our only object is to remove all moisture from them, without interfering, in the slightest degree, with their constitutional water, or any other essential constituent. To accomplish this object, it is absolutely requisite that we should know the properties which the substance under examination manifests in the dry state, and whether it loses water or other constituents at a red heat, or at 212° F., or in dried air, or even simply in contact with the atmosphere. These data will serve to guide us in the selection of the process of desiccation best suited to the substance under examination.*

The following classification may accordingly be adopted :—

a. Substances which yield water even in simple contact with the atmosphere; such as sulphate of soda, crystallized carbonate of soda, &c. Substances of this kind turn dull and opaque when exposed to the air, and finally crumble wholly or partially to a white powder. They are more difficult to dry than many other bodies. The process best adapted for the purpose, is to press the pulverized salts with some degree of force between thick layers of fine blotting-paper, until the last sheets remain absolutely dry.

It is generally advisable to subject the pulverized salts, in the course of this operation, once more to the process of trituration.

b. Substances which do not yield water to the atmosphere (unless perfectly dry), but effloresce in artificially dried air; such as sulphate of magnesia, tartrate of potassa and soda (Rochelle salt), &c. Salts of this kind are reduced to a fine powder, which, if it be very moist, is pressed between sheets of blotting-paper, as in a; after this operation, it must be allowed to remain for some time spread in a thin layer upon a sheet of blottingpaper, effectually protected against dust, and shielded from the direct rays of the sun.

$ 27.

c. Substances which undergo no alteration in dried air, but lose water

The dried substance should always at once be transferred to a well-closed vessel; glass tubes, sealed at one end, and of sufficiently thick glass to bear the firm insertion of tight-fitting smooth corks, are usually employed for this purpose. It is advisable to put tinfoil under the cork.

at 212° F.; tartrate of lime, for instance. These are finely pulverized; the powder is put in a thin layer into a watch-glass or shallow dish, and the latter placed inside a bell-glass, in which the air is kept dry by means of sulphuric acid. This process is usually conducted in one of the apparatus illustrated by Figs. 22 and 23, which are technically termed desiccators, and subserve still another purpose besides that of drying, viz., to allow hot crucibles, dishes, &c., to cool in dry air.

In Fig. 22, a represents a glass plate (ground glass plates answer the purpose best), b a bell jar, with ground rim, which is greased with tallow; c is a glass basin with sulphuric acid; d a round iron plate, supported on three feet, with circular holes of various sizes, for the reception of the watch-glasses containing the substances to be dried, or of crucibles in course of cooling, &c.

In Fig. 23, a represents a beaker with ground rim, and filled to onefourth or one-third with concentrated sulphuric acid; b is a ground-glass plate, the rim of which is smeared with tallow; c is a bent wire of lead, which serves to support the watch-glass

containing the substance to be dried.

Fig. 24 represents a portable exsiccator, used more particularly to receive crucibles in course of cooling, and carry them to the balance. The instrument consists of a box made of strong glass; the lid must be ground to shut airtight; the place on which it joins is greased with tallow. The outer diameter of my boxes is 105 mm.; the sides are 6 mm. thick. The aperture has a diameter of 80 mm.; the box up to the groove is 65 mm. high; the lid has the same height; the rim above the groove is 15 mm. high, and ground to a slightly conical shape. A brass ring, with grooved rim fitting close under the glass, fits exactly into the aperture; the upper border of the ring must not project above the glass. The ring bears a triangle of iron, or, better, platinum wire, intended for the reception of crucibles, &c.

Fig. 24.

Fig. 25 represents the desiccator constructed by A. Schrötter, upon the principle of affording free egress to the air, which expands as soon as hot crucibles are placed inside the apparatus; the expanding air escapes, in the first place, through the little tube a, then through the two lateral apertures in the lower part of b; the air-bubbles lastly, which ascend through the sulphuric acid in c, make their escape from the bulb d, which is filled with chloride of calcium. When the apparatus

is cooling, perfectly dry air re-enters by the same way. The operation may be considered at an end when no more air-bubbles ascend through the sulphuric acid. The little tube e serves to catch the sulphuric acid that might be carried down through a; this tube must not close the lower orifice of the bell-jar air-tight, and the cork holding it must be channelled serves as stand for the bell-glass. This desiccator affords the advantage that the substances placed in it are cooled in dry air of the common pressure, and have accordingly, when removed from the apparatus, no tendency to attract moist air, which cannot be said of substances cooled in air slightly rarefied by heat. The body which it is intended to dry is kept exposed to the action of the dry air in the glass, until it shows no further diminution of weight. Substances upon which the oxygen of the air exercises a modifying influence are dried in a

similar manner, under the exhausted receiver of an air-pump. Substances which, though losing no water in dry air, yet give off ammonia, are dried over calcined lime, mixed with some chloride of ammonium in powder, and consequently in an anhydrous ammoniacal atmosphere.

§ 28.

d. Substances which at 212° F. completely lose their moisture, without suffering any other alteration, such as bitartrate of potassa, sugar, &c. These are dried in the water-bath; in the case of slow drying substances, or where it is wished to expedite the operation, with the aid of a current of dry air.

Fig. 26 represents the water-bath most commonly used.

It is made either of

Fig. 26.

sheet tin, or, better, of sheet copper, and is soldered with brass, to adapt it for use also as an oil-bath. The engraving renders a detailed explanation unnecessary. The inner chamber, c, is surrounded on five sides by the outer case or jacket, de, without communicating with it. The object of the apertures g and h is to effect change of air, which purpose they answer sufficiently well. When it is intended to use the apparatus, the outer case is filled to about one-half with rain-water, and the aperture a is closed with a perforated cork, into which a glass tube is fitted; the aperture b is entirely closed. If the apparatus is intended to be heated over charcoal, it should have a length of about 20 centimetres from d to f; but if over a gas-, spirit- or oil-lamp, it should be only about 13 centimetres long. In the former case, the inner chamber is 17 centimetres deep, 14 centimetres broad, and 10 centimetres high; in the latter case, it is 10 centimetres deep, 9 centimetres broad, and 6 centimetres high. The temperature in the iuner chamber never quite reaches 212° F.; to bring it up to 212° F., F. Rochleder has lately suggested to close b with a doublelimbed tube, the outer longer limb of which dips into a cylinder filled with water; a is in that case closed with a perforated cork bearing a sufficiently tall funnel tube, which fits air-tight in the cork. The lower end of this tube reaches down to one inch from the bottom. In large analytical laboratories water is usually kept boiling all day long, for the production of distilled water. The boilers used in my own laboratory have the shape of somewhat oblong square boxes, about 120 centimetres long, 60 centimetres broad, and 24 centimetres high; the front side of the boiler has soldered into it, one above the other, two rows of dryingchambers, or closets, of the kind shown in Fig. 26. This gives a sufficient number of drying-chambers to enable me to afford the special use of one of them to almost every student in the laboratory. Most of these little apparatus are from 11 to 12 centimetres long and broad, and 8 centimetres high; some of them, however, are 16 centimetres long and broad, to fit them to receive also larger-sized dishes. The substances to be dried are usually put on double watch-glasses, laid one within the other, which are placed in the inner chamber; the apparatus is then closed. In the subsequent process of weighing, the upper glass, which contains the substance, is covered with the lower one. The glasses must be quite cold before they are placed on the scale. In cases where we

have to deal with hygroscopic substances, the re-absorption of water upon cooling is prevented by the selection of close-fitting glasses, which are held tight together by a clasp (Fig. 27), and allowed to cool

Fig. 27.

with their contents under a bellglass over sulphuric acid (see Fig. 22). These latter instructions apply equally to the process of drying conducted in other apparatus.

The clasps used for keeping the watch-glasses pressed togetherand which in all cases where it is intended to ascertain the loss of

weight which a substance suffers on desiccation, are to be looked upon as part and parcel of the glasses, and must accordingly be weighed with them are constructed of two strips of thin brass plate, about ten centimetres long, and one centimetre wide, which are laid the one over the other, and soldered together at the ends, to the extent of 5 to 6 millimetres.

The following apparatus serve for drying substances in a current of air.

In Fig. 28 (A) the current is caused simply by heating the air, which renders this apparatus very convenient for use.

ab is a case of sheet copper, or sheet tin, into which the canal cd is soldered; the latter communicates with the chimney ef; this is surrounded on three sides by the cover g h, which again communicates with a b. The cover has no opening at the top. At i is a round aperture leading into the canal,

and which may be closed with a cork; lk is provided with a well-fitting sliding lid running in grooves.

The operation is commenced by filling the case a b to one half with water, through the aperture m-the aperture n, which serves to let off the water, being closed with a cork-and heat is then applied to raise the temperature of the water to the boiling-point. The watch-glasses with the substances to be dried are placed into the hollows of the sliding shelf, B, and the latter is introduced into the canal c d at l k, which is then closed by the sliding lid.

The air in the canal, becoming heated by the surrounding steam, ascends, and the cold atmospheric air flowing in through the aperture i and passing over the drying substances, carries away with it the evaporating moisture. There is a slight disadvantage attending this mode of proceeding, viz., the cold air which passes through i keeps the drying substances always a little below 212° F.; this may be easily remedied, however, by conducting the air, in the first place, through a tube running along under the bottom of the canal down to lk, and returning