water is added; the filter is then filled up with pure water, hot or cold, according to circumstances; and fresh quantities are added, as the filter becomes empty, until the solid is sufficiently washed. It is advisable in washing a solid, to

Fig. 50.

bring the whole of the substance together into one mass, and not to allow it to remain as a coating upon the sides of the filter. This may be conveniently effected by forcing a jet of water against the sides until it is detached and carried down by the stream. The apparatus employed for obtaining this jet is the syringe, or washing-bottle. There are two or three forms of washing-bottles; the most convenient is constructed in the following manner: a narrow-mouthed bottle or flask is fitted with two tubes, the one bent in the . form of a siphon, the long arm of which passes nearly to the bottom of the bottle, the extremity of the short arm being drawn out to a narrow point (so as to deliver a fine stream of water); the other tube is bent at an angle of about 100°, its two arms are nearly equal (the longest about three inches in length); the shorter arm is fitted into the cork, so as just to project inside the bottle. On filling the latter with a liquid, inserting the cork and tubes, and blowing into the bottle through the short tube, the liquid is forced up through the long one, and passes out at the short arm with considerable force, in a small stream. By inverting the bottle, the air will enter through the long tube, and the water pour out of the smaller one, the opening of which is not contracted; some force may be imparted to the current of water thus obtained by blowing into the bottle through the narrow opening of the long tube. By throwing a jet of water against the sides of the filter, and varying its direction by a movement of the hand in which the bottle is held, the portion of solid that adheres to them may be detached and washed down towards the point of the funnel.1

Some substances adhere so obstinately to the sides of the filter as not to be easily removed by the washing-bottle; they should, in that case, be detached by means of a glass rod with a round extremity, great care being taken not to damage the filter. With some substances, it is also necessary to stir up the mass upon the filter with a glass rod, in order to disturb the fissures that form in it, which would otherwise allow the water poured upon the filter to pass through them, without penetrating the greater portion of the substance.

In filtering off or washing a substance, it is advisable to keep the funnel covered with a glass plate, which serves to exclude any impurities that might otherwise fall into the funnel, and also to prevent evaporation if the liquid filtered is volatile (particularly in hot filtrations). Large crystals, or considerable masses of substance, may be expeditiously and almost completely separated from liquid, by throwing them upon a funnel, into the neck of which a small piece of tow, or asbestos, or glass rod is introduced, allowing the liquid to drain off

§ 52. DECANTATION.-Some solids may be separated from fluids, or purified by a process termed decantation. This consists in allowing the solid to subside to the bottom of the vessel (tall jars being most convenient), and removing the clear supernatant liquid either by pouring it off slowly, or by decanting it with a siphon or pipette. Glass siphons are best adapted for this purpose; their size and bore should be suited to the quantity of material operated upon; but little need be said with regard to their use. The mouth of the short arm should not be allowed to approach the solid too closely, lest a portion of it should be

1 The washing-bottle is also very useful for adding small quantities of water or other solvents to a solid, in effecting its solution, or for washing it down to the bottom of any vessel when it adheres to the sides.

stream.

sucked into the siphon by the force of the ascending current. For small operations, where the liquid is to be decanted to a great nicety, it is advisable somewhat to contract the orifices of the siphon, in order to reduce the size of the The long arm should be held by one hand, near the extremity, in such a manner that the thumb may be directly opposite its mouth when it is necessary to stop the current. When the supernatant liquid has been decanted from a substance, the vessel containing the latter is filled up with water, the substance is thoroughly suspended in it by stirring the two well together with a glass rod, and it is then allowed to subside perfectly, and the water decanted when clear; this operation being repeated until the substance is perfectly washed.

Fig. 51.

Small quantities of liquid may be decanted from solids with greater safety by means of the pipette, which consists of a narrow glass tube, contracted at one extremity, and provided with a large bulb at about three inches from the other end, which is bent, above the bulb, at an angle of 120°. By immersing the constructed extremity in the liquid, and applying suction to the other end with the mouth, it may be made to ascend into the bulb; when the latter is filled, the tongue is pressed tightly against the opening of the bent end, the pipette is then removed from the vessel, and the liquid transferred to a receiver.

Care must be taken not to allow the liquid to flow back when once in the pipette, as this would occasion a disturbance of the particles of solid, and thus prevent for a time the removal of the liquid.

This tube is also of great use for the separation of two immiscible liquids. The operation is similar to that just described, the pipette being introduced into the upper or lower liquid, so that its extremity reaches nearly to the bottom of the layer. It is well, in separating liquids in this mauner, to introduce them first into long narrow glass jars or tubes, so as to contract the area of the column of liquid by increasing its height, and thus rendering the separation of the liquids by the pipette far easier and more effectual. This operation may also be effected in various other ways. Oils may be separated from water, in which they are insoluble, by throwing them upon a wet filter. The water will pass through, and the oil may then be removed by piercing a hole in the bottom of the filter. Glass funnels, of which the stem is furnished with a tightly-fitting stopcock (separating funnels), are also very useful for separating liquids. The stopcock being closed, the liquids are poured into the funnel, and allowed to separate perfectly. By then opening the cock, the lower liquid may be removed from the upper, care being taken to allow it to run through very slowly as it decreases in quantity, and to close the cock directly the upper liquid approaches it. These funnels are frequently made in the form of globes, provided with an opening at the top; the loss of liquids by evaporation is much decreased by their use. The siphon may also be used for the separation of liquids.

Fig. 52.

EVAPORATION.

§ 53. The apparatus necessary for evaporations are the same as those required for solution, with the addition of watch-glasses and small slips of glass. The dishes employed should, however, if possible, be more shallow than those used for making solutions.

Evaporation takes place at almost any temperature. When a liquid is allowed to evaporate slowly at the common temperature, it is said to undergo spontaneous evaporation. This species of evaporation is had recourse to principally in effecting crystallization, and in the desiccation of some substances to be presently mentioned. The vessel containing the liquid is covered with filtering-paper, to exclude any extraneous impurities, and then placed in a dry situation, where there is a continual access of air to remove the vapor as it gradually forms. Spontaneous evaporation may be very much assisted by the use of desiccators.

Fig. 53.

2

The method most generally adopted is to place. the vessel containing the liquid under a bell-jar, together with another containing sulphuric acid, which has a great affinity for water; or better still, to place these two vessels together under the receiver of an air-pump.

As the air is exhausted, the water passes readily into vapor, which is directly absorbed by the sulphuric acid, thus giving place to a fresh quantity of vapor, which is in turn absorbed. The vessel containing the sulphuric acid should present a much larger surface than that containing the liquid to be evaporated, and they should be so placed that the latter is supported by the former. Other substances having a great affinity for water, may be substituted for sulphuric acid, such as chloride of calcium, fused potassa, or quicklime.

When the evaporation is effected by the aid of heat, it is applied in the manner directed for the solution or distillation of substances. When the evaporation is to be conducted at a constant temperature, recourse is had to the open water-bath already described, or to a similar bath filled with oil, if a temperature above 212° is required, a thermometer being retained in the bath to indicate the temperature during the operation.

Evaporation effected below ebullition should be conducted in dishes (which should be kept covered by a piece of filtering-paper of proper size, through which a glass rod has been passed to act as a support): any crusts which may form upon the surface of the liquid should be disturbed from time to time, to prevent their retarding vaporization.

Evaporations at the boiling temperature are most safely conducted in flasks, as the liquid is thus prevented from spirting over the sides of the vessel: evaporation, when thus conducted, proceeds naturally much more slowly, as the vapor generated cannot escape so readily, and is also continually condensed to some extent. The same precautions must be attended to in the application of heat in evaporations as in distillation and solution.

In evaporating a solution to dryness, the residual mass should be diligently stirred, while the last portion of water is passing off; this considerably promotes evaporation, and lessens the risk of loss of substance from spirting. It is

generally advisable to expose the residue for a lengthened period to a comparatively low temperature, in order to expel the last traces of moisture.'

§ 54. The expulsion of moisture from solid substances, is termed desiccation. This is effected at various temperatures, according to the nature of the substance. The greater portion of moisture may be removed mechanically from many substances, by pressing them, in a state of powder, between folds of blotting paper. Some bodies require perfect desiccation at the ordinary temperature; these are exposed over a desiccating agent, in vacuo, until they no longer part with any moisture. Other substances are desiccated in the wateroven; a double metal box, between the sides of which water is contained, the temperature being maintained at 212° by means of a lamp placed underneath the inner box is provided with apertures through which a current of air may circulate. For desiccation at higher temperatures we employ an air-bath (of which va rious forms exist), also provided with draught

Fig. 54.

holes and with a thermometer, so that the temperature may be properly regulated. Previously to desiccation, the substance is reduced to as minute a state of division as possible.

A crude test of its dryness after desiccation, is that of holding a cold clean glass plate over it while warm, and observing whether any film of moisture be deposited thereon. The most accurate test is, however, that of weighing the substance before desiccation, again ascertaining its weight after it has been exposed to the proper temperature for about an hour, and repeating the weighings at intervals of half an hour, until the last two agree with each other.

Some rather volatile substances may be desiccated by maintaining them at a moderate temperature, and at the same time passing a current of air over them. The substance is contained in a bulb-tube, one end of which is connected with a long chloride of calcium drying-tube (or with a wash-bottle containing oil of vitriol), the other with a tube leading into the top of a large closed vessel filled with water, and having one or two openings at the top for the reception of tubes (being kept closed when not in use), and a cock fitted into its side, near to the bottom. Such a vessel is termed an aspirator. Upon turning the cock, the water will run out of the vessel and become replaced by air, which first passes through the chloride of calcium tube (where it is dried), thence over the heated substance into the aspirator, carrying with it any portions of moisture that may be given off by the substance operated upon. It is necessary to introduce a second drying-tube between the aspirator and tube containing the substance, lest the suspension of the operation should allow moist air to pass back into the tube. When a substance has been perfectly dried, it should be at once transferred to a well-closed bottle or tube, in which it is preserved until required; unless this precaution is taken, the substance will absorb moisture to a greater or less extent in almost every case; some bodies absorb moisture rapidly at common temperatures, i. e. are deliquescent; these should be transferred as rapidly as possible

It is often found convenient, in evaporating to dryness, to place the evaporating dish upon an empty tin pot, heated over a flame, the application of heat being thus rendered more uniform and gradual. Should the residue adhere firmly to the basin in which it has been evaporated, it is most easily removed by means of a spatula.

while warm. Crystals and precipitates may be conveniently and expeditiously dried by spreading them upon two or three folds of blotting-paper, and then placing the latter upon a porous tile, which is replaced by a fresh one as it becomes saturated with moisture. In many cases these tiles may be first gently heated; they then effect the desiccation of the substance by promoting vaporiza

tion.

CRYSTALLIZATION.

§ 55. The property possessed by many substances of assuming definite and peculiar crystalline forms, frequently serves as a characteristic by which they may be recognized and distinguished from one another.

Crystallization also affords an easy and effectual method of purifying chemical compounds; it is generally effected by means of solution, fusion, or vaporization. CRYSTALLIZATION BY SOLUTION.-A substance is crystallized from its solvent in one of two ways; by cooling the hot saturated solution, or by spontaneous evaporation. The first method is always resorted to when it is wished to effect rapid crystallization, the size and forms of the resulting crystals being of less importance than the purification of the substance. Those bodies which are less soluble in hot than in cold solvents are crystallized by the second method, which is also employed for procuring good crystals by gradual deposition.

A hot saturated solution, prepared according to the prescribed method (§ 50), is filtered, if necessary, as rapidly as possible (a hot-water funnel being employed when required), and allowed to cool undisturbed and gradually. If the solution is very concentrated, or has been agitated, or cooled rapidly, the crystals deposited are small, confused, and irregular in their forms; if it is wished to obtain regular crystals from a hot solution, the latter should not be too strong, and the above precautions should be attended to, in addition to which it is advisable to cover the opening of the vessel in which the crystallization is effected, to prevent evaporation on the surface of the liquid, and the consequent formation of a crust of confused crystals.

When the crystallization is to be effected by spontaneous evaporation, a saturated solution of the substance is prepared at the common temperature, and placed in a moderately shallow evaporating basin, which must afterwards be carefully covered with blotting-paper, unless the solution can be preserved in some place where no dust or impurity can reach it. Crystallization by spontaneous evaporation may be much assisted by placing the solution under a bell-jar, together with some rapid absorbent of moisture; or by introducing it under the receiver of an air-pump, together with a desiccating agent, and exhausting in the manner described under evaporation.

A

If crystallization is resorted to for the purification of a substance, the crystals obtained are separated from the solution (or mother-liquor) when perfectly cool, by gently decanting as much as possible from the crystals, and then throwing them upon a strainer or filter, and allowing the mother-liquor to drain off. small quantity of the solvent employed is then poured upon them, and allowed to drain off, so as to remove the mother-liquor still retained by them. If, on applying the proper tests to a small portion of the substance, it is now found sufficiently pure, it may be at once dried for use. Should it require further. purification, the crystals may be freed still more perfectly from the mother-liquor by pressing them between folds of blotting-paper, and then recrystallizing. This operation may be repeated until the resulting crystals are sufficiently pure.

There are some substances which lose more or less of their water of crystallization at ordinary temperatures, i. e. are efflorescent; it is, however, always necessary, when they are required anhydrous, to submit them to desiccation by one of the above methods.