latter together with the solvent, and observing whether, after the liquids have been properly mixed, any separation takes place, or whether the substance to be dissolved has decreased in bulk. The solubility of a gas may be ascertained by observing whether the bubbles passing through the solvent diminish in size, or whether, on agitating a portion of the gas in a closed vessel together with the solvent, absorption takes place. The solvent power of a liquid, at ordinary temperatures, is generally increased more or less by the application of heat.

Numerous kinds of vessels are employed for effecting solution. When the application of heat is not required, the substance may be dissolved in stout glass vessels of various forms, test-glasses or lipped glasses, and jars may be used; should the substance require the aid of much agitation to effect its solution, it may be shaken together with the solvent in a stoppered or corked bottle of such dimensions as not to be more than two-thirds filled by the liquid.

The apparatus employed in the solution of substances by the aid of heat, are dishes and capsules, beakers, flasks, and stirrers.

DISHES.-These are made of various materials; those of earthen and Wedg wood ware will answer for the solution of many substances not requiring the application of a very high temperature; great care must, however, be taken that they are compact in substance, so as not to absorb any solution that may be introduced into them. They should not become stained by solutions of sulphate of copper or of indigo. The best dishes are those made of Berlin or Meissen porcelain, since they are very compact, and thoroughly glazed. They should be lipped, and should be selected as thin as possible, varying from one-eighth to one-fourth of an inch, according to their size. When employed for effecting solution, these dishes should be chosen as deep as possible. The substance, and particularly the glazing, of porcelain and earthenware dishes, is attacked, and sometimes to a great extent, by the solutions of various chemical compounds; in such cases it is necessary to have recourse to dishes of silver, or even small dishes or capsules of platinum. These should be provided with a lip.

BEAKERS.-These glass vessels are exceedingly useful for effecting solutions, since they are generally very thin and uniform in substance, and made of well annealed glass.

They are also very tall in proportion to their diameter, which is likewise a great advantage. They may be had of various sizes, from two inches to ten or twelve inches in height, and of proportionate diameter. Great care must be taken that the bottoms be not thicker in substance than the sides, and that they are without the knot or punty-mark at the bottom. A flatter kind of beaker, provided with a lip, is also imported from Germany, and is very convenient for effecting solutions.

FLASKS. The cheapest (and, in most cases, superior) flasks for dissolving substances, are the Florence oil-flasks; they have the advantage of being generally very thin at the bottom. When larger flasks are required, they should be of flint-glass; those of German manufacture are generally the best, on account of their uniformity of substance, and the goodness of the glass. It is more convenient to have flat-bottomed flasks for general purposes; the bottom should be uniform in thickness with the upper part of the flask, if not thinner; the necks should be rather wide, and provided with a projecting ring of glass, by which they may be securely held. If the flasks are round-bottomed, they may be conveniently supported on the table by the list-rings already referred to.

The STIRRERS employed are generally made of solid glass rod, from one-sixth to one-third of an inch in diameter, and four to ten inches in length. Their extremities should be carefully rounded off by fusion before the blowpipe. Some may be flattened, and others provided with a button of glass at the extremities. § 48. Solution may sometimes be effected in a mortar, by following the direc tions prescribed for levigation, and continuing the process until the whole of the

soluble substance has been removed from the mortar. Solution is always aided by previous mechanical division of the substance, particularly if only a portion of the body operated upon be soluble, the surface presented to the solvent being greatly increased thereby. Heat assists solution by increasing the power of the solvent, and also by establishing currents in the liquid, and thus continually exposing the substance to the action of fresh portions of the solvent. When the application of heat for effecting or promoting solution has to be continued for some time, it is advisable to make use of flasks which will retain more completely, and partially condense, the vapors evolved.1

Some substances require continued boiling, or treatment at lower temperatures with the solvent, in order to effect their solution, or the separation of soluble from insoluble portions. In the latter case, the process is called digestion. Dishes and beakers are preferable when the substance requires agitation in the solvent by means of stirrers, or when portions have to be added or removed during the operation.

The vessel in which the solution is effected should be heated very gradually; it is always safer to protect it from the flame or fire by the sand-bath or sandpot. When a temperature below 212° is required for solution or digestion, the vessel should be placed upon a water-bath.

All the precautions recommended in heating retorts in distillation should likewise be attended to in effecting solutions.

The addition of cold liquid to the hot contents of a basin should be effected very gradually (the latter having in all cases been previously removed from the source of heat), as, in descending rapidly, it would suddenly change the temperature of the bottom of the dish, and thus probably crack it.

A flask or dish is supported over the flame of a lamp by a tripod, or the ring of a retort-stand. Should the ring or tripod be too large, a triangle of strong iron wire may be placed across it, and the vessel firmly supported thereby. Should the contents of a flask be in danger of boiling over, the flask must be lifted away from the flame or sand-bath, the portion of it above the surface of the liquid being cooled down at the same time by blowing upon it; the vapor within will thus be, to a certain extent, condensed, and the ebullition diminished.

Small quantities of a substance may be dissolved in a test-tube. The latter should not be filled above two-thirds with liquid; it should then be held in an oblique direction in the upper part of the flame of a lamp; the fingers may be protected from the heat either by wrapping a thick piece of paper or cloth round the portion of the tube where it is held, and twisting the end together so as to form a handle, or by supporting it in the flame by means of a small metal clasp, with sliding ring and wooden handle, which is termed a tube-holder. In boiling liquids in test-tubes, the evolution of vapor is often very irregular and sudden, particularly when any dense solid is operated upon, the contents of the tube, or a portion of them, being frequently ejected in consequence; a slight and rapid agitation of the tube backwards and forwards in the flame will, to a great extent, prevent this.

When solvents boiling at low temperatures are employed, much economy of material is effected by partially closing the upper end of the tube with the forefinger, the vapor being thereby prevented from escaping, and allowed to condense in the cooler portion of the tube. The tube should not, in such cases, be filled more than about one-third, and the flame of the lamp need only be applied at intervals, to keep up ebullition.

If the solution of a substance is accompanied by effervescence, it should be

1 When somewhat volatile or precious solvents are employed, it is advisable to adapt a long wide tube to the mouth of the flask, in order that the condensed vapor may return to the latter.

effected in tall jars, or in flasks, in order to prevent the liquid from spirting over the sides of the vessel; the solvent should only be added gradually, that the action may not be too violent, and cause the liquid to froth up and overflow.

The very gradual addition of the proper amount of solvent, when considerable accuracy is necessary, may be effected by the washing-bottle, which will be described under the head of edulcoration.

Much difficulty is frequently experienced in pouring solutions from a flask, dish, or beaker, to another vessel, without spilling some portion. This may be

avoided by the following simple means: A glass rod, first wetted with the solution, is applied, in an almost vertical position, to the edge of the vessel containing the liquid, and its lower extremity allowed to dip into the vessel which is to receive the liquid; the full vessel is then gradually inclined, so as to allow the liquid to run down the rod in a steady stream; upon restoring the vessel to its original position, when the requisite quantity has been decanted, the drop of liquid that would otherwise run down the side of the vessel from the edge, is completely withdrawn by the rod.1

Various terms are applied to the solution of organic substances, and their extraction from vegetable matters. Infusion is effected by pouring hot water upon the substance, and straining off the liquid; decoction, by digesting the substance for some time with the solvent, by the aid of heat; and maceration by pouring hot or cold water upon the substance, and allowing it to digest for some time. Soluble constituents may be conveniently removed from porous bodies by lixiviation.

The substance is introduced, in the state of a coarse powder, into a large funnel, closed, at first, at the lower end of the neck, with a cork, and at the upper

end with a glass plate. Some large fragments of the substance to be dissolved, or a little plug of asbestos, or tow, are placed at the bottom of the funnel; these serve to retain the finer particles. The solvent is then poured upon the contents of the funnel, so as to cover the whole mass; the liquid, as it penetrates the particles, gradually takes up the soluble portions, and becoming dense, descends, making room for fresh portions of the solvent. After a time, the cork is withdrawn from the neck of the funnel, and the liquid collected as it drips through, being replaced in the funnel by the introduction of fresh quantities of the solvent, until the substance is exhausted. The use of the cork is in many cases unnecessary, when the soluble portion is easily extracted. This method is very convenient for the removal of vegetable principles from seeds and plants, and various forms of apparatus are in use for effecting extraction in this manner, under the name of displacement-apparatus, or percolators, the principal advantage of which over the simple funnel is that they prevent loss of the solvent by evaporation during the process.

§ 49. SATURATION.-A liquid is saturated with a solid or gas, when it has been charged with as much of either as it is capable of dissolving. The method of saturating liquids with gases has been described under the head of solution of gases ($36).

SATURATION OF A LIQUID WITH A SOLID.-A hot saturated solution is obtained by dissolving a moderate amount of the solid in the liquid, heated to the proper temperature, and then adding fresh quantities of the substance to the solution at intervals, until it refuses to dissolve any more, a quantity of the substance, consequently, remaining undissolved. Liquids generally dissolve a larger quantity of a solid, with the aid of heat, than they do at the ordinary temperature; the most expeditious way, therefore, of making a solution saturated when cold, is to prepare a hot saturated solution, to allow this to stand until perfectly cool, and to separate the portion of substance that has remained undissolved, or been deposited in the cooling of the liquid.

In order to ascertain whether a solution is saturated while hot, a drop of the liquid is transferred, by means of a glass rod, to a cold watch-glass, or piece of glass-plate; the deposition of crystals of a solid substance indicates the saturation of the solution.

This deposition may be promoted by stirring or agitation.

With some substances the application of heat is not admissible in the preparation of a saturated solution: in such cases the solvent is placed in contact with the powdered substance in the cold, in a stoppered bottle, or vessel in which it can be agitated. Should the whole of the substance first added dissolve after agitation, a fresh portion is placed in contact with the solution, and this is persevered in, either until there is no perceptible diminution in the bulk of the solid in the vessel, or, if accuracy is required, until the weight of a certain amount of the solid is not diminished by being left some time in contact with the clear solution.

FILTRATION, EDULCORATION, DECANTATION, AND SEPARATION OF LIQUIDS.

50. The separation of solids from liquids is effected by filtration or decantation. The apparatus required for this purpose are funnels and funnel-stands, tall jars or beakers, stirring-rods, glass plates, and filtering-paper.

Filtrations on the large scale are performed in conical bags of flannel, or in pieces of linen cloth of moderate fineness, which are loosely strained over wooden frames, having been previously well soaked in water.

The form of glass and earthenware funnels should vary according to the ope

ration to be performed with them. If it is wished to filter a liquid rapidly, the funnels should be ribbed; or, if plain funnels are used, the cone and neck should not join at a sharp angle. When the substance to be separated from a solution is in a state of very minute division, it is necessary that the cone and neck of the funnel should join at an angle of about 130°, and the former should taper gradually, and not bulge at the sides, so that the filter, when placed in it, may touch it at all points.

The funnel may be supported by the ring of a retort-stand, by tripod-stands, or by a wooden stand consisting of a flat board supported on four feet, about twelve inches high, and provided with round holes of various sizes. The lower extremities of the feet may be fixed into a piece of board corresponding to the top, to impart firmness to the stand.

Various kinds of filtering-paper are employed by chemists, according to the nature of the operation required. That most generally used, is the best white blotting-paper. When greater strength is required, a coarser, and much thicker kind of blotting paper is used. All ordinary filtering-paper is, to some extent, contaminated with mineral substances, which do not, however, materially interfere in ordinary operations. At times, however, particularly in quantitative analysis, it is necessary to have the paper as nearly chemically pure as possible; the best paper of this description is imported from Sweden. The purity of filtering paper is indicated by the quantity of ash which it leaves upon being thoroughly burnt. It is very convenient to keep the paper ready cut into circular pieces of various sizes. By folding these twice in opposite directions, the ordinary filter is obtained.

Rapid filtrations are effected by means of ribbed filters. If the filtering-paper employed is very porous, or the solid operated with heavy, or difficult to separate from the liquid, it is well to employ double filters.

PRECAUTIONS IN FILTERING.-The filter should in no case protrude beyond the funnel; its edge should be at least about a quarter of an inch below that of the latter.

Before throwing any substance upon a filter, the paper should be moistened. with water (or any other solvent employed), by which means it is somewhat expanded, and the small pores existing in it thereby considerably contracted; if the substance is poured upon the filter without first attending to this precaution, a portion of the solid will frequently pass through the paper; and small particles are more liable to fill up the pores in the latter to such an extent as to cause the liquid to run through very slowly. The filter should never be quite filled with liquid, and the substance to be filtered should be poured on gradually and against the sides of the filter. Should the first portions of a liquid not pass through the filter quite bright, they must be returned thereto; and this must be repeated until the liquid is perfectly bright, when it is collected in a clean receiver. The spirting occasioned by the fall of the liquid as it drips from the funnel into the receiver, may be avoided by approaching the beak of the funnel to the side of the vessel, when the liquid will trickle down as it filters through. It is necessary, at times, to filter solutions rapidly while hot. In such cases, it is advisable to fit the funnel into a larger one, by introducing into the beak of the latter a perforated cork, adapted to hold the neck of the smaller funnel, and to fill up the space between the two with hot water. A metal case, with a hollow cylindrical arm projecting laterally, and closed at the bottom, is frequently substituted for the large funnel, and is very convenient, as heat may be applied to the arm, which is, of course, filled with water, and the latter kept at the boiling point while the filtration lasts.

$51. Many solids are purified from soluble substances, by washing them with water or other solvents, either by filtration or decantation. In the former case, the solution is allowed to drain off the substance as much as possible before