53.]

COHESION OF LIQUIDS.

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(53) Cohesion of Liquids.-Iu liquids, notwithstanding the facility with which their particles slide one over the other, and the unlimited freedom of motion of each molecule within the mass of liquid, a very appreciable amount of cohesion still exists, and is displayed in the rounded form assumed by every detached drop. This same form of cohesion is also beautifully shown in the case of two liquids which do not dissolve each other, but which have precisely the same density, as is the case with oil and spirit of wine of a certain degree of dilution: if a little oil be poured into such diluted spirit, it remains suspended within it in the form of a perfectly spherical mass. In the drops of dew which fringe every leaf in a fine summer morning, we have an admirable natural illustration of this fact. A striking exemplification of cohesion in the particles of liquids is also afforded by blowing a large soapbubble upon the end of a glass tube: upon presenting the open end of the tube to a lighted taper, whilst the bubble is still attached to the other end, the contraction of the film expels the air with sufficient velocity to extinguish the taper.

.mm.

FIG. 25.

The fol

The researches of Donny (Ann. Chim. Phys. 1846 [3], xvi. 167) have added many curious facts to our knowledge of the cohesion of liquids. lowing form of one of his experiments may be cited as an illustration:-A tube, A, fig. 25, about 1 metre or 40 inches long, and 25' or I inch in diameter, is bent at its middle to an angle of about 60°; it is sealed at one end, and filled with distilled water, which, when the tube is closed, is to oc

SV

cupy about two-thirds of its capacity; the water is thoroughly boiled for an hour, and the tube is then hermetically closed while boiling. In this condition the tube contains only water and the vapour of water. After it has been carefully reversed, as at A, it may be brought into the position represented at B, and the water will nevertheless be supported above the level of the liquid in, the other limb by adhesion to the surface of the glass, and by the cohesion among its own particles. If now the tube be inclined in such a manner that a minute bubble of aqueous vapour is made to pass up into the full limb, the column of water, having its continuity broken, at one point, immediately falls, and the level of the liquid in both limbs becomes the same.

The same phenomenon is often observed in the filling of barometer tubes. In order to remove the air completely, the mercury is boiled in the tube (52), and it not unfrequently happens that, when the tube is inverted after cooling, the mercury adheres so strongly to the glass that the column does not fall to the usual barometric height. A concussion separates the mercury from the glass,

80

INFLUENCE OF SURFACE ON ADHESION.

[53.

but on inclining the tube so as to allow the liquid to rise to the top, the adhesion is not again manifested.

(54) Influence of Surface on Adhesion.-Since adhesion takes place solely between the surface of bodies, it is evident that any circumstance which increases the extent of that surface must materially facilitate the exertion of this force. Minute subdivision, by thus increasing the extent of surface, greatly exalts the effect of adhesion :—for example, a cube of 1 centimetre in the side exposes a surface of 6 square centimetres; i.e., there is a square centimetre upon each of its 6 faces; if this cube be subdivided into a number of smaller cubes, each of which is only T of a centimetre in the side, it would furnish 1,000,000 of these minute cubes. Now as each little cube has 6 sides, the surface which it will expose is of a square centimetre, or 10,000 of them will expose 6 square centimetres ; that is, as much surface as a solid cube of a centimetre in the side: the 1,000,000 cubes will consequently expose 100 times as great a surface, or 600 square centimetres. The adhesion, therefore, by such a subdivision, should be increased somewhat in this proportion.

:

The

The influence of this kind of subdivision in exalting the effect of adhesion is strikingly exhibited in the case of charcoal. structure of the wood from which the charcoal is procured is cellular when heated in vessels from which air is excluded, the volatile constituents of the wood are expelled; and the charcoal, which does not fuse, remains behind in a very porous condition, retaining the form of the wood which furnished it. Mitscherlich has calculated that the cells of which a cubic inch of boxwood is formed expose a surface of not less than 73 square feet.

Adhesion occurs between charcoal and other bodies in very different degrees. For many colouring matters of vegetable and animal origin this adhesion is extremely energetic; so that if these bodies be dissolved in any liquid and agitated with charcoal, nearly the whole of the colouring matter will be retained by the charcoal, and on separating the latter by filtration, the liquor wili run through colourless. Ordinary vinegar and port-wine may thus be obtained in a colourless condition. Advantage is taken of this fact in the refining of sugar, in which process the syrups are deprived of colour by filtration through a column of charcoal 30 or 40 feet (12 metres or more) in thickness. The species of charcoal which is most extensively employed for this purpose is that obtained by burning bones in closed vessels; and it is hence termed bone black, or ivory black, or frequently animal charcoal. The charcoal is in this case in a state of extreme sub

54.]

SURFACE ACTION OF CHARCOAL.

81

division; it does not constitute above a tenth or a twelfth of the weight of the mass; the remainder consists of earthy matters; chiefly calcic phosphate and carbonate. When bone black has been used for filtering liquids, and has ceased to take up any more colouring matter, it is thrown aside and allowed to ferment: if then it be well washed, and re-burned, it may be used again with nearly equal effect. Other animal matters, especially dried blood, furnish, when calcined and well washed, a charcoal which is still more efficacious. The addition of potassic carbonate to the mass before calcination, still further increases the decolorizing power.

Many other matters besides those possessed of colouring properties have likewise this peculiarity of adhering strongly to charcoal. Graham has shown that metallic oxides in solution in potash or ammonia, arsenious acid in water, and bodies generally of feeble solubility, possess this property; a variety of vegetable matters, and especially the bitter principles, are thus affected. If porter be agitated with charcoal and filtered, it will not only be deprived of colour, but also of much of its bitterness. It was formerly the practice, after the active principles of medicinal plants had been separated from the woody fibre and most of the extraneous matters with which they are associated, to free them from the colouring matters with which they were contaminated, by digestion with animal charcoal; so large a proportion of the active principles themselves, however, was found to be retained by the charcoal, that the plan was abandoned. In consequence of this property, animal charcoal has been administered with good effect in some instances of poisoning with vegetable matters: in such cases it can never be unsafe, and may often be of great value. I have found that very dilute aqueous solutions of salts of lead are decomposed by filtration through a column of animal charcoal plumbic nitrate, acetate, and chloride, each part with their metallic base, which is retained by the charcoal, probably as a basic salt; whilst free nitric, acetic, or hydrochloric acid is found in the filtered liquid.

Many finely-divided substances besides charcoal, such as hydrated ferric oxide, hydrated alumina, hydrated antimonious sulphide, hydrated bone phosphate (tricalcic diphosphate, Ca, P2O), as well as plumbic iodide and sulphide when freshly precipitated, also exert powerful decolorizing actions. The decolorizing. power varies for each substance with the nature of the colouring principle: thus tincture of litmus yields its colouring matter more readily to calcic phosphate, and to hydrated ferric oxide, than it

82

SOLUTION.

[54.

does to animal charcoal freed from bone phosphate by the action of acids. On the other hand, the colouring matter of red wine and of molasses is more readily absorbed by animal charcoal than it is by hydrated calcic phosphate, or ferric oxide. (Filhol, Ann. Chim. Phys., 1852 [3], xxxv. 206.)

(55) Solution.-Adhesion is frequently manifested between solids and liquids sufficiently to overcome cohesion, and the substance is then said to become dissolved, or to undergo solution. In this manner sugar or salt is dissolved by water, camphor or rosin by spirit of wine, lead or silver by mercury. Anything that weakens cohesion in the solid favours solution. For instance, if the substance be powdered, it becomes dissolved more quickly, both from the larger extent of surface which it exposes, and from the partial destruction of cohesion. In the same way, heat, by increasing the distance between the particles of the solid, lessens its cohesion, and probably thus contributes so powerfully to assist in producing solution. If a solid body be introduced in successive portions into a quantity of a liquid capable of dissolving it, the first portions disappear rapidly, and as each succeeding quantity is added, it is dissolved more slowly, until at length a point is reached at which it is no longer dissolved. When this occurs, the cohesion balances adhesion, and the liquid is said to be saturated. It is important to remark, that in cases of simple solution, the properties both of the solid and of the liquid are retained. Syrup, for instance, retains the sweetness of the sugar and the liquid form of water. So, when camphor is dissolved in spirit of wine, the resulting tincture partakes of the properties of both, having the smell and taste both of camphor and of spirit. Solution is, in this respect, distinguished broadly from those cases in which a solid disappears under the influence of a liquid owing to the exertion of a chemical force between the particles of the two bodies; as when copper is dissolved by nitric acid, or iron by sulphuric acid. Solution usually occurs more readily when the solvent and the body dissolved present some general resemblance in properties: for example, mercury dissolves many of the metals, alcohol dissolves resins, oils dissolve fatty bodies and each other. Mere solution is attended by depression of temperature, but where the formation of a hydrate (or definite chemical compound with water) occurs, elevation of temperature is produced, a circumstance which, as Graham remarks, indicates an essential difference between solution and chemical combination. It is, however, possible that in cases which appear to be merely those of solution a chemical action

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VARIATIONS IN SOLUBILITY.

83

takes place, but that the heat developed is more than counterbalanced by the quantity absorbed in the conversion of the solid into a liquid. The solution of sodic sulphate in concentrated hydrochloric acid produces intense cold; but it seems probable that the rapid conversion of the solid into a liquid is due to chemical action. Again, the freezing mixtures obtained by mixture of ice with sodic and calcic chlorides would appear to owe their efficacy to the chemical attraction existing between these salts and water, but which cannot take place until the ice is liquefied. The view that solutions are combinations of the dissolved bodies with the solvents, is also supported by the changes which occur in some solutions by the action of heat; thus the diminution of solubility of some salts by heat appears to be due to the existence at the higher temperature of a hydrate containing less water, and the changes of colour of solutions of cobalt salts when heated, may be ascribed to the same cause. In cases of chemical action, on the other hand, that action is most energetic between bodies the properties of which are most widely different; the metals, for example, are dissolved by acids, oils by the alkalies, and silica, if melted with potash or soda, becomes soluble in water. The extent to which different solids are dissolved by the same liquid varies almost indefinitely. In water, baric sulphate is almost absolutely insoluble; calcic sulphate or gypsum is soluble in the proportion of about 1 part in 700 of water; potassic sulphate in about 1 part in 16; while magnesic sulphate may be dissolved to the extent of 2 parts of the crystals in 3 of water. It should be observed that water, after it has been saturated with one salt, will still continue freely to dissolve others.

3

Many substances in which the cohesion amongst their particles is weak are extensively soluble in water, though they have but little adhesion to it. Such substances will often be displaced by adding a solution of another body which adheres more strongly to water. Prussian blue, for example, is dissolved by distilled water which has been acidulated with oxalic acid; but it is precipitated by adding a solution of common salt, or of sodic sulphate, and the blue compound subsides on standing, leaving a clear colourless liquid above it.

Although in the majority of instances the solubility of a substance is increased by heat, it is not uniformly so. Lime and several of its salts offer remarkable exceptions. Water just above the freezing point dissolves nearly twice as much lime as it does when boiling; so that if water, saturated with lime in the cold, be