The Boy's Playbook of Science Part 7

CAPILLARY ATTRACTION.

This kind of attraction is termed capillary, in consequence of tubes, of a calibre, or bore, as fine as hair, attracting and retaining fluids.

If water is poured into a gla.s.s, the surface is not level, but cupped at the edges, where the solid gla.s.s exerts its adhesive attraction for the liquid, and draws it from the level. If the gla.s.s be reduced to a very narrow tube, having a hair-like bore, the attraction is so great that the water is retained in the tube, contrary to the force of gravitation.

Two pieces of flat gla.s.s placed close together, and then opened like a book, draw up water between them, on the same principle. A ma.s.s of salt put on a plate containing a little water coloured with indigo displays this kind of attraction most perfectly, and the water is quickly drawn up, as shown by the blue colour on the salt. A little solution of the ammonio-sulphate of copper imparts a finer and more distinct blue colour to the salt. A piece of dry Honduras mahogany one inch square, placed in a saucer containing a little turpentine, is soon found to be wet with the oil at the top, which may then be set on fire.

Almost every kind of wood possesses capillary tubes, and will float, on account of these minute vessels being filled with air; if, however, the air is withdrawn, then the wood sinks, and by boiling a ball made of beech wood in water, and then placing it under the vacuum of an air pump in other cold water, it becomes so saturated with water that it will no longer float. A remarkable instance of the same kind is mentioned by Scoresby, in which a boat was pulled down by a whale to a great depth in the ocean, and after coming to the surface it was found that the wood would neither swim nor burn, the capillary pores being entirely filled with salt water.

A piece of ebony sinks in water on account of its density, closeness, and freedom from air. A gauge made of a piece of oak, with a hole bored in it of one inch diameter, accurately receives a dry plug of willow wood which will not enter the orifice after it is wetted. Millstones are split by inserting wedges of dry hard wood, which are afterwards wetted and swelled, and burst the stone asunder. One of the most curious instances of capillary attraction is shown in the currying of leather, a process which is intended to impart a softness and suppleness to the skin, in order that it may be rendered fit for the manufacture of boots, harness, machine bands, &c. The object of the currier is to fill the pores of the leather with oil, and as this cannot be done by merely smearing the surface, he prepares the way for the oil by wetting the leather thoroughly with water, and whilst the skin is damp, oil is rubbed on, and it is then exposed to the air; the water evaporates at ordinary temperatures, but oil does not; the consequence is that the [Page 70] pores of the leather give up the water, which disappears in evaporation, and the oil by capillary attraction is then drawn into the body of the leather, the oil in fact takes the place vacated by the water, and renders the material very supple, and to a considerable extent waterproof. In paper making, the pores of this material, unless filled up or sized, cause the ink to blot or spread by capillary attraction. The porosity of soils is one of the great desideratums of the skilful agriculturist, and drainage is intended to remove the excess of water which would fill the pores of the earth, to the exclusion of the more valuable dews and rains conveying nutritious matter derived from manures and the atmosphere.

A cane is an a.s.semblage of small tubes, and if a piece of about six inches in length (cut off, of course, from the joints) be placed in a bottle of turpentine, the oil is drawn up and may be burnt at the top; it is on this principle that indestructible wicks of asbestos, and wire gauze rolled round a centre core, are used in spirit lamps. Oil, wax, and tallow, all rise by capillary attraction in the wicks to the flame, where they are boiled, converted into gas, and burnt.

[Ill.u.s.tration: Fig. 82. Geber's filter. A. The solution of acetate of lead. B. The dilute sulphuric acid. C. The clear liquid, separated from the sulphate of lead in B.]

[Ill.u.s.tration: Fig. 83. Prawn syphon.]

The capillary attraction of skeins of cotton for water was known and appreciated by the old alchemists; and Geber, one of the most ancient of these pioneers of science, and who lived about the seventh century, describes a filter by which the liquid is separated from the solid. This experiment is well displayed by putting a solution of acetate of lead into a gla.s.s, which is placed on the highest block of a series of three, arranged as steps. Into this gla.s.s is placed the short end of a skein [Page 71] of lamp cotton, previously wetted with distilled water; the long end dips into another gla.s.s below, containing dilute sulphuric acid, and as the solution of lead pa.s.ses into it, a solid white precipitate of sulphate of lead is formed; then another skein of wetted cotton is placed in this gla.s.s, the long end of which pa.s.ses into the last gla.s.s, so that the clear liquid is separated and the solid left behind. (Fig. 82.)

In this filter the lamp cotton acts as a syphon through the capillary pores which it forms. On the same principle, a prawn may be washed in the most elegant manner (as first shown by the late Duke of Suss.e.x), by placing the tail, after pulling off the fan part, in a tumbler of water, and allowing the head to hang over, when the water is drawn up by capillary attraction, and continues to run through the head. (Fig. 83.) The threads of which linen, cotton, and woollen cloths are made are small cords, and the shrinkage of such textile fabrics, is well known and usually inquired about, when a purchase is made; here again capillary attraction is exerted, and the fabric contracts in the two directions of the warp and woof threads; thus, twenty-seven yards of common Irish linen will permanently shrink to about twenty-six yards in cold water. In these cases the water is attracted into the fibres of the textile material, and causing them to swell, must necessarily shorten their length, just as a dry rope strained between two walls for the purpose of supporting clothes, has been known to draw the hooks after being suddenly wetted and shortened by a shower of rain.

In order to tighten a bandage, it is only necessary to wind the dry linen round the limbs as close as possible, and then wet it with water, when the necessary shrinkage takes place.

If a piece of dry cotton cloth is tied over one end of a lamp gla.s.s, the other may be thrust into, or removed from the basin of water very easily, but when the cotton is wetted, the fibres contract and prevent air from entering, so that the gla.s.s retains water just as if it were an ordinary gas jar closed with a gla.s.s stopper.

[Ill.u.s.tration: Fig. 84. A. Basin of water. B. Cylinder of wire gauze closed at both ends with gauze. When full of water it may be lifted from the basin by the handle, C.]

A Spanish proverb, expressing contempt, says, "go to the well with a sieve," but even this seeming impossibility is surmounted by using a cylinder of wire gauze, which may be filled with water, and by means of the capillary attraction between the meshes of the copper-wire gauze and the water, the whole is retained, and may be carefully lifted from a basin of water; the experiment only succeeds when the air is completely driven out of the interstices of the gauze, and the little cylinder completely filled with water; this may be done [Page 72] by repeatedly sinking and drawing out the cylinder, or still more effectually, by first wetting it with alcohol and then dipping the cylinder in water.

A balloon, made of cotton cloth, cannot be inflated by means of a pair of bellows, but if the balloon is wetted with water, then it may be swelled out with air just as if it had been made of some air-tight material; hence the principle of varnishing silk or filling the pores with boiled oil, when it is required in the manufacture of balloons.

Biscuit ware, porous tubes for voltaic batteries, alcarrazas, or water coolers, are all examples of the same principle.

Whilst speaking most favourably of the benevolent labours of many gentlemen (beginning with Mr. Gurney) who have erected "Drinking Fountains" in London's dusty atmosphere and crowded streets, it must not be forgotten that pious Mohammedans have, in bygone times, already set us the example in this respect; and in the palmy days of many of the Moorish cities, the thirsty citizen could always be refreshed by a draught of cool water from the porous bottles provided and endowed by charitable Mussulmans, and placed in the public streets.

[Ill.u.s.tration: Fig. 85. Moorish niche and porous earthenware bottle, containing water.]

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CHAPTER IX.

CRYSTALLIZATION.

[Ill.u.s.tration: Fig. 86. Crystals of snow.]

It has been already stated that the force of cohesion binds the similar particles of substances together, whether they be _amorphous_ or shapeless, _crystalline_ or of a regular symmetrical and mathematical figure. The term crystal was originally applied by the ancients to silica in the form of what is usually termed rock crystal, or Brazilian pebble; and they supposed it to be water which had been solidified by a remarkable intensity of cold, and could not be thawed by any ordinary or summer heat. Indeed, this idea of the ancients has been embodied (to a certain extent) in the shape of artificial ice made by crystallizing large quant.i.ties of sulphate of soda, which was made as flat as possible, and upon [Page 74] which skaters were invited to describe the figure of eight, at the usual admittance fee, representing twelve pence.

A crystal is now defined to be an inorganic body, which, by the operation of affinity, has a.s.sumed the form of a regular solid terminated by a certain number of planes or smooth surfaces.

Thousands of minerals are discovered in the crystallized state--such as cubes of iron pyrites (sulphuret of iron) and of fluor spar (fluoride of calcium), whilst numerous saline bodies called salts are sold only in the form of crystals. Of these salts we have excellent examples in Epsom salts (sulphate of magnesia), nitre (nitrate of potash), alum (sulphate of alumina), and potash; the term salt being applied specially to all substances composed of an acid and a base, as also to other combinations of elements which may or may not take a crystalline form. Thus, nitre is composed of nitric acid and potash; the first, even when much diluted, rapidly changes paper, dipped in tincture of litmus and stained blue, to a red colour, whilst potash shows its alkaline nature, by changing paper, stained yellow with tincture of turmeric, to a reddish-brown. The latter paper is restored to its original yellow by dipping it into the dilute nitric acid, whilst the litmus paper regains its delicate blue colour by being pa.s.sed into the alkaline solution. An acid and an alkali combine and form a neutral salt, such as nitre, which has no action whatever on litmus or turmeric; whilst the element iodine, which is not an acid, unites with the metallic element pota.s.sium, and therefore not an alkali, and forms a salt that crystallizes in cubes called iodide of pota.s.sium. Again, cane sugar, which is composed of charcoal, oxygen, and hydrogen, crystallizes in hard transparent four-sided and irregular six-sided prisms, but is not called a salt. Silica or sand is found crystallized most perfectly in nature in six-sided pyramids, but is not a salt; it is an acid termed silicic-acid. Sand has no acid taste, because it is insoluble in water, but when melted in a crucible with an alkali, such as potash, it forms a salt called silicate of potash.

Magnesia, from being insoluble, or nearly so, in water, is all but tasteless, and has barely any alkaline reaction, yet it is a very strong alkaline base; 20.7 parts of it neutralize as much sulphuric acid as 47 of potash. A salt is not always a crystallizable substance, and _vice versa_. The progress of our chemical knowledge has therefore demanded a wider extension and application of the term _salt_, and it is not now confined merely to a combination of an acid and an alkali, but is conferred even on compounds consisting only of sulphur and a metal, which are termed _sulphur salts_.

So also in combinations of chlorine, iodine, bromine, and fluorine, with metallic bodies, neither of which are acid or alkaline, the term _haloid salts_ has been applied by Berzelius, from the Greek ([Greek: _als_], sea salt, and [Greek: _eidos_] form), because they are a.n.a.logous in const.i.tution to sea salt; and the mention of sea salt again reminds us of the wide signification of the term salt, originally confined to this substance, but now extended into four great orders, as defined by Turner:--

ORDER I. _The oxy-salts._--This order includes no salt the acid or base of which is not an oxidised body (ex., nitrate of potash).

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ORDER II. _The hydro-salts._--This order includes no salt the acid or base of which does not contain hydrogen (ex., chloride of ammonium).

ORDER III. _The sulphur salts._--This order includes no salt the electro-positive or negative ingredient of which is not a sulphuret (ex., hydrosulphuret of pota.s.sium).

ORDER IV. _The haloid salts._--This order includes no salt the electro-positive or negative ingredient of which is not haloidal. (Exs., iodide of pota.s.sium and sea salt). To fix the idea of salt still better in the youthful mind, it should be remembered that alabaster, of which works of art are constructed, or marble, or lime-stone, or chalk, are all salts, because they consist of an acid and a base.

In order to cause a substance to crystallize it is first necessary to endow the particles with freedom of motion. There are many methods of doing this chemically or by the application of heat, but we cannot by any mechanical process of concentration, compression, or division, persuade a substance to crystallize, unless perhaps we except that remarkable change in wrought or fibrous iron into crystalline or brittle iron, by constant vibration, as in the axles of a carriage, or by attaching a piece of fibrous iron to a tilt hammer.

If we powder some alum crystals they will not again a.s.sume their crystalline form; if brought in contact there is no freedom of motion.

It is like placing some globules of mercury on a plate. They have no power to create motion; their inertia keeps them separated by certain distances, and they do not coalesce; but incline the plate, give them motion, and bring them in contact, they soon unite and form one globule.

The particles of alum are not in close contact, and they have no freedom of motion unless they are dissolved in water, when they become invisible; the water by its chemical power destroys the mechanical aggregation of the solid alum far beyond any operation of levigation.

The solid alum has become liquid, like water; the particles are now free to move without let or hindrance from friction. A solution, (from the Latin _solvo_, to loosen) is obtained. The alum must indeed be reduced to minute particles, as they are alike invisible to the eye whether a.s.sisted by the microscope or not. No repose will cause the alum to separate; the solvent power of the water opposes gravitation; every part of the solution is equally impregnated with alum, and the particles are diffused at equal distances through the water; the heavy alum is actually drawn up against gravity by the water.

How, then, is the alum to be brought back again to the solid state? The answer is simple enough. By evaporating away the excess of water, either by the application of heat or by long exposure to the atmosphere in a very shallow vessel, the minute atoms of the alum are brought closer together, and crystallization takes place. The a.s.sumption of the solid state is indicated by the formation of a thin film (called a _pellicle_) of crystals, and is further and still more satisfactorily proved by taking out a drop of the solution and placing it on a bit of gla.s.s, which rapidly becomes filled with crystals if the evaporation has been carried sufficiently far (Fig. 87).

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After evaporating away sufficient water, the dish is placed on one side and allowed to cool, when crystals of the utmost regularity of form are produced, and, denoted by a geometrical term, are called octohedral or eight-sided crystals, when in the utmost state of perfection (Fig. 88).

[Ill.u.s.tration: Fig. 87. R R. Ring-stand. S S. Spirit-lamps. A. Flask containing boiling solution of alum.--_Solution._ B. Funnel, with a bit of lamp-cotton stuffed in the bottom.--Filtration. C. Evaporating dish.--Evaporation. D. Drop on gla.s.s.--Crystallization.]

[Ill.u.s.tration: Fig. 88.]

The science of crystallography is too elaborate to be discussed at length in a work of this kind; the various terms connected with crystals will therefore only be explained, and experiments given in ill.u.s.tration of the formation of various crystals.

When the apices--_i.e._, the tips or points of crystals--are cut off, they are said to be truncated; and the same change occurs on the edges of numerous crystals.

If some of the salt called chloride of calcium in the dry and amorphous state is exposed to the air, it soon absorbs water, or what is termed _deliquesces_: the same thing occurs with the crystals of carbonate of potash, and if four ounces are weighed out in an evaporating dish, and then exposed for about half an hour to the air, a very perceptible increase in weight is observed by the a.s.sistance of the scales and grain weights. _Deliquescence_ is a term from the Latin _deliqueo_, to melt, and is in fact a gradual melting, caused by the absorption of water from the atmosphere. The reverse of this is ill.u.s.trated with various crystals, such as Glauber's salt (sulphate of soda), or common washing soda (carbonate of soda); if a fine clear crystal is taken out of the solution, called the mother liquor, in which it has been crystallized, wiped dry, and placed under a gla.s.s shade, this salt may remain for a long period [Page 77] without change, but if it receive one scratch from a pin, the door is opened apparently for the escape of the water which it contains, chemically united with the salt, and called water of crystallization; the white crystal gradually swells out, the little _quasi_ sore from the pin-scratch spreads over the whole, which becomes opaque, and crumbling down falls into a shapeless ma.s.s of white dust; this change is called _efflorescence_, from _effloresco_, to blow as a flower--caused by the abstraction from them of chemically-combined water by the atmosphere. With reference to the preservation of crystals, Professor Griffiths recommends them to be oiled and wiped, and placed under a gla.s.s shade, if of a deliquescent nature; or if efflorescent, they are perfectly preserved by placing them under a gla.s.s shade with a little water in a cup to keep the air charged with moisture and prevent any drying up of the crystal.

Deliquescent crystals may be preserved by placing them, when dry, in naphtha, or any liquor in which they are perfectly insoluble. Some salts, like Glauber's salts, contain so much water of crystallization that when subjected to heat they melt and dissolve in it, and this liquefaction of the solid crystal is called "watery fusion." Other salts, such as bay salt, chlorate of potash, &c., when heated, fly to pieces, with a sharp crackling noise, which is due sometimes, to the unequal expansion of the crystalline surface, or the sudden conversion of the water (retained in the crystal by capillary attraction) into steam; thus nitre behaves in this manner, and frequently retains water in capillary fissures, although it is an anhydrous salt, or salt perfectly free from combined water. The crackling sound is called _decrepitation_, and is well ill.u.s.trated by throwing a handful of bay salt on a clear fire; but this property is destroyed by powdering the crystals.

Many substances when melted and slowly cooled concrete into the most perfect crystals; in these cases heat alone, the antagonist to cohesion, is the solvent power. Thus, if bis.m.u.th be melted in a crucible, and when cooling, and just as the pellicle (from _pellis_, a skin or crust) is forming on the surface, if two small holes are instantly made by a rod of iron and the liquid metal poured out from the inside (one of the holes being the entrance for the air, the other the exit for the metal); on carefully breaking the crucible, the bis.m.u.th is found to be crystallized in the most lovely cubes. Sulphur, again, may be crystallized in prismatic crystals by pursuing a similar plan; and the great blocks of spermaceti exhibited by wax chandlers in their windows, are crystallized in the interior and prepared on the same principle.

There are other modes of conferring the crystalline state upon substances--viz., by elevating them into a state of vapour by the process called sublimation (from _sublimis_, high or exalted), the lifting up and condensation of the vapour in the upper part of a vessel; a process perfectly distinct from that of _distillation_, which means to separate drop by drop. Both of these processes are very ancient, and were invented by the Arabian alchemists long antecedent to the seventh century. Examples of sublimation are shown by heating iodine, and especially [Page 78] benzoic acid; with the latter, a very elegant imitation of snow is produced, by receiving the vapour, on some sprigs of holly or other evergreen, or imitation paper snowdrops and crocuses, placed in a tasteful manner under a gla.s.s vessel. The benzoic acid should first be sublimed over the sprigs or artificial flowers in a gas jar, which may be removed when the whole is cold, and a clear gla.s.s shade subst.i.tuted for it. (Fig. 89.)

All electro deposits on metals are more or less crystalline; and copper or silver may be deposited in a crystalline form by placing a sc.r.a.ped stick of phosphorus in a solution of sulphate of copper or of nitrate of silver. The phosphorus takes away the oxygen from the metal, or deoxidizes the solution, and the copper or silver reappears in the metallic form. The surface of the phosphorus must not be sc.r.a.ped in the air, but under water, when the operation is perfectly safe.

A singular and almost instantaneous crystallization can be produced by saturating boiling water with Glauber's salt, of which one ounce and a half of water will usually dissolve about two ounces; having done this, pour the solution, whilst boiling hot, into clean oil flasks, or vials of any kind, previously warmed in the oven, and immediately cork them, or tie strips of wetted bladder, over the orifices of the flasks or vials, or pour into the neck a small quant.i.ty of olive oil, or close the neck with a cork through which a thermometer tube has been pa.s.sed. When cold, no crystallization occurs until atmospheric air is admitted; and it was formerly believed that the pressure of the air effected this object, until some one thought of the oil, and now the theory is modified, and crystallization is supposed to occur in consequence of the water dissolving some air which causes the deposit of a minute crystal, and this being the turning point, the whole becomes solid. However the fact may be explained, it is certain that when the liquid refuses to crystallize on the admission of air, the solidification occurs directly a minute crystal of sulphate of soda, or Glauber's salt, is dropped into the vessel.