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 glass, the surface is not level, but cupped at
the edges, where the solid glass exerts its adhesive attraction for the
liquid, and draws it from the level. If the glass 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 glass placed close together, and then opened like a
book, draw up water between them, on the same principle. A mass 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air is withdrawn, then the wood sinks, and by boiling a ball made of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 assemblage 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.
[Illustration: 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.]
[Illustration: 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 glass, which is placed on the highest block of a series of three,
arranged as steps. Into this glass is placed the short end of a skein
[Page 71] of lamp cotton, previously wetted with distilled water; the
long end dips into another glass below, containing dilute sulphuric
acid, and as the solution of lead passes into it, a solid white
precipitate of sulphate of lead is formed; then another skein of wetted
cotton is placed in this glass, the long end of which passes into the
last glass, 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 Sussex), 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 glass, 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 glass retains water just as if it were an
ordinary gas jar closed with a glass stopper.
[Illustration: 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.
[Illustration: Fig. 85. Moorish niche and porous earthenware bottle,
containing water.]
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