Sponge is a very porous substance, and when thrown into water it does not disappear or dissolve, because the water has no chemical attraction for the particles of which it is composed; but it will mechanically attract and retain a very large quantity of the liquid, because sponge may be considered as consisting of myriads of small tubes interwoven or interlaced in all directions, and exerting capillary attraction for the water, as energetically as did the perfectly straight tubes or pores of the cane.

Take another example, bearing yet more strongly upon the rationale of capillary attraction in nature ;— a heap of sand consists of countless solid atoms of silica, not in absolute contact, as its examination by a magnifier will immediately prove, but separated by interstices; or, in other words, a heap of sand is full of capillary pores, and these will become filled upon the addition of water.

To render this more intelligible, or to represent, upon a magnified scale, the grains of sand and their inter

Fig. 5.

stices, a large glass may be filled with marbles; whilst to denote the water (whose liquid particles, philosophers

have likewise every reason to believe are not in absolute contact) a small glass may be filled with rape-seed, this, when poured among the marbles, will fill their interstices, as the water did those of the sand.

Place a shell or a piece of tile over the aperture in the lower part of a flower-pot, fill it with dry sand, then pour water upon its surface, and remark the large quantity that may be added before any portion flows from the aperture; when it does so, the capillary pores of the sand are saturated, and the excess of water passes through, in obedience to the attraction of gravitation.

When the mould in a flower-pot becomes excessively dry, we know that a great quantity of water may be poured upon its surface before any escapes from the aperture into the saucer beneath this depends upon the capillary attraction of the pores of the mould, as just exemplified in the case of sand.

Suppose the saucer become filled with water, we find it soon vanish if the plant in the flower-pot be in full vigour.

Why does the water vanish? Not by drying away,in the common meaning of the term,-but in consequence of being attracted upwards—as was the water by the porous cane to supply the roots of the plant, in which capillary vessels exist, and these transfer it to the stem, leaves, and flowers, from whose surfaces it ultimately dries away or evaporates; therefore, in place of watering the surface of the mould, we very frequently pour

water into the saucer, and depend, as now shown, upon capillary attraction for the rise of the liquid to maintain the life of the plant.

A porous substance has the power of raising water to a very remarkable extent, as may be proved by the following simple experiment.

Select a lump of common salt, cut it with a saw into a regular shape, about six inches long, two inches broad, and one inch thick, then set it to dry perfectly before a fire. Fill a shallow plate with water, containing sufficient red ink to impart a deep tint, and when the lump of salt is dry, place it upright in the plate, as here shown.

Fig. 6.

The salt, though apparently solid, is in fact full of capillary pores; these will instantly attract the reddened water, and the salt will become powerfully stained to a very considerable height. The red ink is only added to render the result more striking than it would be if water alone were employed.

In the course of a few seconds the salt will fall, on

4

account of the chemical solvent power of the water undermining its lower part, or in other words, on account of the solid particles of the salt being now received by the interstices between the individual atoms of the water, which action is regarded as solution; but it does not necessarily follow that a substance capable of exerting capillary attraction, should thus be soluble. A lump of porous sandstone, for example, similarly placed, will attract water, but will not fall, even when perfectly saturated, because it is an insoluble sub

stance.

In nature, it frequently happens that capillary attraction and chemical solution act simultaneously, and produce highly important results in conveying liquid manure to the roots of plants, by which their growth is wonderfully promoted. Vegetation on the margin of a brook, or pond, is generally more luxuriant than on the soil a few yards distant, because the porous earth of the bank attracts water abundantly, and retains it very forcibly even in time of drought.

Whilst upon this subject, it may be stated, that much of the dampness in the walls of buildings depends upon the porous brick or stone-work of the foundations attracting water from the earth, and it frequently causes great damage and inconvenience.

Oolite, or Bath-stone, as it is popularly called, is much employed in the decorative architecture of interiors, on account of the extreme facility with which it may be sawn and wrought into the most delicate tracery, and in such situations, if protected from damp, it will

remain unimpaired for ages, as exemplified in many abbeys and cathedrals.

Requiring little labour from the masons, and producing a most beautiful effect, architects have been occasionally tempted to employ oolite for external work; but it is not durable, because water is rapidly absorbed by capillary attraction, and this extraordinary liquid, by undergoing expansions and contractions during the changes of atmospheric temperature, and more particularly expansion upon freezing; flakes of the stones are violently detached, and the original sharp outline of the work broken down. This action silently, though certainly, continued for years, the sculptured work becomes undistinguishable.

Turning from the works of Art to those of Nature, as more particularly connected with this inquiry regarding the principal chemical phenomena of the Four Seasons, we discover capillary attraction to be a most active agent in retaining and administering a proper supply of water to the vegetation upon certain soils and rocks.

In sandstone and limestone districts, which particularly abound in England, the absorption and retention of water for the support of vegetation is very remarkaable; we may see luxuriant plants, nay even forest trees, flourishing upon both rocks, the sandstone being porous, the limestone full of cracks or fissures, all readily imbibing water from the heavens, retaining or yielding it in proportion as it is demanded, for the sustenance of vegetable life.