liness—(a housemaid, in short, who never advertises in the “Times,” but is a tradition of the days that are gone)—you must on more than one occasion have found a layer of dust collected on your books, portfolio, or table, dust piled up in the corners of the picture frame, dust covering your microscope case, dust gathering in the carvings of the piano-forte legs, dust on the looking-glasses, dust on the windows, dust everywhere. And this you know must have been transported by the atmosphere. But you are not astonished. The atmosphere is an energetic Pickford. It carries clouds of dust on every highway, and sweeps the sands over the fields and hedges. Nay, it is said to catch up quantities of frogs, and whirl them away to distant spots, where they fall like hailstones of a larger growth. But you are not bound to believe this. Nor need you be more credulous of the showers of herrings which are also recorded. There is evidence enough of the transporting power of the air, without falling into exaggerations. By slow deposits from the air the temples of Egypt, Greece, and Rome are now to a great extent buried below the surface; and you have often to descend a flight of steps to get upon the ancient soil.
It is probable, however, that while you were perfectly familiar with the idea of the atmosphere carrying clouds of dust, on occasions, you never thought of the atmosphere being constantly loaded with dust, which is constantly being deposited, and constantly renewed. This sunbeam has made the fact visible. It has lighted up the tiny cloud of dust, which we see to be restlessly whirling.
Suppose we examine this dust, and see of what it is composed? Restrain your surprise: the thing is perfectly feasible. The dust was invisible and unsuspected till the revealing sunbeam made us aware of its presence; and now the Microscope, which deals with the invisible, shall reveal its nature. For, in consequence of the united labours of hundreds of patient workers, we can now distinguish with unerring certainty whether a tiny blood-stain is the blood of a man, a pig, a bird, a frog, or a fish; whether a single fragment of hair is the hair of a mole or of a mouse, of a rabbit or of a cat, of a Celt or of a Saxon; whether a minute fibre is of cotton, or linen, or silk; whether a particle of dust is of flint, chalk, or brick; and we do this with the same precision as if we were distinguishing one animal from another, or one substance from another. If the characters are not sufficiently marked to the eye, we call in the aid of chemical tests. Equipped thus with a knowledge of marks by which to distinguish the separate particles, let us place a layer of dust, large enough to cover the surface of a fourpenny piece, under the Microscope, and begin the examination.
The composition of this dust will always be of two kinds—inorganic and organic, that is to say, mineral particles, and the skeletons of animalcules, or the skeletons and seeds of plants. The mineral particles will of course depend on the nature of the soil, and position of the spot whence the dust was derived. It may be swept in from the gravel walks of a garden, from the highroad, or from the busy street. The grinding of vehicles, the wear of busy feet, the disintegration everywhere going on, keeps up a constant supply of dust. The smoke of chimney and factory, steamship and railway, blackens the air with coal-dust. If the rocky coast is not a great way off, we shall find abundance of particles of silica, with sharp angles, sometimes transparent, sometimes yellow, and sometimes black.
And this silica will occasionally be in so fine a powdered condition that the granules will look like very minute eggs—for which indeed many microscopists have mistaken them. In this doubt, we have recourse to chemistry, and its tests assure us that we have silica, not eggs, before us. Besides the silica, we may see chalk in great abundance; and if near a foundry, we shall certainly detect the grains of oxide of iron (rust), and not a little coal-dust.
Our houses, our public buildings, and our pavements, are silently being worn away by the wind and weather, and the particles that are thus tom off are carried into the dust-clouds of the air, to settle where the wind listeth and the housemaid neglecteth. The very rocks which buttress our island are subject to incessant waste and change. The waters wash and scrub them, the air eats into them, the mollusc and the polyps rasp away their substance; and by this silent, but inevitable destruction, dust is furnished. Curious it is to trace the history of a single particle. Ages ago it was rock. The impatient waves wore away this particle, and dashed it among a heap of sand. The wind caught it in its sweeping arms, and flung it on a pleasant upland. The rain dragged it from the ground, and hurried it along water-courses to the river. The river bore it to the sea. From the sea water it was snatched by a mollusc, and used in the building of his shell. The mollusc was dredged and dissected; his shell flung aside, trampled on, powdered, and dispersed by the wind, which has brought this particle under our Microscope, serving us for a text on which to preach “sermons in stones.”
Equally curious is the history of this tiny particle of silk thread. A silkworm feeding tranquilly under the burning sun of India converts some of its digested plant-food into a cocoon of silk, in which it comfortably houses itself for a prolonged siesta. The silk is unwound, is carried to England or France, is there woven into a beautiful fabric, and after passing through many hands, enriching all, it forms part of the dress of some lovely woman, or the neck-tie of some gentlemanly scoundrel. Contact with a rough world, or a stiff shirt-collar, rubs off a minute fibre; the wind carries it away; and, after more wanderings than Ulysses, it comes to the stage of our Microscope. Beside it is a cotton-thread, brilliant in colour, of which a similar history might be told; and perhaps, also, there will be the hair of a dog, or of a plant; a fibre of wood, or the scale of a human epidermis; the fragment of an insect’s claw, or the shell of an animalcule. Very probably we shall find the spore of some plant which only awaits a proper resting-place, with the necessary damp, to develop into a plant. You must not expect to find all these things in one pinch of dust; but you may find them all, if you examine dust from various places.
