Once a Week (magazine)/Series 1/Volume 2/The dust in a sunbeam

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You must frequently have watched the whirling cloud of dust in the sunbeam aslant a somewhat darkened room; and perhaps were a little staggered at this sudden revelation of the invisible air not being quite so pure as you had imagined. It is true that unless your housemaid is a woman of stern conscientiousness, the mortal enemy of spiders, implacable on the subject of cleanliness—(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.

There is one thing which will perhaps be found in every place, and in every pinch of dust, and you will be not a little surprised to learn what that is. It is starch. No object is more familiar to the microscopist than the grain of starch. It is sometimes oval, sometimes spherical, and varies in size. The addition of a little iodine gives it a blue colour, which disappears under the influence of light. There seems to be no difference between the starch grains found in the dust of Egyptian tombs and Roman temples, and that found in the breakfast-parlour of to-day. They both respond to chemical and physical tests in the same way.

But there is one curious fact which has been observed by M. Pouchet of Rouen, namely, that in examining the dust of many centuries he has sometimes found the starch grains of a clear blue colour; and he asks whether this may not be due to the action of iodine in the air, traces of which M. Chatin says always exist in the air. The objection to this explanation is, that if iodine is always present in sufficient quantities to colour starch, the grains of starch should often be coloured, whereas no one but M. Pouchet has observed coloured grains, and he but rarely.

M. Pouchet tells us that, amazed at the abundance of starch grains which he found in dust, he set about examining the dust of all ages and all kinds of localities—the monuments and buildings of great cities, the tombs of Egyptian monarchs, the palaces of the age of Pharaoh; nay, he even examined some dust which had penetrated the skulls of embalmed animals. In all these places starch was found. But a moment’s reflection dispels the marvellousness of this fact. Starch must necessarily abound, because the wheat, barley, rice, potatoes, &c., which form everywhere the staple of man’s food, are abundant in starch; the grains are rubbed off, and scattered by the winds in all directions.

So widely are these grains distributed that a careful examination of our clothes always detects them. Nay, they are constantly found on our hands, though unsuspected until their presence on the glass slide under the Microscope calls attention to them. It is only necessary to take a clean glass side, and press a moistened finger gently on its surface, to bring several starch grains into view. Nay, this will be the case after repeated washing of the hands; but if you wash your hands in a concentrated solution of potash, no grains will then be found on pressing the moistened finger on the glass. This persistent presence of starch on our hands is not astonishing when we consider the enormous amount of starch which must be rubbed from our food, and our linen, every instant of the day; and when we consider, on the one hand, the specific lightness of these grains, which enables them to be so easily transported by the air, and, on the other hand, the powerful resistance they offer to all the ordinary causes of destruction, one may safely affirm that in every town or village a cloud of starch is always in the air.

And hereby hangs a tale. Starch is a vegetable substance, and, until a very few years ago, it was believed to have no existence in the animal tissues. But the great pathologist Virchow discovered that in various tissues a substance closely resembling starch was formed, which he considered to be a morbid product. The discovery made a great sensation, and many were the ingenious theories started to account for the fact. At last it came to be maintained that starch was a normal constituent of animal tissues; and there is no doubt that investigators might easily find starch in every bit of tissue they handled, since their fingers, as we have seen, are plentifully covered with grains. If, however, proper precautions be taken not to touch the tissue with the fingers, nor the glass slide on which it is placed, no starch will be found. It is because of the starch-clouds in our atmosphere that grains are found on our persons and on almost every microscopical preparation.

But are the starch-clouds all that the sunbeam reveals? By no means. Some animals will be found there; not always, indeed, nor very numerously, but enough to create astonishment. And these animals are not insects disporting themselves, they are either dead or in a state of suspended animation. A few skeletons of the infusoria, scales of the wings of moths and butterflies, and fragments of insect-armour, may be reckoned as so much dust; but there is also dust that is alive, or capable of living. You want to know what that dust is? It is always to be found in dry gutters on the housetops, or in dry moss growing on an old wall; and Spallanzani, the admirable naturalist to whom we owe so much, amazed the world with announcing what old Leeuwenhoek had before announced, namely, that these grains of dust, when moistened, suddenly exhibited themselves as highly-organised little animals—the Rotifers and Tardigrades. Water is necessary to their activity. When the gutter is dried up, they roll themselves into balls, and patiently await the next shower. If, in this dried condition the wind sweeps them away with much other dust, they are quite contented; let them be blown into a pond, they will suddenly revive to energetic life; let them be blown into dusty corners, and they will patiently await better times. It may happen that the wind will sweep them into your study, and there they will settle on the gilt edges of Rollin’s Ancient History, or some other classical work which every gentleman’s library should be without; and in this position it has a fair chance of remaining undisturbed throughout the long years of your active career. But you die. Your widow has probably but an imperfect provision, and a very imperfect sympathy with Rollin and Co.; your books are sold by auction; the dust is shaken from them, and is blown into the street—from the street into the gutter, or the river, and there the dried Rotifers suddenly revive, to fight, feed, and propagate as of old. It is said that the Rotifer may be dried and revived fifteen times in succession. And if this be so, you may imagine what a history would be that of a single Rotifer under a fortunate juncture of circumstances. It might have seen life in a gutter at Memphis, or a pond at Thebes; been blown as dust to Carthage, and carried as dust to Rome; from thence to Constantinople; and, after being shaken from the robe of Theodora, or the code of Justinian, it might have accompanied the Crusaders to Jerusalem; from which place Mrs. Brown, after a two months’ Eastern scamper, might have brought it back to London, where a chance breeze wafted it into the room which the very sunbeam I am discoursing about illuminates. From Memphis to my Microscope, what a course! And during this adventurous course our Rotifer has fourteen times shaken off the cerements of death. Dead? Not he:

I’ve not been dead at all, says Jack Robinson.

Such are some of the things found in the dust of a sunbeam, and you will probably have been too much astonished at some of the facts to have made the reflection that among all these objects not a single egg has been named. A few spores of plants are, indeed, frequently found. Knowing that many plants are fertilised by the agency of the wind, one expects to find pollen grains abundant. Indeed, when we consider how rapidly bread, cheese, jam, ink, and the very walls of the room, if damp, are covered with mould, which is a plant; when we consider how impossible it is to keep decaying organic substance free from plants and animalcules, which start into existence as by magic, and in millions, we have no difficulty in accepting the hypothesis of an universal diffusion of germs—eggs or seeds—through the atmosphere. No matter where you place organic substance in decay, if the air in never so small a quantity can get at it, mould and animalcules will be produced. Close it in a phial, seal the cork down, take every precaution against admitting more air than is contained between the cork and the surface of the water; and although you may have ascertained that no plants or animalcules, no seeds or eggs, were present when you corked the bottle, in the course of a little while, say three weeks, on opening the bottle you will find it abundantly peopled.

To explain this, and numerous other facts, the hypothesis of an universal diffusion of germs through the air has been adopted; and the known fecundity of plants and animalcules suffices to warrant the belief that millions of millions of germs may be constantly floating through the air. Ehrenberg computes the rate of possible increase of a single infusory, Paramecium, at two hundred and sixty eight millions a month. And it is calculated that the plant named Bovista giganteum will produce four thousand million of cells in one hour. As the mould plants are single cells, and as they multiply by spontaneous division, the rapidity with which they multiply is incalculable.

From all this you see how naturally the idea of universal diffusion of germs has become an accepted fact. If it is a fact, we must feel not a little astonished at finding the dust we examine so very abundant in starch, coal, silica, chalk, rust, hair, scales, and even live animals, and so strangely deficient in this germ-dust! The germs are said to be everywhere: millions upon millions must be diffused through tho air; every inch of surface must be crowded with them. Do we find them?

We find occasional pollen grains and seeds. But we find no animalcule eggs, and no animals, except the Rotifers and Tardigrades. We find almost everything but eggs. “Oh!” you will perhaps remark, “that is by no means surprising; if they are diffused in such enormous quantities through the air, it stands to reason that they must be excessively minute, otherwise they would darken the air; and if they are excessively minute, they escape your detective Microscope—that’s all.” Your remark has great plausibility; indeed, it would have overwhelming force, were there not one fatal objection to the assumption on which it proceeds. If the eggs of animalcules were so excessively minute, as you imagine them to be, there would be no chance of our detecting them. But it happens that the size of the eggs of those animalcules which are known (and of many we are utterly ignorant) is, comparatively speaking, considerable; at any rate, the eggs, both from size and aspect, are perfectly recognisable inside the animalcule; and if we can distinguish these eggs when the parent is before us, or when we have crushed them out of her body, it will be difficult to suppose that we could not distinguish them among the other objects in a pinch of dust, when a drop of water has been added.

It will be seen from these remarks that I do not believe in the hypothesis of universal diffusion of germs through the air. I believe that almost all the eggs of animalcules are too easily destroyed to resist desiccation; and that in the air they would become dust and cease to be eggs. At any rate we find no trace of eggs in the air.

The dust which our sunbeam has lighted up is a various and varying cloud of inorganic and organic matters—a symbol of the wear and tear of life—a token of the incessant silent destruction to which the hardest or the most fragile substances are exposed. The sunbeam has not only lighted up that, but many other obscurities, and shown us in what a world of mystery we move.