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;