The Filter.
Even in a state of uncivilisation man has been driven to invent a Filter of some kind.
The simplest kind of Filter is that which is used by the Bosjesman women when procuring water for the use of their families. When, as often happens, the only water to be obtained is to be found in muddy pools which have been trampled and perturbed by thirsty animals, the women have recourse to a simple, though rather repulsive, expedient.
Each woman is furnished with empty ostrich egg-shells by way of water-vessels, and she also takes a couple of hollow reeds. Over the end of one of these reeds she ties a bundle of grass, and then plunges it as deeply as she can into the mud. After a little while she sucks up the water through the tube, the grass acting as a filter, and she then discharges it by the second tube into the egg-shells. In this way the women will obtain water, where none but themselves could have procured it. As to the repulsive mode of obtaining it, no one can be fastidious when dying of thirst. Sir S. Baker mentions that when he was on his travels he managed in a halt to save up enough water for a bath for himself and his wife. He was about to throw away the soapy water, when the vessel was snatched from his hands by two of his attendants, and the contents eagerly drunk.
The different varieties of the Filter which we use at the present day are too familiar to need description. Whether they be made principally of charcoal, which is a powerful disinfectant, or of merely stones, gravel, and sand, they are all constructed on the same principle, namely, the straining out solid substances, and allowing only the pure water to pass through the interstices.
As to the Filters of Nature, they are almost innumerable. In the first place, the Earth itself is the primary filter of all, taking into itself all kinds of decomposing substances, separating them for the use of vegetation, and delivering the pure, bright, and sparkling spring water which we so highly and rightly value. The whole human body, again, is practically a collection of the most elaborate and effective filters that the mind of man can conceive. But we will pass to the more obvious examples of filters as seen in animal life.
On the upper left-hand portion of the illustration may be seen a long, fat, hairy creature, called popularly the Sea-mouse, and known to zoologists as Aphrodite aculeata. Although it inhabits the mud—and sea-mud is about as noisome a substance as can be imagined—it is clothed with a garment of such beauty that the rainbow itself can scarcely rival, and not surpass it. The hairs with which it is so profusely covered glitter and sparkle with every imaginable hue, among which red and green seem to be predominant.
These hairs occupy the sides of the body, but in the upper surface there is a thick coating of felted hairs, interwoven with each other so closely that they can with difficulty be separated. These hairs form a natural filter, strain away the mud from the water, and allow the latter to pour itself upon the organs of respiration. If, therefore, a specimen be examined when it is first brought up by the dredge, the felted hair will always be found to contain a considerable amount of mud, and much washing is needed before the creature can be introduced into an aquarium where the water is intended to be transparent.
I may here mention that the name of Aphrodite is a singularly happy one. It signifies something that arises from the foam of the sea, and was given to the goddess of beauty, because in the ancient myths she was said to have sprung from the foam of the sea. Unpoetical as it may appear, the German word Meerschaum, which is so familiar to us in connection with pipes, is the exact equivalent of Aphrodite.
Below the Aphrodite is a figure representing the filtering apparatus which is found in the beak of the duck. This singularly beautiful apparatus is well worthy of examination, and the more important details of its structure can easily be made out by the unassisted eye.
In the first place, the upper half of the beak, or upper mandible, as it is scientifically called, is furnished along its edges with a row of curved horny projections, very like the teeth of a comb, and each of them coming to a point. There are some fifty or sixty of these teeth on each side, and they are regularly graduated in size, being longest in the middle of the beak, and becoming very short at either end. They are set diagonally, with the tips pointing backwards. The edges of the lower mandible are turned up in a sort of fold, on the outside of which is a row of grooves corresponding with the teeth of the upper mandible, and, like them, being set diagonally.
These teeth and grooves would of themselves make a very efficient filter, but they are further aided by the tongue. This is thick, fleshy, and very mobile; so much so, indeed, that when the mouth is opened the tongue is automatically thrust forward. The edges of the tongue are, like those of the mandibles, furnished with a filtering apparatus. Instead, however, of being horny and stiff like those of the mandibles, they are membranous and exceedingly delicate. Indeed, in order to see them properly, it is necessary to place the tongue under water, so that the membranous filaments shall be floated apart instead of clinging together by their own weight.
The whole of this apparatus is abundantly supplied with nerves, and is evidently a most exquisite instrument of touch. The reader will now understand the peculiar movements of a duck’s beak while feeding. Although the bird can and does eat solid food, such as barley, and, by reason of its superior width of beak, will very much defraud the poultry in a yard where ducks and hens are kept together, it is chiefly fitted for extracting nourishment from water, and will find abundant subsistence where a hen would die of starvation.
When the beak is plunged into the water, the mandibles are rapidly opened and shut, the tongue incessantly working backwards and forwards between them. Consequently, not only are the solid parts of the water strained between the comb of the upper beak and the grooves of the lower, but they undergo a further sifting or filtering from the delicate fibrils which fringe the edge of the tongue.
Another familiar example of the Filter is to be found in the jaw of the Greenland Whale. In this animal, as well as in its congeners, the “whalebone,” or “baleen,” as it is more properly called, is so formed that it allows liquids to pass through it, while it retains solids. Feeding as it does upon small marine matters, it would starve but for the filtering power of the baleen, which enables the animal to take into its vast mouth the sea-water with its inhabitants, and to expel the water through the plates and fibres of the baleen, while retaining the animals.
The process of filtering, as well as the structure of the baleen, is so familiar that it does not need further description.
We will now proceed to another filter, which is used in the air, and not in water, namely, the Mouth-guard or Respirator of the fork-grinder.
There is, perhaps, no trade which is more destructive of human life than that of the fork-grinder was until the peculiar respirator was made obligatory. The minute particles of steel thrown off by the grindstone fill the air, and were necessarily inhaled. Now, the human lungs are capable of enduring very bad treatment, but the introduction of steel-dust into them is more than they can bear. Consequently the duration of human life was very short, consumption almost invariably setting in at an early age, and carrying off the men before they had achieved middle age.
Nor did the mischief end there. It was bad enough that life should be shortened, but far worse that it should be wasted, as was mostly the case. The men, knowing what their fate must be, were simply reckless, and plunged into all kinds of debauchery, under the plea of “a short life and a merry one.” They knew no better, and could scarcely be blamed for their mode of living. And, as a matter of course, each succeeding generation was worse, smaller, and feebler than the preceding.
Then there came the invention of the Magnetic Respirator, by which the fork-grinder’s trade was rendered as healthy as any other. It was made of steel-wire gauze, and magnetised, so that the floating particles of steel were not only stopped in their progress to the lungs, but arrested by the magnetism, and, so to speak, taken prisoners by it.
Even a well-made respirator of several layers, like those which are used by persons suffering from weak lungs, would have been useful, but the addition of magnetism doubled the efficacy while greatly diminishing the cost, a single layer of wire being quite adequate to the office, and was, in fact, quite a stroke of genius.
The value of this invention is at once shown by the many complaints which the workmen made when the Respirator was first introduced. They complained that the apertures of the Respirator became so choked that they could not breathe. This was perfectly true, but the complaint showed the real value of the instrument.
It was necessary for the workmen, every now and then, to clear off the innumerable particles of steel which adhered to the magnetised wires, and impeded respiration. But they never seemed to realise the fact that, if it had not been for these wires, all the particles would have been drawn into the lungs, and gradually choked them up, brought on inflammation, and extinguished their life altogether. And, with the usual repugnance to new ideas which is inherent in undeveloped minds, the men stoutly resisted the introduction of the Respirator, and did their best to reject an invention which doubled the length of their lives, and enabled them to find long happiness in the world instead of brief pleasure ended by sure and painful death.
Now, we will see how the principle of the Respirator is carried out in Nature.
On the left hand of the illustration is drawn one of the most perfect Respirators, or air-filters, if we may use the term, that can be imagined. Perhaps some of my readers may know that insects do not breathe as we do. They have no lungs, but their entire system is permeated by air-vessels, just as is our system with blood-vessels, and therefore the air, instead of being restricted to the lungs, is conveyed to every part of the insect, the air-vessels extending to the very tips of the wings and antennæ, and to the claws of the feet.
Neither does the insect receive the air through mouth or nostrils as we do. Along the sides of the body are certain oval apertures called “spiracles,” from the Latin word spiro, which signifies breathing. These spiracles can easily be seen by examining an ordinary silkworm. They are situated in the soft and flexible skin which connects the rings or segments of which all insects are composed, and pass directly into two large air-tubes which run on either side of the body.
It is evident that since an insect is so thoroughly permeated with air, it must be furnished with means to render that air as pure as possible, and at all events to preserve the respiratory system from being choked with dust or other adventitious substances.
How important the air is to an insect can easily be seen by dipping it in oil, or even brushing an oiled feather on its sides so as to fill up the spiracles. A man under the hands of the hangman or garotter could not die more swiftly, so much does an insect depend on air. In fact, an insect is almost wholly composed of air-tubes, but for which the great thick-bodied dor-beetles could never use their organs of flight.
Of course, although the spiracles can act as filters as far as the air is concerned, they cannot be analysts, and consequently insects are peculiarly sensitive to a bad atmosphere. There is, for example, the well-known “laurel-bottle” of entomologists. A few young laurel-leaves are crushed and placed in a bottle. As soon as an insect is introduced, it breathes the prussic acid which is exhaled from the leaves, and at once dies.
So it is with the more delicate “death-bottle,” into which a little cyanide of potassium is introduced, and covered with plaster of Paris. The plaster prevents the poison from touching the insects and damaging their beautiful colours. It permits the deadly vapour to roll through its interstices; consequently, even the large-bodied moths, which are tenacious of life almost beyond credibility, can barely run round the bottle, when they roll over, and expire almost without a struggle, the venomous atmosphere having saturated the entire body.
All entomologists know that the spiracles act as sieves, preventing any extraneous objects from gaining admission into the breathing-tubes. But, unless they have had personal experience, they cannot appreciate the efficacy of the spiracle when acting as a respirator. Even the microscope, though it may magnify the object to any extent, does not show the wonderful filtering power of the spiracle. The figure in the illustration represents a spiracle of the common “blue-bottle” fly, and any one who wishes to examine such an object for himself can have but little difficulty in doing so, especially in the warm season of the year.
How effectual is the barrier thus interposed by Nature between the external world and the interior of the insect may be inferred from the following narrative:—
Many years ago, while absorbed in the comparative anatomy of insect structure, I believed myself to have hit upon a plan for injecting the minutest of tubes with mercury. So I took a male cockroach, placed a vessel of mercury in the receiver of an air-pump, and suspended the cockroach exactly over it. As the reader will fully have surmised, my idea was, first to exhaust the air from the inside of the insect, then to plunge it into the mercury, and then to admit the air, which, at a pressure of fifteen pounds to the square inch, was likely to drive the mercury into the smallest of tubes. Such a plan was very successful with ordinary tissues, and might succeed with insects.
Accordingly, I exhausted the air from the vessel in which the cockroach was placed, and kept it in a state of exhaustion for a whole day, so as to prove that every particle of air was withdrawn from the insect. I then plunged the cockroach deeply beneath the mercury, and admitted the air, hoping that the severe pressure would drive the mercury into the respiratory vessels. But not one particle of the mercury could pass through the wonderful filter with which the cockroach had been provided, and, except that I had learned the power of the spiracle, I might have saved both the time and trouble.
It is worthy of notice that, almost countless as are the species of insects, no two of them possess exactly the same structure of the spiracles, the individuality being marked as clearly in these tiny organs as in the entire insect.
USEFUL ARTS.
CHAPTER V.
THE PRINCIPLE OF THE SPRING.—THE ELASTIC SPRING.—ACCUMULATORS.—THE SPIRAL SPRING.
Springs and their various Structure.—The Elastic Spring.—The Boy’s Catapult and its Powers.—The Pistolograph, its Principle, and Uses to which it can be put.—Leaf-rolling Caterpillars, and their Way of Work.—The Carriage Spring.—The Horse’s Hoof and its complex Structure.—Fungi and their united Power.—The Chinese Cross-bow.—The ancient Balista.—Skull of the Crocodile.—Bones of young Children.—The Spiral Spring and its many Uses.—The Toy-gun.—The Needle-gun.—Valved Brass Instruments.—Watch and Clock Springs.—The Bed Spring.—Parallels in Nature and Art.—Buffers of Railway Carriages.—Spring Solitaires.—The Bell Spring.—Spiral Springs in Vegetable Tissues.—Poison Cells of various Marine Animals.—Effects of the Spiral Springs.