« AnteriorContinuar »
which nature throws her simple materials. This is placed in a more striking light when we contemplate the different forms which even the same substance can assume. To select one of the many examples which there are of this :—the diamond, the rarest, the hardest, most transparent, and most resplendent mineral known, consists of exactly the same material as the dull, opaque, black, earthy substance called charcoal. When burnt, it produces the same matter as when charcoal is burnt; and both produce the same substance, namely, steel, when they are made to unite with iron. To take yet another example of the wonderful powers which have been impressed upon matter, — let us look at the compounds of two gases, oxygen and nitrogen. These two thin, transparent, and invisible bodies, united in certain proportions, constitute the air which we breathe and are perpetually in contact with, upon which our existence depends, and of which we take in a supply, about a thousand times every hour. The same materials, in different proportions, form the intoxicating gas, which produces such peculiar effects when inspired. The same ingredients, in other proportions, form a curious gas, transparent and colourless, which, whenever it meets the air, also transparent and colourless, becomes of a dark red colour, by depriving the air of that part of it which we use in breathing. The beauti. ful red fumes, thus formed, are composed of the same two gases. Lastly, the same substances, with a little water added, are the ingredients of aquafortis, one of the most virulent and corrosive poisons known, which instantly corrodes the skin, and produces a deep ulcer when applied to it, and would cause the most excruciating torture and sudden death, if it could be taken internally. By subjecting atmospheric air to the influence of electricity, it has actually been converted into aquafortis. Let us pause, for a moment, to reflect on the wonders which a knowledge of nature unfolds to us. Electricity is constantly present in the earth and atmosphere, and it can convert the atmosphere into a deadly poison. Here then we are surrounded on all sides by the elements of destruction. Yet this substance, the air, and this singular agent, electricity, which, by a slight change in their mutual relations, might destroy us in an instant, are constantly, and in a thousand ways, contributing to our support and well-being. So true it is, that we are fearfully and wonderfully made.
LESSON IV. CHANGES IN THE SURFACE OF THE EARTH. From the quiet and regular succession of natural events to which we are accustomed, and the repugnance we feel to the idea that it is possible for the course of nature to suffer interruption, we might, without due investigation, almost persuade ourselves that the physical features and condition of the globe possess an unchangeable character. So far, however, is this from being the case, that there is no country wherein traces are not discoverable of the violent revolutions of which the earth has formerly been the theatre; and even yet it is experiencing changes of a very perceptible kind. Of the several agents which contribute to these changes, water has the widest sphere of activity. Streams which descend along the flanks of elevated grounds, carry along with them some portion of the materials of their respective slopes, especially when swelled into violence by rains or the melting of snows; and such as come from mountains, sweep down with them even some of the fragments of rock that have been collected in the high valleys. In proportion, however, as these streams reach the more level country, and their channels become more expanded, they deposit the fragments and stones, till at last their waters convey along only particles of mud of the minutest kind. If, therefore, these waters do not run too rapidly into the sea, or the particles in question do not previously settle in some lake through which the rivers pass, the mud is deposited on the sides of their mouths, forming low grounds, by which the shores are prolonged and encroach upon the sea; and when the waves, by casting up sand upon them, assist in their increase, whole provinces are created, capable, from their rich soil, of yielding, in the highest degree, to the support of man, and of being made the seats of wealth and civilization. It has been concluded, with reason, that the greater part of Lower Egypt owes its formation to the alluvial matter brought down by the Nile, aided by the sand cast up by the sea. The Delta of the Rhone is undergoing a similar augmentation; and it would appear that the arms of that river have, in the course of 1800 years, become longer by three leagues; and that many places which were once situated on the brink of the sea, or of large pools, are now several miles distant from the water. In Holland and Italy, the Rhine and the Po, since they have been banked up by dykes, raise their beds and push forward their mouths into the sea with great rapidity. Such, indeed, has been the increase of new land formed by the latter, that the city of Adria, which, there is no doubt was, at a very remote date, situated on the coast of the Adriatic, is now more than fifteen miles distant from the nearest part of it. At the same time, the river has, in consequence of embankments made to confine it, been so much raised in the level of its bottom, that the surface of its waters is higher than the roofs of the houses in Ferrara ; and the Adige and the Po are higher than the whole tract of country lying between them. The same cause produces the alterations perceived to be taking place in many of those lakes which are traversed by rivers. The matter brought down by the rivers easily settles in the still waters of the lakes, and the necessary result is, that the basins of the latter are gradually undergoing a diminution. Lake Erie, one of the vast bodies of water in North America, is every
year becoming shallower from the influx of pebbles and earth, and the constant accumulation of reeds and shells; and the diminution of the beautiful lake of Geneva is also said to have been considerable within the memory of man.
LESSON V. FORMATION OF NEW ISLANDS. The formation of new islands constitutes another distinct and interesting class among the changes to which the surface of the globe is subject. Those which have been raised up by volcanic agency are comparatively few; but those of coral, which owe their origin to marine insects, (of the class of zoophytes, or plant animals,) are innumerable. Of the different coral tribes, the most abundant is that named the madrapore. It is most common in the tropical seas, and decreases in number and variety towards the poles ; it surrounds, in vast rocks and reefs, many of the islands of the South Sea and Indian Ocean, and increases their size by its daily growth. The coasts of the islands of the West Indies, of those of the east of Africa, and the shores and shoals of the Red Sea, are encircled with rocks of coral. Several navigators have furnished us with accounts of the curious manner in which these formations take place; the following is extracted from Captain Basil Hall's narrative of his voyage to the Loo Choo Islands:
“The examination of a coral reef, during the different stages of one tide, is particularly interesting. When the tide has left it for some time, it becomes dry, and appears to be a compact rock, exceedingly hard and rugged; but as the tide rises, and the waves begin to wash over it, the coral worms protrude themselves from holes which were before invisible. These animals are of a great variety of shapes and sizes, and, in such prodigious numbers, that, in a short time, the whole surface
of the rock appears to be alive and in motion. The most common of the worms at Loo Choo is in the form of a star, with arms from four to six inches long, which are moved about with a rapid motion, in all directions, probably to catch food. Others are so sluggish, that they may be mistaken for pieces of the rock, and are generally of a dark colour, and from four to five inches long, and two or three round. When the coral is broken, about high-water, it is a solid, hard stone; but if any part of it be detached at a spot which the tide reaches every day, it is found to be full of worms of different lengths and colours; some being as fine as a thread, and several feet long, of a bright yellow, and sometimes of a blue colour; others resemble snails, and some are not unlike lobsters in shape, but soft, and not above two inches long
“ The growth of the coral appears to cease when the worm is no longer exposed to the washing of the sea. Thus, a reef rises in the form of a cauliflower, till its top has gained the level of the highest tides, above which the worm has no power to advance, and the reef, of course, no longer extends itself upwards. The other parts, in succession, reach the surface, and there stop, forming, in time, a level field with steep sides all around. The reef, however, continually increases, and being prevented from going higher, extends itself laterally in all directions. But this growth being as rapid at the upper edge as it is lower down, the steepness of the face of the reef is still preserved. These are the circumstances which render coral reefs so dangerous in navigation; for, in the first place, they are seldom seen above water ; and, in the next, their sides are so steep, that a ship's bows may strike against the rock, before any change of soundings has given warning of the danger.”
Another navigator gives the following succinct account of the manner in which, after being raised up, the coral islands gradually acquire a soil and vegetation: -"To be constantly covered with water seems neces