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but also that a vast variety of the more useful plants are so distributed as to induce commercial intercourse; and thus, while they render nations mutually dependent on one another for many of the comforts and conveniences of life, they are an indirect means of promoting the advancement of civilization, and, above all, of the spread of the gospel. Why, however, a certain species should only be able to flourish in a certain soil, and under a certain amount of heat and moisture, is a problem we cannot as yet solve; it doubtless depends on special peculiarities in the internal structure, but in what those peculiarities consist, we are ignorant.
Some plants, too, are very widely distributed. The daisy, for example, is spread throughout Europe almost universally, in Australia, in Northern Asia, in some parts of Africa, and in South America. In India and North America, however, it is entirely absent, and can only be preserved as a choice exotic, tended with the most zealous care in botanic gardens. The cereals, that is, wheat, barley, oats, rye, etc., are endowed with a very great power of adaptation. Though their native country is scarcely known, and they are rarely found wild, yet they possess a power of enduring such a variety of temperature, that they have been introduced by cultivation over a large portion of our globe. They can withstand the cold of 62° N. lat., and though they will not in general bear the heat of the tropics, yet even in such places they are sometimes cultivated during the winter season. The potato, again, though only known wild as a native of the western coast of South America, is now cultivated almost universally, particularly in Europe and North America, and has become one of the most important articles of food, especially among the poor.
On the other hand, many species (and probably the great majority of plants) are very limited in their abodes. Cactus tribe, so generally cultivated in our green-houses, and so remarkable for the singularity of their growth, the absence of leaves, and the splendour of their flowers, contains 800 species, all of which are peculiar to America, and not a single species is a native of Europe, Asia, or Africa. 533 species of the beautiful genus Erica, or heath, are found at or near the Cape of Good Hope, and nowhere else in the world. The species of Cinchona, too, which yield the Peruvian bark,
grow only on the eastern declivity of the Andes, as far as 18o s. lat; and the cedar of Lebanon is indigenous to that mountain alone. It would be easy to multiply instances, but this is needless.
These facts will enable us readily to understand, that there are numerous botanical districts on the surface of our globe, each of which has its own vegetation, a considerable number, perhaps the majority of the species, being peculiar to the particular district, while others are found in common with other localities. Thus, the Flora of the United States of North America is totally different from that of Europe, even in places where the annual temperature is the same. Of 2,891 species of flowering plants found in the United States, only 385 are common to them and the corresponding latitudes of Europe. In St. Helena, of thirty flowering plants, only one or two are native elsewhere. In the Galapagos1 islands, out of 180 plants which have been collected, 100 are found nowhere else; and of twenty-one species of composite plants, all but one are peculiar to that group. Some few species make the most remarkable leaps, being common to countries at a great distance from each other, while absent, or nearly so, from the intervening ones. Thus in the Falkland2 islands, more than thirty plants, natives of Britain, are found wild. The common quaking-grass has been found in the interior of the country at the Cape of Good Hope; and almost all the lichens brought from the southern hemisphere by Sir James Ross, amounting to 200 species, are found in the northern hemisphere, and chiefly in Europe. Several of our commonest plants, as the bull-rush, the reed, the marsh-mallow, the bird's-foot trefoil, the knot-grass, with several others, are found again in Australia. For this various distribution of plants it is difficult to account, but we are inclined to say with Milton,—
And Earth, an infant, naked as she came
Of verdure with ten thousand glorious flowers,
1 Galapagos; a group of islands in the Pacific Ocean, off the coast of Colombia, and immediately under the Equator.
2 Falkland Islands; a group in the S. Atlantic Ocean, to the E. of the straits of Magellan, consisting of two large and a number of small islands. They are rocky, but abound with seals, and contain large and safe harbours. A British settlement has been formed there.
Exhaling incense; crown'd her mountain-heads
1. State the number of distinct species of plants known.
2. What are included in this number? 3. What modifies greatly the diffusion of plants?
4. Where is vegetation most luxuriant? 5. Where does vegetable life become extinct?
6. In tropical regions, is man able to work very laboriously?
7. Name some vegetable productions found there.
8. Where do the corn-plants succeed best?
9. Mention various wise ends served by this arrangement of the vegetable productions of our globe.
The Geography of Plants.
10. Say where the daisy is found, and where not.
11. What mean you by cereals? 12. Up to what degree of latitude will they grow?
13. Name the native country of the po. tato.
14. What tribe of plants belongs exclusively to America?
15. What about the plant that yields Peruvian bark?
16. What about the flowering plants in the United States?
17. What about those in St. Helena, the Galapagos, and the Falkland Islands? 18. Repeat the lines of Milton about the first production of plants.
George. There are a good many sorts of metals,' are there not?
Tutor. Yes, several; and if you have a mind I will tell you about them, and their uses.
G. Pray do, Sir.
Harry. Yes; I should like to hear it of all things.
T. Well, then. First let us consider what a metal is. Do you think you should know one from a stone?
1 There are forty-three metals now known; some of them, however, are more curious than useful. We shall give you the names of the twelve malleable metals, viz., platinum (or, as it is frequently called, platina), gold, silver, mercury, lead, copper, tin, iron, zinc, palladium, nickel, and cadmium.
G. A stone! Yes I could not mistake a piece of lead or iron for a stone.
T. How would you distinguish it?
G. A metal is bright and shining.
T. True-brilliance is one of their qualities. But glass and crystal are very bright, too.
H. But one may see through glass, and not through a piece of metal.
T. Right. Metals are brilliant, but opaque, or nor transparent. The thinnest plate of metal that can be made, will keep out the light as effectually as a stone wall.
G. Metals are very heavy, too.
T. In general they are; but there are some metals which are lighter than water; these light metals, however, are difficult to be procured, and are more curious than useful. Well, what else?
G. Why, they will bear beating with a hammer, which a stone would not, without flying in pieces.
T. Yes; that property of extending or spreading under the hammer is called malleability; and another, like it, is that of bearing to be drawn out into a wire, which is called ductility. Metals have both these, and much of their use depends upon them.
G. Metals will melt, too.
H. What! will iron melt?
T. Yes; all metals will melt, though some require greater heat than others. The property of melting is called fusibility. Do you know anything more about them?
G. No; except that they come out of the ground, I believe.
T. That is properly added, for it is the circumstance which makes them rank among minerals. To sum up their character, then, a metal is a brilliant, opaque, heavy, malleable, ductile, and fusible mineral.
G. I think I can hardly remember all that.
T. The names may slip your memory, but you cannot see metals at all used without being sensible of the things.
G. But what are ores? I remember seeing a heap of iron ore which men were breaking with hammers, and it looked only like stones.
T. The ore of a metal is the state in which it is generally met with in the earth, when it is so mixed and combined
with stony and other matters, as not to show its proper qualities as a metal.
H. How do people know it then?
T. By experience. It was probably accident that in the early ages discovered that certain minerals by the force of fire might be made to yield a metal. The experiment was repeated on other minerals; so that in length of time all the different metals were found out, and all the different forms in which they lie concealed in the ground. The knowledge of this is called Mineralogy,' and a very important science it is.
G. Yes, I suppose so; for metals are very valuable things Our next neighbour, Mr. Sterling, I have heard, gets a grea deal of money every year from his mines in Wales.
T. He does. The mineral riches of some countries are much superior to that of their products above ground, and the revenues of many kings are in great part derived from their mines.
H. I suppose they must be gold and silver mines.
T. Those, to be sure, are the most valuable, if the metals are found in tolerable abundance. But do you know why they are so?
H. Because money is made of gold and silver.
T. That is a principal reason, no doubt. But these metals have intrinsic properties that make them highly valuable, else probably they would not have been chosen in so many countries to make money of. In the first place, gold and silver are both perfect metals, which are indestructible in the fire. Other metals, if kept a considerable time in the fire, change by degrees into a powdery or scaly matter, called a calx, or oxide. You have melted lead I dare say.
G. Yes, often.
T. Have you not, then, perceived a drossy film collect upon its surface after it had been kept melted a while.
T. That is an oxide; and in time the whole lead would change to such a substance. You may see, too, when you have heated the poker red-hot, some scales separate from it, which are brittle and drossy.
1 Mineralogy, that branch of natural history which makes us acquainted with the relations and properties of minerals. Metallurgy, is the art of separating metals from their ores, comprising the processes of assaying, refining, and smelting.