英文科學(xué)讀本 第六冊·Lesson 43 Ivory

Lesson 43 Ivory

Ivory is a substance resembling bone and, like it, of considerable importance in the arts and manufactures. The closeness of its grain and the high polish which it is capable of taking are sufficient to distinguish ivory from bone. A still more remarkable difference, however, is seen by a careful comparison of the two substances. If, for example, some polished articles of ivory and bone be examined side by side, the former will show a number of beautiful, regular, curved markings on the surface, but the bone has no such marks. These curves may be readily seen, as they are of a slightly different shade of color from the rest of the substance of the ivory. They cross each other, something like the curves on the back of an engine-turned watch-case, forming small lozenge-shaped spaces between them. No specimens of bone—indeed, no other animal substance of any kind—has these markings. Hence this is a sure test for distinguishing ivory from bone.

The readiness with which ivory can be cut, carved, and turned; the beauty of its hard, white, polished surface; and its great durability in all climates—render it specially suitable for many purposes in the arts, both useful and ornamental. It is chiefly employed for making knife-handles, backs for brushes, billiard-balls, chessmen, paper-knives, fans, combs, piano forte and organ keys, and a large variety of fancy and ornamental articles. Cut into thin plates, it is also used in bookbinding as covers for books, and for writing-tablets.

The ivory of commerce is furnished by the teeth and tusks of various animals. Those of the elephant, hippopotamus, and walrus yield the best quality and the largest supply. Among these elephant ivory holds the foremost place.

The elephant, being a herbivorous animal, has largely-developed molar teeth in both jaws for crushing its food; but the most striking feature of its dentition is presented by the two enormous tusks which project from the upper jaw. Incisors or cutting teeth like those of the horse would be useless to an animal that feeds as the elephant does; hence they are always wanting.

The molar teeth of the elephant are of little commercial value, because they contain, in addition to the hard solid dentine, a large proportion of more loosely-constructed bony matter, known as cement, which has a tendency to crack, and so renders the ivory less durable. They are sometimes used for making knife-handles and small thin plates for writing-tablets.

There is a remarkable difference in the structure of the molars of the African and Asiatic elephants. In both of them the hard enamel of the tooth appears in ridges on the crown. In the tooth of the African elephant these ridges assume a kind of lozenge-shape; in the Asiatic animal they stretch transversely across the crown of the tooth from edge to edge.

The tusks of the elephant are the largest teeth to be met with in the animal kingdom. They grow to their enormous size because of the absence of any teeth in the lower jaw to oppose them.

In most of the mammalia (including man himself) the first set of teeth are shed in due course, and their place is taken by a new set. The renewal occurs but once during the life of the animal, and the teeth do not grow after attaining a certain size. We may therefore say that these teeth are of limited growth. Contrast such teeth with the incisors of the rodents, which grow as long as the animals live, and the enormous tusks of the elephant will cease to be a matter of wonder. The tusks of the elephant, hippopotamus, wild-boar, and walrus, and the long spiral, horn-like tusk of the narwhal, are alike in this respect; they are endowed with perpetual growth, for they continue to grow as long as life lasts.

The African is much larger than the Asiatic elephant; the female as well as the male is furnished with tusks, and the tusks attain immense proportions. A single tusk will sometimes weigh from 150 to 175 lbs. and measure 10 feet in length.

The tusks of the Indian elephant are smaller; they usually weigh from 60 to 80 lbs. each. The elephants of Ceylon and Siam furnish very beautiful and highly-prized ivory, but the tusk does not weigh more than 25 or 30 lbs.

The elephant uses his tusks in uprooting trees, to get at the young herbage growing on the upper branches. They are also very formidable weapons of attack and defence.

Sir Samel Baker, the great African explorer, gives a vivid description of the havoc which he once saw wrought in a mimosa forest by a great herd of wild elephants. These trees grow from 16 to 20 feet in height, and the animals uproot them easily, using their tusks as crowbars and levers. He found the place a complete wreck, the trees not only torn up, but stripped of their leaves and tender green shoots, and even their bark; while the vast herd could be seen in the distance leisurely making off after their meal.

For hundreds of years vast quantities of fossil-ivory of the now extinct mammoth have been dug out of the frozen soil in Siberia and Alaska. It is estimated that about 20,000 lbs. of it finds its way into the Russian markets every year. As many as ten fossil tusks have been dug out in one spot, many of them weighing from 200 to 300 lbs. each.

In spite of the removal of such enormous quantities of fossil-ivory each year, the store does not seem to materially diminish, for in many places the tusks are found lying about in heaps.

The tusks of the hippopotamus yield a very highly-esteemed ivory, which is much harder and whiter than elephant ivory. It keeps its color, too, better than some kinds, which are apt to turn yellow. This ivory was very much in demand till late years by dentists for the manufacture of artificial teeth, and fetched as much as 30s. a lb. It is not used for this purpose now; it has been replaced by other substances.