"I have been looking up the subject of ivory," responded Fred, "and find that the elephant hasn't a monopoly of the business. The tusks of the hippopotamus, the walrus, the narwhal, and some other animals are included under the name of ivory, and have nearly the same uses." "But not exactly the same," was the reply. "Several high authorities contend that the name only belongs properly to the product of the elephant, which has a different composition from that of other animals. A cross-section of an elephant's tusk has the appearance of circular lines, intersecting each other, so as to form lozenge-shaped figures with curved boundaries; this feature exists in no other kind of tusk, and is the distinguishing mark of elephant ivory. "There is a vegetable ivory which is used for making small articles, but it is too soft to wear well, and tarnishes very easily. It is the kernel of a nut that grows in Peru and New Granada. The tree is a species of palm, and the ivory corresponds to the meat of the familiar cocoa-nut. Perhaps somebody will invent a process for converting the cocoa-nut into ivory, and then the article will be a good deal cheaper than it is." "What would be the use?" said Fred. "Haven't we several imitations of ivory already? Have you forgotten celluloid?" "Quite true," replied Frank; "but then celluloid, while greatly resembling ivory, is far from equalling it. Men who play at billiards say the balls made of celluloid have a dead sound when struck, and that the same is the case with all other imitations. However, there is no doubt that celluloid can take the place of ivory for many uses, and the elephants ought to have a mass meeting, and send a vote of thanks to the man who invented it." "Perhaps I've got a question you can't answer," said Fred. "What is the composition of ivory?" "I've informed myself on that point," replied Frank, with a smile. "Ivory contains twenty-four per cent. of animal matter, sixty-four of phosphate of lime, a little more than eleven per cent. of water, and the balance is carbonate of lime and other insignificant ingredients. The water and animal matter are dried away by long exposure, and for this reason ivory is apt to turn yellow and change its form. Billiard-balls require to be turned occasionally to correct these changes. They generally shrink or expand more in the direction of the width of the tusks than in that of their length, and consequently the makers of these articles are accustomed to shape them roughly at first, and then keep them for months in a warm room before finishing.