"It is a curious fact that the uses of rubber which have been discovered in England and America in the present century were known in South America nearly three hundred years ago. In a book published in Madrid in 1615, Juan de Torquemada describes a tree in Mexico yielding a gum from which the natives make shoes and other things, and he also says that the Spaniards used this gum for waxing their canvas cloaks to make them resist water. Herrara's account of the second voyage of Columbus mentions balls which the natives of Hayti use in their amusements; he says they are made from the gum of a tree, and are lighter and bounce better than the wind-balls of Castile." When the above notes were read over to the Doctor he suggested an addition, which was made at once. "By far the most extensive uses of this material at present are in its vulcanized form, as the pure India-rubber can only be employed to a limited extent. The process of vulcanizing was discovered by an American, Charles Goodyear, in 1843, and consists in mixing rubber with sulphur and heating it to a high degree. There are two kinds of vulcanized rubber, one hard and horny, and the other soft and elastic; for the first the rubber is cut into small shreds, mixed with a third of its weight of sulphur, and heated for several hours, the final heat being not less than 300° Fahrenheit. For the elastic rubber the proportion of sulphur and the degree of heat are much less. An endless variety of articles is made from the two kinds of vulcanized rubber." "While we are on this subject," said Frank, "I wonder if there is a cow-tree in this region. The cow-tree is a South American production, is it not?" "Yes," answered the Doctor, "but it is not in this part of the continent, or, at any rate, the most famous of the family does not grow in the lowlands. There are several trees known by that name, but the _Palo do Vaca_ is found principally in Venezuela and the northern part of the continent, generally at an elevation of three or four thousand feet." "Please tell us what it is like." [Illustration: LEAVES, FRUIT, AND FLOWERS OF THE COW-TREE.] "It is a tall, slender tree, with leaves resembling the laurel in shape, but ten or twelve inches long. It grows in rocky places where there is very little moisture, and during the dry season its leaves are withered and the branches appear dead. But as soon as the trunk is pierced it gives forth a rich, nourishing juice that resembles milk in appearance, taste, and qualities, though it differs materially from the milk of animals. It contains a good deal of wax and fibrin, a little sugar and a salt of magnesia, the rest being water." [Illustration: MILKING THE COW-TREE.] "And does it make cream like the milk of a living cow?"