Though the oils (or fats) of plants and animals are very similar to one another in appearance, there are a very large number of them differing chemically from one another. Thus the fat or oil of dozens of different nuts and plant-products and of lower animals and fishes, and of sheep, oxen, pigs, dogs, elephants, and men contain different and special chemical substances, corresponding to the "cetyl" which is present in the fat of the sperm whale's head. Many of them have acquired as a result of experience and tradition special value for some special purpose. Several oils have peculiar fitness and great value for oiling delicate machinery; others are used in curing leather, for burning, and for medicinal ointments, whilst a large variety is used as human food. CHAPTER XIX MUSEUMS The word "museum" is not one of those which explain themselves and give an indication of what the thing to which they are applied should be, when it has ceased to be what it was intended to be. In ancient Greece the word "mouseion" meant "the place of the Muses"--a grove or a temple--and there was such a place on a part of the Acropolis of Athens, the rocky temple-crowned hill around which the city was built. There were other "museums," or seats of the Muses, in ancient Greece; those on the slopes of Mount Helicon and of Mount Olympus were the most famous. In modern times a picture gallery and art collection, that of the Louvre, in Paris, is called "the Musee," whilst "the Museum" (the Latin form of the same word) is the name distinctively applied in Paris to the collections of natural history and the laboratories connected with them in the Jardin des Plantes. In London "the British Museum," founded in 1753, originally comprised the national library as well as collections of antiquities and of natural history. In Heidelberg "the Museum" was the name, when I was there, for a delightful club, with a garden. It belonged to the professors, their families, and their friends in the town, and concerts and dances were given in it. It seems that the Heidelberg "Museum" comes nearest to the original meaning of the word as "a seat of Muses," for nearly all those mythical ladies were remarkable for their special patronage of music, dancing, and song.