In the field of botany Aristotle had a wide knowledge of natural phenomena, and raised general questions as to mode of propagation, nourishment, relation of plants to animals, etc. His pupil and lifelong friend, and successor as leader of the Peripatetic school of philosophy, Theophrastus, combined a knowledge of mathematics, astronomy, botany, and mineralogy. His _History of Plants_ describes about five hundred species. At the same time he treats the general principles of botany, the distribution of plants, the nourishment of the plant through leaf as well as root, the sexuality of date palm and terebinth. He lays great stress on the uses of plants. His classification of plants is inferior to Aristotle's classification of animals. His views in reference to spontaneous generation are more guarded than those of his master. His work _On Stones_ is dominated by the practical rather than the generalizing spirit. It is evidently inspired by a knowledge of mines, such as the celebrated Laurium, from which Athens drew its supply of silver, and the wealth from which enabled the Athenians to develop a sea-power that overmatched that of the Persians. Even to-day enough remains of the galleries, shafts, scoria, mine-lamps, and other utensils to give a clear idea of this scene of ancient industry. Theophrastus considered the medicinal uses of minerals as well as of plants. We have failed to mention Hippocrates (460-370 B.C.), the Father of Medicine, in whom is found an intimate union of practical science and speculative philosophy. We must also pass over such later Greek scientists as Aristarchus and Hipparchus who confuted the theories of Pythagoras and Plato in reference to the relative distances of the heavenly bodies from the earth. Archimedes of Syracuse demands, however, particular consideration. He lived in the third century B.C., and has been called the greatest mathematician of antiquity. In him we find the devotion to the abstract that marked the Greek intelligence. He went so far as to say that every kind of art is ignoble if connected with daily needs. His interest lay in abstruse mathematical problems. His special pride was in having determined the relative dimensions of the sphere and the enclosing cylinder. He worked out the principle of the lever. "Give me," he said, "a place on which to stand and I will move the earth." He approximated more closely than the Egyptians the solution of the problem of the relation between the area of a circle and the radius. His work had practical value in spite of himself. At the request of his friend the King of Sicily, he applied his ingenuity to discover whether a certain crown were pure gold or alloyed with silver, and he hit upon a method which has found many applications in the industries. His name is associated with the endless screw. In fact, his practical contrivances won such repute that it is not easy to separate the historical facts from the legends that enshroud his name.