Physics and chemistry, as well as mathematics and astronomy, owe much in their development to the Arabs. An Arabian scientist of the eleventh century studied the phenomena of the reflection and refraction of light, explained the causes of morning and evening twilight, understood the magnifying power of lenses and the anatomy of the human eye. Our use of the terms retina, cornea, and vitreous humor may be traced to the translation of his work on optics. The Arabs also made fair approximations to the correct specific weights of gold, copper, mercury, and lead. Their alchemy was closely associated with metallurgy, the making of alloys and amalgams, and the handicrafts of the goldsmiths and silversmiths. The alchemists sought to discover processes whereby one metal might be transmuted into another. Sulphur affected the color and substance. Mercury was supposed to play an important part in metal transmutations. They thought, for example, that tin contained more mercury than lead, and that the baser, more unhealthy metal might be converted into the nobler and more healthy by the addition of mercury. They even sought for a substance that might effect all transmutations, and be for mankind a cure for all ailments, even that of growing old. The writings that have been attributed to Geber show the advances that chemistry made through the experiments of the Arabs. They produced sulphuric and nitric acids, and _aqua regia_, able to dissolve gold, the king of metals. They could make use of wet methods, and form metallic salts such as silver nitrate. Laboratory processes like distilling, filtering, crystallization, sublimation, became known to the Europeans through them. They obtained potash from wine lees, soda from sea-plants, and from quicksilver the mercuric oxide which played so interesting a part in the later history of chemistry. Much of the science lore of the Arabs arose from their extensive trade, and in the practice of medicine. They introduced sugar-cane into Europe, improved the methods of manufacturing paper, discovered a method of obtaining alcohol, knew the uses of gypsum and of white arsenic, were expert in pharmacy and learned in materia medica. They are sometimes credited with introducing to the West the knowledge of the mariner's compass and of gunpowder.