36 A TROPICAL DEPENDENCY Al Gazzali, who in 1058 was the forerunner of Descartes; Al-Hazen, the optician, who estabHshed the modern theory of vision, basing it on clear anatomical and geometrical evidence, and who was the first to trace, about the year 1 100, the curvilinear path of rays of light through the air, deducing from his theory of refraction a determination of the height of the atmosphere ; to El Idrisi, the geographer of Roger of Sicily ; the famous Averrhoes, the commentator of Aristotle, who lived at the end of the twelfth century ; and the brilliant schools of medicine and surgery which adorned the thirteenth century : all serve to demonstrate that in the application of the abstract principles of science to natural phenomena, the Arabs luminously opened the path of modern progress. Their studies in chemistry were profound. Geber, or Djajar, who lived in the ninth century, and of whom Roger Bacon speaks at a much later period as the magister magistrorum of chemical science, was the first to describe nitric acid and aqua regia. Before him chemistry had no stronger acid than concentrated vinegar. The properties and preparation of sulphuric acid and phosphorus soon followed. For the composition of gunpowder we get towards the end of the eighth century the following prescription: "Pulverise on a marble mortar one pound of sulphur, two of charcoal, and six of saltpetre." Any one who may have visited the royal gunpowder works at Waltham Abbey will know how little the prescription has altered, except in varying proportions, to the present day. In geology, also, Arabian Investigations were on the sound path of reason. Avicenna says of mountains that they may be due to two different causes. ** Either they are the effect of upheavals of the crust of the earth . . . or they are the effect of water which, cutting for Itself a new route, has denuded the valleys, the strata being of different kinds — some soft, some hard." The Arabs were early acquainted with the properties of the magnet and the theory of gravitation. True conceptions of geology and astronomy led naturally to truer conceptions of the