The natural sciences were cultivated by them with great ardor, and many naturalists among them merit the gratitude of posterity. Botany and chemistry, of which they were in some sort the inventors, gave them a better acquaintance with nature than the Greeks or Romans ever possessed, and the latter science was applied by them to all the necessary arts of life. Above all, agriculture was studied by them with a perfect knowledge of the climate, soil, and growth of plants. From the eighth to the eleventh century, they established medical schools in the principal cities of their dominions, and published valuable works on medical science. They introduced more simple principles into mathematics, and extended the use and application of that science. They added to arithmetic the decimal system, and the Arabic numerals, which, however, are of Hindu origin; they simplified the trigonometry of the Greeks, and gave algebra more useful and general applications. Bagdad and Cordova had celebrated schools of astronomy, and observatories, and their astronomers made important discoveries; a great number of scientific words are evidently Arabic, such as algebra, alcohol, zenith, nadir, etc., and many of the inventions, which at the present day add to the comforts of life, are due to the Arabians. Paper, now so necessary to the progress of intellect, was brought by them from Asia. In China, from all antiquity, it had been manufactured from silk, but about the year 30 of the Hegira (649 A.D.) the manufacture of it was introduced at Samarcand, and when that city was conquered by the Arabians, they first employed cotton in the place of silk, and the invention spread with rapidity throughout their dominions. The Spaniards, in fabricating paper, substituted flax for cotton, which was more scarce and dear; but it was not till the end of the thirteenth century that paper mills were established in the Christian states of Spain, from whence the invention passed, in the fourteenth century only, to Treviso and Padua. Tournaments were first instituted among the Arabians, from whom they were introduced into Italy and France. Gunpowder, the discovery of which is generally attributed to a German chemist, was known to the Arabians at least a century before any trace of it appeared in European history. The compass, also, the invention of which has been given alternately to the Italians and French in the thirteenth century, was known to the Arabians in the eleventh. The number of Arabic inventions, of which we enjoy the benefit without suspecting it, is prodigious.