its distillates in his book "Kitab sirr al-asrar" (book of the secret of secrets.) When choosing a site to build Baghdad's hospital, he hung pieces of fresh meat in different parts of the city. The location where the meat took the longest to rot was the one he chose for building the hospital. Advocated that patients not be told their real condition so that fear or despair do not affect the healing process. Wrote on alkali, caustic soda, soap and glycerine. Gave descriptions of equipment processes and methods in his book Kitab al-Asrar (book of secrets). Mathematics 826901: Thabit ibn Qurra (Latinized, Thebit.) Studied at Baghdad's House of Wisdom under the Banu Musa brothers. Discovered a theorem that enables pairs of amicable numbers to be found. Later, al-Baghdadi (b. 980) developed a variant of the theorem. Miscellaneous c. 810: Bayt al-Hikma (House of Wisdom) set up in Baghdad. There Greek and Indian mathematical and astronomy works are translated into Arabic. 810887: Abbas ibn Firnas. Planetarium, artificial crystals. According to one account that was written seven centuries after his death, Ibn Firnas was injured during an elevated winged trial flight. Tenth Century By this century, three systems of counting are used in the Arab world. Finger-reckoning arithmetic, with numerals written entirely in words, used by the business community; the sexagesimal system, a remnant originating with the Babylonians, with numerals denoted by letters of the arabic alphabet and used by Arab mathematicians in astronomical work; and the Indian numeral system, which was used with various sets of symbols. Its arithmetic at first required the use of a dust board (a sort of handheld blackboard) because "the methods required moving the numbers around in the calculation and rubbing some out as the calculation proceeded." Chemistry 957: Abul Hasan Ali Al-Masudi, wrote on the reaction of alkali water with zaj (vitriol) water giving sulfuric acid. Mathematics 920: al-Uqlidisi. Modified arithmetic methods for the Indian numeral system to make it possible for pen and paper use. Hitherto, doing calculations with the Indian numerals necessitated the use of a dust board as noted earlier. 940: Born Abu'l-Wafa al-Buzjani. Wrote several treatises using the finger-counting system of arithmetic and was also an expert on the Indian numerals system. About the Indian system, he wrote: "[It] did not find application in business circles and among the population of the Eastern Caliphate for a long time." Using the Indian