CH. vti. GEBER DISCOVERS ACTDS. 45 



a vapour which cooled down into a very strong acid, now 

 called nitric add. He used this to dissolve silver, and by 

 mixing it with sal-ammoniac he found it would even dissolve 

 gold. Sal-ammoniac was a kind of salt which was known 

 to the Arabs before Geber's time. They made it by heat- 

 ing the dung of camels, and the name ammoniac was given 

 to it because they made it first in the desert near the temple 

 of Jupiter Ammon. Geber also made sulphuric acid by dis- 

 tilling alum. When we remember that all these experi- 

 ments were made more than a thousand years ago, we must 

 acknowledge that Geber deserves great honour for the dis- 

 coveries which he made. 



Albategnius, 879. We have seen that in chemistry 

 the Arabs learned very little from the Greeks, but in mathe- 

 matics and astronomy they found a great deal written, and 

 the Arabian astronomers spent much of their time in trans- 

 lating Greek works. Unfortunately they mixed up astro- 

 nomy, or the study of the heavenly bodies, with astrology, a 

 kind of magic art, by which they imagined they could fore- 

 tell what was going to happen by studying the stars. It 

 was the Arabs, however, who preserved the knowledge of 

 astronomy for nearly 700 years. In 964 Al Sufi re-estimated 

 the brilliancy of the stars in Ptolemy's catalogue; while 

 Albategnius, born A.D. 879, calculated the length of the 

 year more exactly than Ptolemy had done, making it 365 

 days 5 hours 46 minutes 24 seconds, which was only two 

 minutes shorter than it really is. In A.D. 1008 Ebn Junis 

 drew up several astronomical tables, and in 1437 Ulug Beg 

 made the second original catalogue of stars. 



Ben Musa, 90O. Of mathematicians, one of the most 

 celebrated was Mohammed Ben Musa, who lived about 

 A.D. 900. He is the earliest Arabian writer on Algebra, or 

 the working of sums by means of letters, although we find 



