62 HISTORY OF SCIENCE. 



in Spain during the eleventh century. He is best known by a treatise 

 on optics, containing some original and refined applications of geo- 

 metry. He treats of the refraction of light and the cause of twilight. 

 As an explanation of the well-known fact of the sun and moon ap- 

 pearing (for it is an appearance only) much larger when near the 

 horizon than when high up in the sky, Alhazen suggests that our judg- 

 ment is the result of a comparison made between the terrestrial objects 

 such as trees, buildings, etc., which are visible on the horizon near the 

 luminary ; but when the latter is high up in the sky we think it looks 

 smaller because we unconsciously judge it to be more distant. 



Astrology was not the only pretended science sedulously cultivated 

 by the Arabians. They were great students of alchemy, a science 

 which professed to investigate methods of converting or transmuting 

 common metals into silver or gold. Innumerable trials and experi- 

 ments made with a view to this illusory object were the means by 

 which many entirely new and valuable truths of chemistry were first 

 acquired. It is by the Arabians that we first find recorded some of 

 the most important discoveries in the domain of chemistry, and our 

 history of this branch of knowledge may not improperly commence 

 with the name of the Arabian alchemist GEBER X^S-Qo), who must not 

 be confounded with the mathematician of the same nanie already 

 mentioned. Of course a vast number of facts regarding the reduction 

 of metals, glass-making, preparation of drugs, etc., had been known 

 and acted upon long before chemistry as a science had any existence. 

 But it is only when men have learnt to read the laws of a science in 

 such processes, or in such facts, that these can be considered as be- 

 longing to the science in question. 



Geber was not only a famous alchemist, but he has a fair claim 

 to be considered the founder of chemistry. Little appears to be known 

 wifR certainty as to the place of his birth, his life, or the precise epoch 

 at which he flourished. His name, and a few of his numerous writings, 

 are all that have reached our times. He was undoubtedly the author 

 of important discoveries in chemistry, and he was appealed to as an 

 authority for centuries afterwards by those who cultivated this branch 

 of science. Geber is the first chemist who described a method of 

 preparing nitric arid, a powerful reagent which is now in constant 

 employment in every chemical laboratory. In his works the first in- 

 dications are met with of some knowledge of the part played by gases 

 in chemical phenomena. Geber also describes certain operations 

 which since his time have been everywhere employed by alchemists 

 and chemists. Such operations are dissolving metals in acids, filtering, 

 evaporating, crystallizing, and distilling; and the chemist of to-day 

 employs these more frequently* than any other processes. What is 

 now called crystallization was indeed then termed coagulation, which 

 word was used in a wider signification than with us, and included all 

 cases in which by any process a liquid was converted into a solid. 



