44 SCIENCE OF THE MIDDLE AGES. PT. n. 



You can distil vapours from solid things as well as from 

 liquids : if you heat sugar over a fire, it will soon boil, and a 

 vapour will rise up from it. 



But if you put a piece of camphor in a flask with a 

 stopper to it, and heat it very gently either by placing it in 

 the sun or at some distance above a lighted candle, the cam- 

 phor will gradually disappear from the bottom of the flask, 

 and will collect again in little crystals on the inside of the 

 neck. This is because camphor at an ordinary heat changes 

 at once into a dry invisible gas, without first remaining 

 liquid for a time as ice does. The process by which sub- 

 stances are turned directly from a solid state into a dry gas 

 is called sublimation^ and Geber describes it in his book as 

 ' the elevation of dry things by fire.' He knew that if you 

 take a kind of stone called cinnabar, and heat it, a dry gas 

 rises from it, which you can collect, and which cools down 

 into drops of mercury or quicksilver. 



Geber made another remarkable experiment, though he 

 did not thoroughly understand it. He states in his book 

 that if you take a certain weight, say a pound of iron, lead, 

 or copper, and heat it in an open vessel, the metal will weigh 

 more after it has been heated than it did before, which 

 seems very strange, as we cannot see that anything has been 

 added to it. We shall learn the reason of this when we come 

 to the discoveries of Priestley (Chap. XXVII.) ; but Geber 

 carefully noticed the fact, though he could not explain it. 



The discovery, however, which most of all gives Geber 

 the right to be called the ' founder of chemistry ' was that 

 of strong acids. Most of the chemical experiments we 

 make now would be impossible without acids, but before 

 Geber's time vinegar seems to have been the strongest acid 

 known. He found, however, that by heating copperas (or 

 sulphate of iron) with saltpetre and alum, he could distil off 



