44 SCIENCE OF THE MIDDLE AGES. pt. ii. 



brandy gently, a vapour called alcohol or spirits of wine will 

 rise up, because the alcohol turns into vapour more easily 

 than the other materials of the wine. If you collect and cool 

 down this vapour in another bottle, you will have the liquid 

 spirits- of- wine. This process is called distillation^ and is 

 used by chemists to separate substances which turn readily 

 into vapour from others. which do not boil so easily. 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 collect in little crystals on the inside of the neck. This 

 is because camphor at an ordinary heat changes straight into 

 a dry invisible gas, without first becoming liquid as ice does. 

 The process by which substances are turned directly from 

 a solid state into a dry gas is called sublitnation, and Gefer 

 describes it in his book as ' the elevation of dry things by 

 fire.' He knew that i( 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 



I 



