DISTILLATION 



117 



is condensed again into water in the cooled flask. If some sand and 

 salt were first added to the water in the retort they would be left behind, 

 and the water found in the flask would neither taste of the salt nor be 

 coloured by the sand ; it would be purified from these by distillation. 

 So that impurities which a liquid contains are left behind in the retort, 

 and the liquid obtained by condensing the vapour is pure. 



Liebig's condenser. Many devices have been invented for the 

 more perfect condensation of the vapour during distillation. Most 

 of them consist of some arrangement for cooling a tube through 



FIG. 89. A large metal " still." The vapour of the liquid boiling in B passes 

 through the neck A and the spiral tube in D. The spiral is surrounded with cold 

 water, so the vapour in it is condensed, and trickles in drops of liquid into O. 



which the vapour passes, by surrounding it with cold water. This 

 is the principle of the Liebig's condenser, Fig. 88. It can be used for 

 the separation of a mixture of alcohol and water, advantage being 

 taken of the difference in the boiling points of the two liquids. 



Differences between distilled and ordinary water. It has already 

 been learnt that the water which is found naturally in rivers, lakes, 

 and especially in the sea, contains a large number of substances in 

 solution which it has at some time in its history dissolved out of the 

 earth. When any natural water is heated in a distilling fla?k it is only 

 pure water which is converted into vapour ; the solids dissolved are 

 left behind. When all the water has been thus changed into vapour 

 the solids are left behind on the inside of the flask in the form of a residue. 

 This residue, consisting of substances which were dissolved in water, 

 forms the crust found inside a kettle or boiler which has been used for a 

 long time. The great difference, then, between distilled and ordinary 

 water is that the former contains no dissolved solids. Rain water is 

 really water which has been naturally distilled from seas and other 

 large quantities of water on the earth. 



