116 JUNIOR GRADE SCIENCE 



drops, trickles down the glass and collects on the window frame as liquid 

 water. The water must evidently come from the air in the room. 



What causes condensation ? The air outside a room is in winter 

 much colder than that inside. This cools the glass of the windows 

 very much, and consequently the air next to the cold surface itself 

 becomes cooled and then cannot hold as much vapour as when it is 

 warm, and some of the vapour which can no longer be held by the air 

 is changed into water. So that condensation is caused when air con- 

 taining water vapour is cooled. 



FlO. 88. Distillation of a mixture of alcohol and water. 



Distillation. The change of liquids into vapour by heating them, 

 and the condensation of the vapoui into liquid by cooling it, is employed 

 in an important process called distillation. This plan is frequently 

 made use of for purifying water and other liquids. Perhaps the most 

 useful application of distillation is to obtain fresh water for drinking 

 purposes from sea water or other water not good to drink. Large 

 ships, carrying as they often do more than a thousand people, cannot 

 take enough fresh water on board for the needs, throughout a long 

 voyage, of so many persons. Instead of attempting this difficult task, 

 it is the custom to change sea water into fresh water by distillation. 

 There is no difficulty in understanding this process after the way in 

 which it is done on a small scale in laboratories has been described. 

 Ordinary water may be boiled in a flask or retort, the neck of which 

 fits into a flask continually kept cold by pouring water upon it, as in 

 Fig. 86. As the water boils it is converted into steam, and this steam 



