CHANGE OF STATE LIQUID VAPOUR. 



161 



Distillation. Condensation is used in the process of distillation to 

 obtain pure water. 



The simplest form of still is one in which the steam rises from a 

 vessel, leaving the impurities behind in the water. The steam is con- 

 veyed through a jacket, through which cold water is kept circulating, so 

 that the temperature of the steam falls below the condensing-point, and 

 the condensed water trickles down into a receiver, as in Fig. 86a. 



We may summarise the results already described in the following 

 statement : 



A liquid with a free surface and a space above it will evaporate, or 

 change into gas, into that space, until a pressure is reached definite for 

 each temperature. This limiting pressure is called the maximum vapour- 

 pressure or vapour- tension. Its value is only very slightly affected 

 by the presence of other gases, though these retard evaporation. It rises 



FlG. 86a. Apparatus for Distillation. A, still ; B, receiver. 



with the temperature, and differs greatly for different liquids at the same 

 temperature. 



It may assist us in arranging these facts in our minds, if we seek 

 to give an explanation of them on the kinetic theory of matter. 



Let us first consider the case of a liquid partially filling a closed 

 vessel, the space above being a vacuum to begin with. 



The molecules in a liquid are, in general, within each other's spheres 

 of action, and are entangled with each other ; but there are a great many 

 of the molecules which have sufficient kinetic energy to escape from the 

 groups which they may be near at a given instant, and these move off to 

 become entangled with other groups. We might, perhaps, represent 

 the liquid at any instant as a network of attached molecules, form- 

 ing a loose solid with numbers of gas molecules moving about in the 

 interspaces. But the members composing the network are continually 

 changing places with the freely moving molecules, so that the network 

 is not permanently solid. If, now, one of the freely moving molecules 

 happens to be close to the surface, and moving from the general body of 

 the liquid, it will entirely escape and move into the space above the 

 liquid, that is, it will evaporate. Escapes will take place all over the 

 surface of the liquid in the same way, and very soon the space above will 

 contain a great number of these molecules, which from their kinetic 



L 



