CHANGE OF STATE LIQUID VAPOUR. 



159 



begins to condense. We shall see later that there is probably for every 

 gas a temperature below which it can always be condensed to liquid by 

 sufficient pressure, and above which such condensation is impossible. It 

 has been suggested that the gas should be described as a vapour below 

 this temperature, and as a gas above it. This restriction of the term 

 " vapour " is in accordance with general usage, but it is probably more 

 convenient to use the term " gas " in the general sense, including "vapour" 

 as a particular case. 



If the barometer-tube B of Fig. 84, containing the water be sur- 

 rounded with a water-bath, as in Fig. 85, so that its temperature can 

 be raised, then it is found that the pressure of the vapour rises with the 

 temperature, more and more of the water evaporating into the space 

 above ; and at each temperature there is a definite maximum pressure, 

 which is more or less rapidly attained. 



In the case of water, the following are some of the values of the 

 maximum pressure. "We shall describe later how the accurate values 

 are obtained. 



For other substances the values are very different. 



If the space into which the evaporation takes place contains air or 

 any other gas the evaporation goes on, but at a diminished rate. The 

 pressure exercised by the vapour and, therefore, the total vapour present 



