158 



HEAT. 



place from the surface of ice a fact which is familiar to us in the 

 disappearance of snow in a dry east wind, though the temperature does 

 not rise to the melting point. Evaporation also 

 absorbs heat, a fact which may be easily illustrated 

 by pouring a few drops of ether on to the hand, 

 when the cooling of the skin as the ether evaporates 

 shows that heat is being rapidly absorbed. 



In studying change of state in detail, we shall 

 consider first the change into the gaseous condition 

 and the converse, because, with the aid of the kinetic 

 theory of gases, we are able to give a more com- 

 plete account of it than is as yet possible in the 

 other cases. 



Liquid-Gas Change. Let us suppose that an 

 ordinary barometer-tube, held with the closed end 

 downwards, is nearly filled with mercury, the re- 

 maining space being filled with water. On invert- 

 ing the tube in the ordinary way, and unclosing the 

 end in a cistern of mercury, the water floats up to 

 the top, and some of it evaporates into the Torri- 

 barometer; EandG, cellian vacuum. Allowing for the pressure of the 

 barometers with small small quantity of water, and comparing the height 

 quantities of water of the column thus corrected with that of a baro- 

 above the mercury. meter made with dry mercury, it is found to be 

 somewhat less, or a certain pressure is exerted on 

 the top of the water Testing on the mercury column. 

 This pressure is due to the water-gas, or, as it is usually 

 termed under such circumstances, the water - vapour, 

 present in the space which would otherwise be a Torri- 

 cellian vacuum. If the temperature is maintained 

 constant, and if there is sufficient water, this pressure 

 rapidly attains a definite value, quite independent of the 

 volume of the tube above the mercury. This may easily 

 be shown by raising this tube or depressing it in the 

 cistern, or by tilting it, or by using different tubes. 

 Thus in Fig. 84 A is an ordinary barometer, B and 

 other barometers, each with a small quantity of water at 

 the top. The height of the columns in B and is the 

 same, and less than that in A. 



In an ordinary tube, the evaporation is very rapid, 

 so that the steady pressure of the vapour is soon attained. 

 In this state the space above the mercury is said to be 

 saturated with vapour, and the pressure exerted is termed 

 the " maximum vapour-pressure " or " vapour-tension " for 

 the given temperature ; maximum, because any decrease 

 in the volume only leads to condensation of the vapour, 

 not to increase in its pressure. 



Use of the Terms " Gas " and " Vapour." In 



ordinary language a gas is usually described as vapour 

 when thought of as given off by a liquid or solid, 

 especially when it is not far from the maximum pressure at which it 



. Baro- 

 meter with 

 Water above 

 the Mercury 

 and surround- 

 ed by a Heat- 

 ing Bath. 



