160 



HEAT. 



in a given space when the steady state is attained, is still very nearly 

 the same, though it appears to be always slightly less than in a space 

 containing only the vapour. The practical equality may be shown by 

 an arrangement similar to that in Fig. 86. 



A is a piece of wide glass-tubing, several inches long, closed at its 

 upper end by a tap, above which is a small funnel. A is connected to 

 the open tube B by a piece of strong flexible tubing. The tap t is 

 opened, and mercury is poured into B until it reaches a certain mark in 

 A, standing, of course, at the same level in B. The tap is then closed, 

 and ether is poured into the funnel. B is lowered so as to reduce the 

 pressure in A, and when t is opened, a small quantity 

 of the liquid is pressed into A without the escape of 

 r~~1 any air ; t is now closed, and when the steady state is 

 $^\t reached an excess of liquid being still present B is 

 raised till the level of the mercury in A is at the original 

 mark. The air occupying its original volume, still exerts 

 the atmospheric pressure. The difference of levels of 

 the mercury in the two tubes is, therefore, due to the 

 pressure of the ether-vapour, and this is found to be 

 equal to its value when the air is absent. The same 

 arrangement may easily be used to find this value by 

 opening t and raising B till all the air is driven out of 

 A, a small quantity of liquid being still left in the tube 

 above the mercury and below the tap. If we close t and 

 lower B, then as soon as the surface in B is a depth 

 below that in A equal to the difference between the 

 atmospheric pressure and the pressure of ether vapour, 

 evaporation takes place. When this begins, even if B 

 is slowly lowered still further, A also falls so that the 

 difference between the levels in A and B will remain 

 constant. 



This evaporation into % space already containing air 



R6 E is, of course, continually occurring from water on the 



ration into an surface of the earth. The pressure of the vapour present 

 Air Space. in the air, in fine weather, is always less, below the cloud 

 level, than the maximum for the existing temperature. 

 Evaporation, therefore, takes place from the surface of any water, or from 

 any damp material present, and tends to bring about saturation. But fresh 

 drier air is continually being brought over the water or damp material. 

 This may take place by winds, or it may be brought about by convection, 

 for the water-vapour is lighter than the air it replaces, and the vapour- 

 charged air tends to rise, colder air from above, containing less vapour, 

 taking its place. The evaporation, therefore, continues, but probably 

 owing to continual renewals of the air, the maximum pressure is never 

 produced by evaporation from the surface of the earth alone. 



As a converse to evaporation, we continually have condensation 

 occurring in the atmosphere in the formation of clouds and fogs. These 

 are, probably, always produced by cooling. A mass of air containing 

 water-vapour, not very far from the maximum pressure, becomes cooled, 

 and normally as soon as the temperature falls to that for which the 

 maximum pressure of water- vapouris equal to the existing pressure, conden- 

 sation begins in the form of cloud> which is well described as water-dust. 



