SATURATED VAPOUR. 807 



If the water-hammer is held in the position shown in 

 fig. 397, and bulb and tube are alternately inclined down- 

 wards, the water will flow from, one side to the other, so 

 as to be always at the same level in both parts of the 

 apparatus. The pressure of the steam which fills the 

 spaces above the liquid must thus always be equal on 

 both sides, whether the spaces or volume occupied by 

 steam become larger or smaller, provided the tempera- 

 ture is on both sides the same. Vapours show this 

 behaviour only when they are not mixed with air and 

 when they are in contact with their liquid. In a 

 vacuum a liquid forms vapour instantaneously, but for 

 any given temperature there is a limit to the quantity 

 of vapour which can be formed in a given space ; when 

 this limit is reached the space is said to be saturated. 

 When the space is diminished a portion of vapour cor- 

 responding to the diminution of volume returns to the 

 liquid state; when, on the other hand, the space is 

 increased, a portion of the liquid present vaporises, 

 and the space occupied by the vapour is again satu- 

 rated. 



If the space contains air, the same quantity of vapour 

 may be formed in it as is formed in an equal space, free 

 from air and at the same temperature, if a sufficient 

 quantity of liquid be present; but the formation of 

 vapour is not instantaneous, as it is in a vacuum; it 

 proceeds much more slowly, and the saturation of an 

 enclosed space containing air may require several 

 hours. 



The vapour in the water-hammer always saturates 

 the space in which it is contained. Very different is 

 the behaviour of vapour of which there is less in a given 



