PROCEEDINGS OF THE COTTESWOLD CLUB 279 



of pressure in the flask, due to the increased tension of the confined 

 vapour ; and the boiling is checked, though the temperature of the 

 water rises several degrees above its previous boiling point. The lamp 

 is taken away, and the formation of steam-bubbles soon ceases. The 

 stopcock is then opened ; steam escapes, the pressure within the flask 

 is lessened, and the water again begins to boil violently. 



I have thus shown that water, though at a temperature 

 much above that at which it boils under ordinary 

 conditions, is prevented from boihng by increasing the 

 pressure upon it, and that, if the pressure is lessened 

 without alteration of the temperature, violent boiling 

 commences at once, and the temperature sinks (owing to 

 the heat-energy abstracted in the formation of vapour) 

 down to the boiling point corresponding to the reduced 

 pressure. 



We are now in a position to understand the mechanism 

 of a geysir. Far below the surface of the earth, at a 

 depth of 100 feet or more, there is a cave or reservoir 

 into which water is continually flowing; the surface 

 drainage making its way downwards through fissures in 

 the rocks. One such fissure, however, serves, not so 

 much to convey water down, as to form a free communi- 

 cation between the reservoir and the external air; and this, 

 as water accumulates in the reservoir, becomes filled with 

 a column of water reaching nearly or quite to the surface. 

 The water in the reservoir, from its contact with intenselv 

 heated rocks, soon rises in temperature to what would be 

 its boiling point in the open air ; but it is prevented from 

 actually boiling by the pressure of the column of water in 

 the outlet-pipe, which acts in addition to the atmospheric 

 pressure. Still the communication of heat goes on, 

 until the temperature of the water in the reservoir and 

 pipe reaches the boiling point corresponding to the 

 pressure existing at the lower end of the pipe. Then a 

 great bubble of steam forms there, and heaves up most 

 or all of the column of water in the pipe, throwing it out 



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