364 ELEMENTARY GEOLOGY 
in certain cases, it is a lava mtrusion that has not 
quite reached the surface. In a geyser there is appar- 
ently a narrow tube extending into the earth; and in 
this the water is heated in one part more than else- 
where. If the tube were broad enough, this would 
merely produce boiling; but being of small compass, 
the circulation of water and 
steam is prevented, and _ so 
energy is gathered for an ex- 
plosion. Geysers may be made 
artificially by heating water im 
a long glass tube until the steam 
causes an explosion. 
At the surface of the ground, the 
boiling-point of water is about 212°; 
but the pressure of a column of 
7 Fia. 222. water raises the boiling-point, and 
Diagram to illustrate the heat- so, at a depth of some feet below 
Ane, (Oy. Senet, Ae 2. tis surface, the boiling-point in the 
mass) of the water in a gey- ’ ‘ fi 
son tule. geyser tube is higher than 212°. 
Suppose that at this place there is a 
constant supply of heat, which raises the temperature of the 
water so high that the boiling-point for that particular depth 
is reached. The expansion of the steam generated in the tube, 
then raises the water column and causes it to overflow. By 
this means some of the water is removed, and therefore the 
pressure upon the point which we have been considering 1s 
slightly lessened. This release immediately lowers the boiling- 
point nearer 212°; and so the water, already heated above this 
point, changes immediately to steam, causing an eruption. 
