334 NOTICES OF THE MEETINGS [June 3, 
becomes condensed and produces explosions similar to those pro- 
duced on a small scale. when a flask of water is heated to boiling. 
Between the interval of two eruptions, the temperature of the water 
in the tube towards the centre and bottom gradually increases. 
Bunsen succeeded in determining its temperature a few minutes 
before a great eruption took place; and these observations furnished 
to his clear intellect the key of the entire enigma. A little below 
the centre the water was within two degrees of its boiling point, 
that is within two degrees of the point at which water boils under 
a pressure equal to that of an atmosphere, plus the pressure of the 
superincumbent column of water. The actual temperature at thirty 
feet above the bottom was 122° centigrade, its boiling point here is 
124°. We have just alluded to the detonations and the lifting of the 
Geiser column by the entrance of steam from beneath. These de- 
tonations and the accompanying elevation of the column are, as before 
stated, heard and observed at various intervals before an eruption. 
During these intervals the temperature of the water is gradually 
rising ; let us see what must take place when its temperature is near 
the boiling point. Imagine the section of water at 30 feet above 
the bottom to be raised six feet by the generation of a mass of 
vapour below. The liquid spreads out in the basin, overflows its rim, 
and thus the elevated section has six feet less of water pressure upon 
it ; its boiling point under this diminished pressure is 121°; hence in 
its new position, its actual temperature (122°) is a degree above the 
boiling point. This excess is at once applied to the generation of steam ; 
the column is lifted higher, and its pressure further lessened; more 
steam is developed underneath; and thus, after a few convulsive 
efforts, the water is ejected with immense velocity, and we have the 
Geiser eruption in all its grandeur. By its contact with the atmos- 
phere the water is cooled, falls back into the basin, sinks into the 
tube through which it gradually rises again, and finally fills the basin. 
The detonations are heard at intervals, and ebullitions observed ; 
but not until the temperature of the water in the tube has once 
more nearly attained its boiling point is the lifting of the column 
able to produce an eruption. 
In the regularly formed tube the water nowhere quite attains the 
boiling point. In the canals which feed the tube, the steam which 
causes the detonation and lifting of the column must therefore be 
formed. These canals are in fact nothing more than the irregular 
continuation of the tube itself. The tube is therefore the sole and 
sufficient cause of the eruptions. Its sufficiency was experimentally 
shewn during the lecture. A tube of galvanized iron six feet long 
was surmounted by a basin; a fire was placed underneath and one 
near its centre to imitate the lateral heating of the Geiser tube. 
At intervals of five or six minutes, throughout the lecture, eruptions 
took place ; the water was discharged into the atmosphere, fell back 
into the basin, filled the tube, became heated again, and was dis- 
charged as before. 
