Royal Institution. 155 



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 thirty 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 atmosphere 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 

 shown 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. 



Sir Geo. Mackenzie it is well known was the first to introduce the 

 idea of a subterranean cavern to account for the phenomena of the 

 Geiser. His hypothesis met with general acceptance, and was even 

 adopted undoubtingly by some of those who accompanied Bunsen to 

 Iceland. It is unnecessary to introduce the solid objections, which 

 might be urged against this hypothesis, for the tube being proved 

 sufficient, the hypothetical cavern disappears with the necessity 

 which gave it birth. 



From the central portions of the Geiser tube downwards, the 

 water has stored up an amount of heat capable, when liberated, of 



