244 66 



On the basis of the preceding remarks we can sum up tlie conditions of the 

 periodic eruptions as follows. 



A mixture of hot water and spring gases saturated with vapour is conveyed 

 through the spring channel, which we must assume to be situated in solid strata, 

 and to be well defined, at least in the upper part, i. e. near the surface of the earth. 



If the gas bubbles, while a state of equilibrium rules, do not cause the water 

 pressure to decrease on their way up the channel, we have then a continually 

 ebullient spring. 



If, on the other hand, the gas bubbles, while the spring is in a state of 

 equilibrium, would produce a decrease in the pi-essure, especially by filling out 

 parts of the channel, the spring is periodic. 



In the latter instance, the surface tension, and adhesion, will be of considerable 

 consequence, especially in regard to the size of the gas bubbles. 



The constantly boiling springs are therefore hot springs with a fixed equili- 

 brium, while the periodic springs have no fixed equilibrium. I understand that a 

 spring has attained its state of equilibrium, when the flow of water, steam and 

 spring gases, passing through the channels of the spring, becomes constant, and 

 the temperature in all parts of the channels has also become constant. 



We have assumed that water is conveyed through the spring channels, for 

 that is a necessary condition of the periodic springs. Where no water is to be 

 found in the spring channel, an eruption can hardly take place. But it is suffi- 

 cient if there is water in the upper part of the spring channel. In this connection 

 it matters little if there is only steam farther down the channel, for the cause of 

 the eruption is only to be sought in the parts of the channel where there is water. 



In the theory of the periodicity of the hot springs, it is immaterial whether 

 we assume that the heat energy is supplied by steam or by highly heated water 

 which, on account of the great pressure to which it is subjected, cannot boil and 

 evaporate. But, in my opinion, the heat energy is most probably supplied by 

 highly heated steam and spring gas. Direct proofs of this are not forthcoming, 

 but there are many things that seem to indicate that it is correct: amongst others, 

 that I have never been able to detect any radium worth mentioning in the sedi- 

 ments or mud of the springs; and since radium emanation is always found in the 

 exhalations of the springs, either the spring water or the steam must have been in 

 contact with radium. If it were the spring water, it would dissolve a little radium, 

 and carry it in this manner up to the earth's surface, where it ought to be found 

 in the deposits of the springs, as is the case with many springs in Italy and 

 France. 



But this is not the case with any of the hot springs I have investigated in 

 Iceland. This is most probably on account of the spring water having, somewhere 

 down under the earth's surface, passed into a state of vapour, and thus being 

 hindered from conveying any ladium. On the other hand the vapour must con- 



