THE GEOLOGY OP MINEEAL SPEINGS. 
213 
the convulsive ebullition which takes place in a deep and narrow 
test-tube. The intermittence of other springs is due to other 
causes. Sometimes it is produced by the irregular evolution 
of carbonic acid gas in the waters of the spring, as in those 
of Nauheim, Kissingen, and Carlsbad; at others it is owing 
to a connection existing between the spring-bearing strata 
and the sea. At the flood-tide, the pressure exerted by the 
sea upon the water in the strata presents an obstacle to its 
exit ; and hence it rises in the springs, whose flow is in- 
creased at these periods. At the ebb this state of things 
is reversed. Such intermittent springs exist at Skoga- 
pordar, in Iceland ; at Noyell, in the department of the Saone ; 
and in Artesian wells at Dieppe and Fulham. Another variety 
of intermittent spring is exhibited in our second illustration, 
where we have a cavity A in the substance of a mountain, 
into which water finds its way by the three orifices h, c, cZ, and 
from which it passes by a siphon, which may have its outlet 
either at g or h. As soon as the water has accumulated in the 
cavity as high as the level of/, it will begin suddenly to flow 
by either g or li, in consequence of the well-known property 
of a siphon ; but directly it has sunk to the level K, the flow 
from g will cease : and when it has fallen below eZ, that from 
h will also discontinue. No flow will occur again from either 
outlet until the water has reaccumulated to the level of /, 
when the same order of events will be repeated. 
As we have already exceeded the limits of the space allotted 
to us, we have no room left to speak of the chemical con- 
stituents of mineral springs, a subject which is full of interest, 
both in a mineralogical and geological point of view. 
