372 Prof. Bischof on the Natural History of 



effects a decomposition and solution of the stone, and hence 

 arises an acidulous spring, rich in carbonic acid and carbo- 

 nates. The deeper the meteoric water penetrates, the warmer 

 it becomes. Rising springs of water are then produced in 

 this cleft, through which the concentrated mineral water form- 

 ed beneath at c, rises to h. If here the direction of the 

 slaty or stratified surface (Schieferungs oder Schichtungs 

 Flache) leads down to d, which either has an immediate exit 

 in the section of the valley a d e^ or runs at a slight depth 

 below the surface, then the mineral spring will issue, owing to 

 the pressure of the column of water a h. While the rising 

 streams of warm water take the course c h d, the originally 

 concentrated mineral water becomes diluted by the fresh water 

 flowing down from above ; the carbonic acid gas, absorbed in 

 great quantity beneath, is gradually disengaged as the water 

 rises, and consequently the hydrostatic pressure is diminished, 

 and thus free carbonic acid gas is evolved at d with the acidu- 

 lous spring. It is clear, that the carbonic acid gas, which is 

 constantly disengaged from the rising water during its whole 

 course, not only moves on with the water on the surface of the 

 stratum h d, but fills all the intervals of the clefts in the whole 

 clay-slate rocks, so that the gas will be evolved wherever these 

 clefts are open at the surface. If these fissures open above the 

 bottom of the valley, and therefore are not filled with water, at 

 least not up to the opening, then the gas will escape from them 

 with a hissing noise. If, on the other hand, they open from 

 beneath the bottom of the valley, and are therefore filled with 

 water, then the gas will escape bubbling through the water, 

 and present entirely the appearance of a mineral spring. 

 If, lastly, these fissures be covered by alluvium, which, never- 

 theless, does not form an air-tight covering, then the gas will 

 escape silently from the ground, and such places are recognised 

 from the scanty vegetation which exists there. I know but 

 one of the first description of fissures in that district, which is 

 found close to the first mineral spring called Fehlenbor, in the 

 valley of Bw'gbrohl, between Tonnisstem and Buj-ghrohL Such 

 a fissure is found also in the Eifel^ in the Brudeldreis, as it is 

 called, not far from Bireshorn. Fissures filled with water, 

 from which gas is evolved, are tolerably numerous, as, for ex- 

 ample, in tlie valley of Burgbrohl, I formerly considered these 



