106 Krug von Nidda on the Mineral Springs of Iceland. 



considerable quantity. The proofs of the existence of such a 

 combination of silica and soda, or of the saturation of the one 

 by the other, are, that the original water of the Geyser does not 

 perceptibly affect turmeric paper, although the alkali is contain- 

 ed in it in such abundance, and that, when the silica is separated 

 and the soluble salts diluted *with distilled water in the same 

 degree as in the original water, the turmeric paper is strongly 

 coloured. 



Black's opinion of the existence of the soda in a caustic con- 

 dition is therefore confirmed by these experiments of Faraday. 



But even though Klaproth's view should prove to be correct, 

 viz. that the soda exists as a carbonate in the water of the Geyser, 

 yet the quantity of carbonic acid would still be only sufficient to 

 neutralize the soda, and the remarkable phenomenon would still 

 remain, of the presence, in such minute quantity, of an acid which, 

 in other springs, is the prevailing one, and is the solvent of the 

 lime and magnesia. 



The solubility of the silica in such considerable quantity in 

 the hot springs of Iceland, remained for a long time a puzzling 

 phenomenon, until that property of silica was discovered which 

 it has in common with phosphoric acid, viz. of forming two 

 isomeric modifications, of which one possesses the property of 

 being insoluble in the moist way, and its combinations are fre- 

 quently insoluble in the strongest acids. The other modification 

 is, on the contrary, distinguished by the property of being solu- 

 ble in acids, and even in considerable quantity in water. After- 

 wards, a larger or smaller quantity of silica was discovered in 

 all mineral waters that were submitted to an accurate analysis ; 

 and it was to the alkalies which, combined with sulphuric, mu- 

 riatic, or carbonic acids, are abundantly contained in the mineral 

 waters, that the power was ascribed of converting the silica from 

 one modification to the other : the presence of the silica and the 

 alkalies was considered as the consequence of a dissolving pro- 

 cess produced by the action of the mineral waters on the rocks 

 during their passage to the surface. Experiments in our labo- 

 ratories do not seem to contradict the opinion, for if we boil 

 finely powdered silica with a solution of carbonate of soda or 

 potash, it is gradually converted into the second modification and 

 is dissolved. We remark a similar dissolving process in the wea- 



