144 CHEMISTRY OF NATURAL WATERS. [IX. 



we may conjecture that it was in former times abstracted from 

 the sea, either directly or through the intervention of organic 

 bodies (as in the case of potash, which is separated and fixed 

 by means of algae, 5). Experiments after the manner of those 

 of Way and Voelcker may throw light upon this interesting 

 question. We are aware that insoluble combinations of solu- 

 ble chlorides with silicates of alumina are found under certain 

 conditions, as appears in sodalite, eudialyte, and the chlorifer- 

 ous micas, and it is not improbable that the soluble iodides 

 may give rise to similar compounds. By such a process might 

 be explained the rarity of this element in modern seas, while 

 the occasional re-solution of the iodine from these insoluble 

 compounds by infiltrating waters would help to explain the 

 variable and often large proportions in which this element is 

 met with in some of the waters noticed above. 



61. SULPHATES. In the preceding sections we have already 

 discussed the principal facts in the history of those neutral 

 waters in which sulphates predominate, or prevail to the ex- 

 clusion of chlorides ( 50, 51). The history and the probable 

 origin of those curious springs which contain free sulphuric 

 acid has also been considered ( 31, 48, 49) ; and it now re- 

 mains to notice the relation of sulphates to the muriated waters. 

 The first fact that excites our attention is that of the total 

 absence of sulphates from numerous springs of the first, sec- 

 ond, and third classes, as shown in the preceding analyses, and 

 also in the observations of Lenny and others on the saline 

 waters over a great area in western Pennsylvania ( 40). 



The elimination of sulphate in the form of gypsum from 

 evaporating waters containing an excess of chloride of calcium 

 has already been discussed in 37 ; but the bitterns resulting 

 from such a process still retain small portions of sulphates; 

 while it is to be remarked that the saline waters under consid- 

 eration contain no traces of sulphates, and in many instances 

 hold portions of baryta and strontia, bases incompatible with 

 the presence of sulphates. The modes in which this complete 

 elimination of sulphates may be effected are two in number. 

 The first has already been suggested in 10, and depends upon 



