BORACIC LAGOONS OF TUSCANY. 145 



whence it is collected by the inhabitants. In 

 Italy they are also common ; and the city of 

 Milan is illuminated with the product of such 

 springs. The surface of the sea near the Cape 

 de Verde Islands has been occasionally seen 

 covered with a film of mineral oil, which had 

 probably exuded from the bed beneath. 



At the foot of volcanic mountains we have 

 often indications of great changes taking place 

 within the earth's crust in the appearance of 

 different sorts of springs. Frequently the 

 waters of some are quite sour, being charged 

 with sulphuric acid, and sometimes are of the 

 colour of yellowish milk, from the presence 

 of the powder of sulphur, abundantly diffused 

 through the waters. Sometimes, also, there 

 are springs, the waters of which contain a 

 quantity of dissolved silex, which petrifies the 

 objects upon which the spray falls. All our 

 chalybeate, sulphureous, and other medicinal 

 waters, indicate the occurrence of constant che- 

 mical changes in the earth's crust. 



In further illustration of the subject of this 

 chapter, an instance of a highly interesting 

 nature may be selected, which gives us a most 

 lively and pleasing picture of the importance 

 to man of several of the chemical phenomena 

 occurring in regions to which he has no pos- 

 sibility of access. The substance commonly 



L 



