32 MINERALOGY. 



it acquired the name of Seneca oil, which it still retains. 

 Naphtha is a light, limpid fluid of a yellowish color, and 

 is a purer article than petroleum, from which it can be 

 obtained by distillation. It is a hydrocarbon ; that is, 

 it is composed of hydrogen and carbon. As there is no 

 oxygen in its composition, it is used, as you learned in 

 Part II., by the chemist to preserve potassium and so- 

 dium in their metallic state. 



47. Carbonic Acid. The qualities of this gas you 

 learned in Part II., Chap. III. It is constantly furnished 

 to the air from the respiration of animals, from fires, and 

 from the decomposition of vegetable and animal matters. 

 As it is constantly absorbed by the leaves of plants, its 

 undue accumulation in the atmosphere is prevented. 

 There are localities where this gas is produced in the 

 earth in large quantities. In some mineral springs, as at 

 Saratoga, it gives briskness and a slight pungent taste to 

 the waters. This large production of it appears in the 

 neighborhood of some volcanoes. A cavern near Na- 

 ples, the Grotto del Cane, has a world-wide reputation on 

 account of the abundance of this gas in it, which fills the 

 cavern up to the level of the lower margin of its en- 

 trance. This gas has a great agency in relation to one 

 of the most important and extensive of the rocks of the 

 earth, the limestone, which is a combination of this gas 

 with lime. The carbonate of lime of the rocks is contin- 

 ually being dissolved in the water that comes in contact 

 with them ; and then the shell-fish and the coral animals 

 appropriate this to make their shells and skeletons, which 

 eventually become, as you have seen ( 8), limestone 

 rock, thus returning the carbonate of lime to the source 

 from which it came. Now it is the carbonic acid in the 

 water which enables it to dissolve sufficient of the lime- 

 stone to supply the shell-fish and the corals with this ma- 

 terial. For a full elucidation of this interesting point I 

 refer you to Part II., 306. 



48. Carbureted Hydrogen. This, which is the common 



