300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



tion and other color or structural features desirable in stones used 

 for decorative purposes. The collection comprises upwards of 2,600 

 specimens and includes representations of all the leading quarries in 

 the United States and those which are important from abroad. (See 

 pi. 20, fig. 1.) 



The entire exhibition series, metallic and nonmetallic, of economic 

 minerals, numbers upwards of 9,300 individual specimens. 



MINERAL WATERS. 



From a strictly scientific standpoint, any water is a mineral water, 

 since water is itself a mineral — an oxide of hydrogen. Common 

 usage has, however, tended toward the restriction of the name to 

 such waters as carry in solution an appreciable quantity of other 

 mineral matter. Of the various salts held in solution, those of 

 sodium, calcium, and iron are the most common. More rarely, or 

 at least in smaller amounts, occur those of potassium, lithium, mag- 

 nesium, strontium, silicon, etc. The most common of the acids is 

 carbonic, and the next sulphuric. 



According to their temperatures, as they flow from springs, the 

 waters are divided into (A) thermal and (B) nonthermal; a thermal 

 water being one, the mean annual temperature of which is 70° F. or 

 more. Each of these groups is again subdivided according to the 

 character of the acids and their salts, as below: 



Class I. Alkaline. 



Sulpha ted. 



Muriated. 



Sulphated. 



Muriated. 



Sulphated. 



Muriated. 



„„. [Sulphated. 



Siliceous | Muriated 



The classes may be further subdivided according to the predomi- 

 nating salt in solution, as (1) sodic, (2) lithic, (3) potassic, (4) 

 calcic, (5) magnesic, (6) chalybeate, or (7) aluminous. 



Any spring water may be characterized by the presence or ab- 

 sence of gas, when it is designated by one of the following terms : 

 (1) nongaseous, (2) carbonated, (3) sulphureted, (4) azotized, or 

 (5) carbureted. 



The water of springs is meteoric and has acquired its mineral 

 matter by gradually percolating downward through the rocks from 

 which it dissolves a certain amount of matter, the quantity and 

 kind being dependent upon the kind of rocks, the temperature and 

 pressure of the water, as well as the gases which it has absorbed. 



Class II. Alkaline-saline. 

 Class III. Saline 



Class IV. Acid. 



