38 MINERALOGY. 



phate of lime, the acid taking the lime to itself, and set- 

 ting free the carbonic acid gas. See 60, Part II. 



61. Sulphates of Magnesia and Soda. The slender, 

 spicula-like crystals of Epsom salts (sulphate of magne- 

 sia), which are probably familiar to you, are prisms. Mi- 

 nute crystals of this mineral are often present in the 

 earth on the floors of the limestone caves in the western 

 part of this country. In the Mammoth Cave of Ken- 

 tucky feathery masses of the crystals adhere to the roof, 

 looking like snow-balls. The common name of this min- 

 eral came from the fact that at Epsom, in England, the 

 waters of the springs hold it in solution. The crystals 

 of Glauber's salt are also prisms, but they are coarse 

 compared with those of Epsom salts. Both of these 

 salts are present in sea-water. 



62. Sulphate of Baryta. This is so heavy a mineral 

 that it is called heavy spar. The crystals, which are 

 translucent, sometimes transparent, are often very beau- 

 tiful. It is found abundantly in some localities in this 

 country, as Cheshire, Conn. ; Pillar Point, N". Y. ; near 

 Fredericksburg, Va. It is extensively ground up for use 

 in paints, being mixed with white lead. The mixture 

 has various names, according to the proportions of the 

 various ingredients: Venice White, when there are equal 

 parts of the two ; Hamburg White^ when the lead is half 

 the weight of the sulphate of baryta ; and Dutch White, 

 when it is one third. 



63. Sulphuric and Sulphurous Acids. That intense 

 acid, sulphuric acid, or oil of vitriol, as it is commonly 

 called, is occasionally found near volcanoes and sulphur 

 springs. Sulphurous acid, the pungent and suffocating 

 gas which is produced whenever sulphur is burned, is 

 often very abundant about volcanoes when they are in 

 action. 



64. Sulphureted Hydrogen. This very offensive gas is 

 common at sulphur springs, and it is by its agency that 

 articles of silver are so readily blackened in such locali- 



