MANUAL OF NATURAL HISTORY. 595 



In the following arrangement the letter H signi- 

 fies hardness, and the specific gravity is denoted by 



sp. gr. 



I. ORDER HYDKOGENEXIDES. 



Hydrogen, either pure or combined with other 

 substances ; liquid or gaseous. 

 1. FAMILY. Hydrogenides. Characters as in 

 Order. Occur as 1, Pure Hydrogen ; gaseous, 

 colourless, inflammable, sp. gr. 0*0694; escapes 

 in volcanic districts, from coal beds and stag- 

 nant pools ; 2, as Sulphuretted Hydrogen, 

 or "Hydro-sulphuric acid m " gaseous, sp. gr. 

 1*177, smelling like rotten eggs, contained in 

 mineral springs or issuing in volcanic vicini- 

 ties ; 3, as Water: liquid, sp. gr. 1., limpid, 

 colourless, inodorous, tasteless when pure, or 

 combined with various substances, as salts of 

 sodium and magnesium in the sea, or with 

 other compounds in mineral springs, which 

 are variously denominated according to the 

 temperature and the prevalent ingredient as 

 cold or thermal, acidulous, alkaline, saline, 

 sulphureous, siliceous, calcareous, or chaly- 

 beate ; 4, as Hydrochloric or Muriatic acid, 

 gaseous, sp. gr. 1*284, colourless, pungent, 

 taste acid ; escaping from volcanic crevices ; 

 5, as Phosphuretted - Hydrogen, gaseous, 

 sp.gr. 1*761, odour alliaceous, taste bitter, 

 often inflaming on coming in contact with 

 atmospheric air, given off in churchyards 

 and marshes, and popularly known as " Jack 



