PREGIPITATIOX IN NATUBE. 435 



o-alena by tlie solvent action of water supersaturated witli sulpliydric acid 

 in closed tubes at high temperatures and pressures, and also obtained ruby 

 silver, both arsenical and antimonial, by heating alkaline sulpharsenites with 

 silver salts dissolved in solutions of acid sodic carbonate at temperatures of 

 from 250'^ to 350°. There is, therefore, nothing strange in the fact that lead 

 and silver accompany the other minerals at Steamboat Springs. 



Natural solutions and precipitations. — The forBgoiug aualyses and experiments 

 show that there is a series of compounds of mercury of the form HgS, «Na-S, 

 one or the other of which is soluble in aqueous solutions of caustic soda, 

 sodic sulphydrate, or sodic sulphide, and apparently also in pure water at 

 various temperatures. These solutions subsist, or subsist to some extent, 

 in the presence of sodic carbonates, borates, and chlorides. There is the 

 strongest e^'idence that the waters of Steamboat Springs contain mercury 

 in this form and that the waters of Sulphur Bank still carry it in solution. 

 Sulphides of iron, gold, and zinc form double sulphides with sodium, which 

 appear to be entirely analogous to those of mercury. Copper also forms a 

 soluble double sulphide, but combines more readily with sodic sulphydrate 

 than with the simple sulphide. All of these soluble sulphosalts may exist 

 in the jjresence of sodic carbonates. 



Mercuric sulphide is readily precipitated from these solutions. Any 

 substance is more soluble in hot solutions than in cold ones, provided that 

 increase of temperature does not resolve the fluid molecules into others 

 which are less soluble, as happens with sodium chloride, neutral sodium car- 

 bonate, etc. Diminishing temperature is thus a cause of precipitation, and 

 diminishing pressure appears to act in a similar way. At Sulphur Bank 

 cinnabar is precipitated at a short distance from the surface, partly at least 

 in consequence of the. action of ammonium salts. There are also other 

 methods of precipitation which may be carried out under natural conditions. 

 If a natural solution of mercury comes in contact with a strong solution of 

 borax, or with sulphydric acid or any stronger acid, it will lose a portion 

 of the mercuric sulphide in solution, and, if the precipitation be a rapid one, 

 the black sulphide will probably be thrown down. At Steamboat Springs 

 and Sulphur Bank large quantities of sulphuric acid are found near the sur- 

 face and, percolating downward, must precipitate mercury. The acid waters 



