Cadmium^ and Mercury. 381 



Let us see how these field observations agree with the facts 

 worked out in the laboratory. First, it was found that cinna- 

 bar very readily forms at moderately low temperatures (80°- 

 100°) by the action of dilute alkali sulphide solutions on mer- 

 curic salts, lppen* obtained good crystals at 45° in two 

 months. JSIever under any conditions have we observed meta- 

 cinnabar as a product of alkaline solutions. 



Black crystalline mercuric sulphide was obtained by the slow 

 precipitation of mercuric salts in acid solutions by the action 

 of soluble thiosulphate. The crystals were not measurable and 

 being opaque could not be positively identified. However 

 there were some indications that they were regular and the 

 identity of the two appears probable. The association of natural 

 metacinnabar with marcasite in the deposits of the western 

 United States is important in this connection because both are 

 regarded by geologists as characteristic secondary minerals, i. e. , 

 they were precipitated directly or indirectly by the action of 

 hydrogen sulphide from solutions which had been formed by the 

 oxidation of sulphides near the surface. Such waters would of 

 course contain the metals in the form of sulphates, and also 

 free sulphuric acid if pyrite or any other sulphide of that 

 chemical type were one of the minerals oxidized. 



Marcasite has already been formed synthetically from such 

 a solution, f 



Chemical and geological relations of deep-sealed and surface 

 waters. 



The difference in chemical character between "ascending" 

 and "descending" natural waters is well known. As previ- 

 ously stated, our knowledge of the composition of hot springs, 

 and the chemical behavior of common minerals with hot water, 

 lead to the conclusion that the former class must be generally 

 alkaline. Chlorides, bicarbonates and sulphides, especially of 

 the alkali metals, are the characteristic constituents. On the 

 other hand, surface waters in the vicinity of sulphides naturally 

 contain sulphates as oxidation products and are generally acid 

 on account of the frequency of pyrite and marcasite. 



Now it is a remarkable fact that the crystalline form of a 

 number of minerals is determined by the acid or alkaline 

 nature of the solutions from which they crystallize. We find 

 three pairs of minerals, pyrite and marcasite (FeS,), sphalerite 

 and wurtzite (ZnS), and cinnabar and metacinnabar (HgS), one 

 member of which crystallizes from acid solutions only, the other 

 member from alkaline solutions as well. Furthermore it is 



*Tscher. Mitt., xiv, 114, 1895. 



f Allen, Crenshaw and Johnston, this Journal, xxxiii, 179, 1912. 



