28 SULPHUR BACTERIA 



These facts are in accord with the conclusions of Doss, 

 who states that black muds containing abundant organic 

 material and impregnated with colloidal hydrous ferrous 

 sulphide occur in many localities, both in enclosed seas and in 

 gulfs and bays connected with the open ocean. He instances 

 the eastern Mediterranean Sea, the Black Sea, the Sea of Azov, 

 the Caspian Sea, and the Dead Sea. Similar muds occur 

 around Oesel Island in the Gulf of Riga, and around the 

 mouths of the Elbe and Weser. According to Andrussow (3) 

 the mud from the bottom of the Black Sea contains an 

 abundance of ferrous sulphide, and this compound occurs 

 generally in the black shore muds and sands of the Limans 

 that border the Black Sea. Omelianski (i) states that this 

 mud when exposed to air becomes oxidized to a grey colour. 

 Harder regarded the black mud found on Russian coasts as 

 ferrous sulphide, and the lighter mud as a mixture of ferrous 

 and ferric sulphides. He did not, however, advance any proof 

 in support of this statement. Neither was it possible to con- 

 firm his statement that the ferrous sulphide existed in a 

 colloidal state. Even when the sand is wet and black, the 

 black particles are large enough to be easily distinguished 

 from the particles of silica. 



(5) Cause of Disappearance of Black Colour on Exposure 

 to the Atmosphere. — It is generally held that the change in 

 colour is due to the oxidation on exposure to air of black 

 ferrous sulphide to oxy-sulphur compounds of iron which are 

 not black in colour. It is also believed that the iron is 

 dissolved out of iron-bearing minerals, and is removed as 

 ferrous carbonate, and that it is later deposited and converted 

 into ferrous sulphide by bacteria. The author considers that 

 the reaction between FeCOg and HgS is reversible, and that 

 the light or black condition is determined by whichever 

 of the two compounds FeCOg or FeS preponderates. The 

 reaction is as follows : — ■ 



FeCOg + H2S ^ FeS + HoCOg. 



Here we have a delicately balanced state of equilibrium. 

 With the production of HgS under moist conditions ferrous 



