REVIEWS 507 



Wisconsin zinc mines; it was also found in other mine-waters, probably 

 being carried downward by surface waters that descended through the 

 soil and rock for many hundreds of feet. The form is especially 

 abundant in the water of Vermilion Lake, Soudan, Minnesota. All 

 these bacteria precipitated ferric hydroxide. 



Besides these there are other iron bacteria which precipitate ferric 

 hydroxide or ferrous sulphide. The former group is especially wide- 

 spread, and though its members do not require iron in solution for their 

 development, still it is thrown down quite rapidly as a waste product. 

 The precipitating action of such forms was studied by means of weak 

 solutions of the slightly acid ferrous ammonium sulphate. 



Whereas some of the sulphide-precipitating bacteria owe this ability 

 to the action of the hydrogen sulphide liberated by them on the ferric 

 salt in solution, others precipitate the sulphide because of their reducing 

 action on the sulphate. When water bearing ferric ammonium citrate 

 was inoculated with hay and soil infusions, there resulted a precipitation 

 of ferric hydroxide not observed in the sterile solution. This ensues even 

 under anerobic conditions. The organisms that induced it were grown on 

 plates of Heyden Naehrstoff agar with ferric ammonium citrate as indi- 

 cator and various types of bacteria were recognized. Slopes of Heyden 

 agar to which no citrate was added showed practically no growth. 

 Other organic salts of iron were also used more or less successfully, and 

 similar experiments showed that no precipitations resulted from the 

 solutions bearing salts of manganese in place of iron. 



On the other hand, no precipitation that could be definitely assigned 

 to organic processes could be obtained from inorganic iron salts such as the 

 bivalent carbonate or sulphate or ferric chloride, the precipitation that 

 did result in these cases being better attributed to the purely inorganic 

 oxidation and (or) hydrolysis. These results do not agree with those of 

 Mumford (Chem. Soc. Jour., Vol. 103, 1913). 



A review of the earlier work of Beijerinck, Van Delden, and Fred 

 on the formation of hydrogen sulphide by bacteria may be summarized 

 as follows : sulphates are formed abundantly by sulphur bacteria from 

 hydrogen sulphide; these sulphates are then reduced by other bacteria 

 to yield sulphides and hydrogen sulphide; if ferrous or manganese 

 sulphides are formed they are precipitated. These observations have 

 been ably and fully discussed in other papers, notably in their bearing on 

 the origin of the Sicilian sulphur deposits. 



The relations of these facts to geologic processes are amplified by 

 the writer. He points out the solubility of iron carbonate; soluble 

 organic compounds, chiefly humides, of iron may also be formed; it is 



