60 AGRICULTURAL BACTERIOLOGY. 



free in the form of sulphuretted hydrogen (H 2 S), a gas which 

 may almost always be detected in decomposing organic mat- 

 ter. Proteids are not the only source of this gas. Some 

 species of bacteria are able to decompose sulphates or sul- 

 phites and other low sulphur compounds. The sulphites and 

 low compounds are easily broken up by a variety of bacteria, 

 but only one definite species is yet known with the power 

 of decomposing sulphates, Spirillum de sulphuric am. Some 

 species of bacteria can even produce H 2 S from pure sulphur, 

 and it thus appears that a great variety of sulphur compounds 

 may, under the influence of different bacteria, liberate sulphur 

 in the form of sulphuretted hydrogen. 



This gas, however, readily enters into new combinations, 

 partly by simple oxidation and partly through the agency of 

 bacteria. It may be directly oxidized by uniting with oxygen 

 and then with water to form sulphuric acid. 



S 2 -f 2H 2 + 3 2 = 2 H 2 S0 4 



(sulphuric acid) 



The sulphuric acid thus formed is an active agent in the soil, 

 especially in the presence of decomposing organic bodies. It 

 will readily combine with various bases to form sulphates. It 

 can unite with ammonia to form ammonium sulphate, or with 

 calcium carbonate to form calcium sulphate, two salts of much 

 significance in the soil and contributing to its fertility. The 

 H 2 S instead of being oxidized may combine with iron to form 

 iron sulphide. 



But this direct oxidation is not, apparently, the chief method 

 by which the H 2 S is converted into sulphuric acid and sul- 

 phates. There are certain bacteria, now known for some fifteen 

 years, which have such a close relation to this sulphur problem 

 that they have been called sulphur bacteria. Some of these, 

 which are the best known, belong to the higher bacteria (see 



