THE SULPHUR CYCLE 239 



sulphuretted hydrogen. They may both be due to decom- 

 position of the albumin of the weed, or on the other hand 

 sulphides may be produced by reduction of the sulphates in 

 the sea water, or in the tissues of the Ulva, and these sulphides 

 are then decomposed by the fatty acids produced in the first 

 fermentation. 



The evidence points to the hydrogen sulphide being derived 

 from the reduction of sulphates, rather than from the decom- 

 position of the albumin, inasmuch as when comparative tests 

 were made, by fermenting the Ulva in sea water, and tap 

 water, respectively, sulphuretted hydrogen was much more 

 readily evolved from the sea water experiment than from the 

 tap water. The reduction of sulphates is clearly, then, a very 

 important source of hydrogen sulphide. 



Before considering this process in detail, however, it should 

 be stated that the objectionable odour evolved, when organic 

 matter is allowed to putrefy, is not solely due to hydrogen 

 sulphide. Under certain conditions, very evil-smelling gases 

 are evolved in which no trace of hydrogen sulphide can be 

 discovered. These are probably organic sulphur compounds, 

 such as mercaptan (C3H5SH), also amines, and substances 

 such as skatol, etc., which are also products of albumin 

 decomposition. It has been further found that the yield of 

 sulphuretted hydrogen can be increased in many cases if a 

 small quantity of flowers of sulphur is added to the fermenting 

 mistiu'e. 



(&) Sulphate reduction. — As already stated, this change 

 has been studied by Beyerinck and Van Delden. Beyerinck 

 inoculated suitable solutions containing sulphates with small 

 quantities of mud from the canals of Delft, and found that 

 the best conditions for sulphate reduction were as follows : — 



(1) No oxygen must be present. 



(2) No acid formation must take place, and consequently 

 little or no sugar should be present in the culture media. 



(3) Phosphates and other suitable solids must be present. 



