THE SULPHUR CYCLE 237 



to nitrates, so the sulphur supplied to the plant as sulphates 

 becomes part of vegetable and animal albumins, which again 

 break down, yielding hydrogen sulphide, and the latter is 

 oxidised, either chemically or biologically, back to sulphate. 



Moreover, just as nitrates are capable of reduction to form 

 nitrites, and finally ammonia, so sulphates are capable of 

 reduction to hydrogen sulphide. 



In the sulphur cycle purely chemical reactions play a 

 greater part than is apparently the case with nitrogen, but in 

 all cases the sulphur transformations are capable of being 

 facilitated by the activities of various organisms. 



The chief workers on this important question of the natural 

 sequence of combinations entered into by sulphur, have been 

 Winogradski in Russia, who has investigated the conditions 

 under which sulphur is oxidised by certain specific sulphur 

 organisms ; Beyerinck and van Delden in Holland, who have 

 studied particularly the reduction of sulphates ; and Letts in 

 Belfast, who, while repeating Beyerinck and van Delden's 

 experiments, has, in conjunction with several of his students, 

 made important original observations upon the conditions 

 under which sulphuretted hydrogen is evolved, in the actual 

 circumstances of certain estuaries. 



It will perhaps be simplest to consider the subject under 

 two heads : 



I. The production of hydrogen sulphide. 

 II. The oxidation of hydrogen sulphide. 



I. Hydrogen sulphide can arise under natural conditions 

 from the following sources : 



(a) The decomposition of albumin, as already stated ; 



(6) The reduction of sulphates. 



Both these changes are due to the action of 

 various organisms. 



