Two groups of phenomena are to be considered in the study of the sulfur cycle: (1) the reduction phenomena and (2) the oxidation phenomena. The first result in the production of hydrogen sulfide and the second in the oxidation of H2S to S and of the latter to H2SO4. Both groups of phenomena result from activities of microorganisms. Winogradsky was the first investigator to have definitely demonstrated the role of microbes in the oxidation of the H2S to S, and of the latter to H2SO4, but it is characteristic that the organisms studied by Winogradsky (Beggiatoa and Thiothrix), never turned the medium acid: this was explained by the presence of sufficient CaCO3 in the culture to neutralize any acid formed. Keil isolated the two organisms in pure culture only about thirty years after the work of Winogradsky was carried out.The sulfur oxidizing bacteria were usually divided into four groups, namely (1) Thread-forming colorless bacteria, accumulating sulfur within their cells. The Beggiatoa and Thiothrix are re...