PREFACE. 



In the last two decades considerable progress has been made in 

 every branch of Bacteriology ; and of the different branches, 

 that dealing with the sulphur bacteria has received much 

 attention from foreign investigators. An attempt is here made 

 to present to English readers the present position of our 

 knowledge of the sulphur bacteria. 



The researches on these organisms are spread over a wide 

 field of investigators, and are described in divers languages. 

 As for the most part such studies have been confined to partial 

 aspects of the subject, there should be room for a book such as 

 the present, which aims at summing up the whole position. 

 l^J^e only section which is scantily treated is that of Ecology, 

 and the reason for this is that this subject is very adequately 

 dealt with in Bavendamm's Die farblosen und roten Schwefel- 

 bakteriek^ 



The study of the sulphur bacteria supplies matters of 

 interest not only to the bacteriologist and the botanist, but 

 also to the chemist who is interested in the analysis of water, 

 in the preservation of the purity of water reservoirs, and in the 

 chemical changes which take place in sewage-laden waters. 



These bacteria form a group of peculiar interest to the 

 botanist because of the possible relationship of many of their 

 species to the animal kingdom. Their study throws con- 

 siderable light on the structure, methods of reproduction, and 

 physiological activities of organisms that stand on the border- 

 line between plants and animals ; and this work has been 



