INTRODUCTION 5 



distributed. They are noticeable in the pools among the rocks 

 on the shore where seaweed is undergoing decomposition, in 

 shallow pools polluted with sewage, at the mouths of sulphur 

 wells, and in many shallow waters, both marine and fresh, in 

 which organic matter is present in solution, and in which 

 the conditions as to oxygen, hydrogen sulphide, etc., are 

 suitable. 



If various putrescent organic masses (remains of animals 

 and plants) be enclosed in bottles, covered with water, and 

 stoppered up, and if these be left exposed to light, the 

 development of purple sulphur bacteria may almost with 

 certainty be observed in some of them after a few weeks. 



Their development on a large scale is seen in shallow land- 

 locked bays in which seaweeds and other plant debris are in 

 a state of putrefaction. Near Copenhagen, where such con- 

 ditions prevail, and where some of the bays on their shore side 

 are filled with decomposing material, the purple sulphur 

 bacteria impart their colour to the whole of the water occupied 

 by the decomposing material. 



The same phenomenon has been observed in various bays 

 in Jamaica, where broad stretches of water off certain of the 

 shores are tinted purple from the same cause. Isolated 

 patches of purple arc comparatively common on most shores 

 on which seaweed is undergoing decomposition. It is not 

 surprising that observations of the occurrence of the coloured 

 sulphur bacteria should date from early times, or that they 

 have been recorded from many sources, for their appearance 

 in mass is sufficiently distinctive to draw the attention of even 

 the least observant. The record of the occurrence of the colour- 

 less sulphur bacteria is not so extensive, owing to the drabness 

 of the colour of their mass cultures. These usually assume a 

 grey felted appearance. Thienemann records that Aristotle 

 remarked on the white patches in certain rivers, and that 

 Pliny called attention to the blood-red colour of the " Vul- 

 sinischen " Sea * in the year 208 B.C. It is doubtful, however, 

 whether the sulphur bacteria were responsible in these cases, 



* Now called Lago di Bolsena, in Etruria, Italy. 



