THE PHYSIOLOGY OF THE lEON-BACTEEIA 161 



the latter are able to thrive when no iron is present in the water, 

 and on the other hand, oxidation into the red compound takes place 

 quite readily when iron-bacteria are altogether absent. Hence the 

 oxidation into the red compound takes place in the water, this com- 

 pound being then attracted chemiotactically by the iron-bacteria if 

 these be present. The deposition is not of a purely mechanical 

 nature, for in many samples it is found that all the ferric hydroxide 

 is located on the bacteria, not a particle being found in an unattached 

 condition. Again, the deposition is selective, for when other 

 organisms, algae, etc., are present in the iron-water, their surfaces 

 are quite devoid of iron. 



The accumulation of ferric hydroxide in water-pipes has on 

 occasions been a source of great trouble, as has been amply shown 

 by the experience of Berlin, Rotterdam, and other towns. Woltering 

 and Sassen have recommended passing the drinking-water of a town 

 through coke towers, before passing the water into the pipes. 

 This would oxidise the iron, which would thus be precipitated 

 before reaching the pipes. This method could not be used on a 

 large scale, and it is doubtful whether even on a small scale it has 

 been completely successful. 



It has been suggested that iron-bacteria are mainly responsible for 

 the decay of white sandstone, which affects so many buildings in 

 this country that are built of this material. So far as the writer's 

 observations go, there is no warrant for the assumption that the 

 decay is due to bacterial action, and it is most certainly not due to 

 the agency of any of the known iron-bacteria. 



