THE IRON-BACTERIA 391 



In considering the relative merits of the second and third 

 theories the following facts, which have been proved, must be 

 borne in mind. 



1 . Iron compounds are not in themselves necessary for growth. 



2. Manganese can equally well replace iron. 



3. Iron-bacteria obtain their nitrogen from organic com- 

 pounds as do the vast majority of bacteria. 



4. The more organic matter there is in solution in ferruginous 

 waters, the more iron these waters are able to hold in solution. 



5 . Bacteria in general do possess chemio-tactic attractions 

 and repulsions for a variety of substances. 



6. Iron is found not only on or in the membrane or sheath, 

 but actually inside the cell. 



7. The growth of Iron-bacteria in ferruginous waters un- 

 doubtedly expedites the accumulation of ferric hydroxide in 

 these waters. 



8. Other minute organisms, e.g. some of the Euglena 

 family, some of the Flagellates, e.g. Anthophysa vegetans and 

 some of the (Edogonia group, as well as the Iron-bacteria, 

 possess the same physiological characteristic. When we now 

 apply the theories of Molisch and of Campbell Brown to these 

 proven facts it will be seen that both theories are able to bear 

 the test. 



Some of the facts, viz. Nos. 2,5, and 8, strengthen Molisch 's 

 theory considerably; on the other hand, Nos. 3, 4, and 6 give 

 greater force to Campbell Brown's theory. They are, however, 

 not necessarily mutually exclusive, and we shall not be far 

 wrong in granting that each contains a certain measure of 

 truth. If these organisms have a chemio-tactic affinity for 

 iron-compounds, and at the same time remove iron out of 

 solution in their absorption and assimilation of organic com- 

 pounds, the effect of their growth in expediting the oxidation 

 of ferrous compounds can readily be imagined. The question 

 has lately arisen as to whether the organisms designated as 

 Iron-bacteria are the only members of the group of Bacteria 

 capable of producing these oxidising changes. Mumford has 

 described an iron-bacterium which oxidises a ferrous compound 

 to ferric hydroxide. This work, however, does not adduce 

 incontestable proof that the effects claimed to be produced 

 by the bacterium were due to that organism and not to other 

 agencies such as chemical oxidation. The absence of control 



