96 MICRO-ORGANISMS AND FERMENTATION. 



large deposits of iron-ochre and meadow-iron ore occurring in 

 nature seem to be the result of the action of such bacteria. 1 



x ln this connection may be mentioned the sulphur bacteria described by 

 COHN, WARMING, ENGLER, and especially WIXOGRADSKY, which occur in 

 water, many of which produce a red-colouring matter. Under the microscope 

 they are distinguished by the roundish bodies they contain, strongly refractive 

 to light and composed entirely of sulphur. They are aerobic, and occur 

 especially in waters containing sulphuretted hydrogen. This substance is 

 oxidised by the bacteria, and the sulphur split off is stored in the cells. 

 Among the thread-like species the Beggiatoa alba may be mentioned. 



An important part in nature is played by bacteria which convert ammoniacal 

 salts into nitrates : they are highly important for the nutrition of many plants. 

 SCHLOESING and MUENTZ first described them ; their observations were con- 

 firmed by WINOGRADSKY, who made use of pure cultures. Among these 

 nitrifying bacteria, as they are termed, there are some which oxidise ammonia 

 into nitrous acid, which is converted by other species into nitric acid. These 

 bacteria also cause the efflorescence of nitre from walls, which often brings 

 about the decay of brickwork, whilst snow-like masses of calcium nitrate are 

 detached from the wall. This evil can, of course, be remedied by means of 

 antiseptics. 



