344 APPLIED BACTERIOLOGY 



at the bottom ; smaller stones are placed above these, then 

 gravel and rough sand, and lastly fine sand. 



The action of the bed is twofold. First it acts as a 

 mechanical strainer ; and its efficiency in this respect is 

 increased by the formation of a slimy layer derived from 

 the water which has been filtered, and consisting principally 

 of organic detritus and living bacteria. This layer in a 

 great measure prevents organisms from being washed 

 through into the filtrate. The second function of this 

 layer is to serve as a culture ground for oxidation bacteria, 

 which to a large extent tend to prevent the multiplication of 

 the other bacteria, and consequently their growth through 

 the filter. It is a matter of common experience among 

 waterworks managers that a far better effect is obtained 

 from the action of a new filter-bed after it has been at 

 work for a few days (i. e., after the layer of slime has had 

 time to form) than when quite fresh. Again, in summer, 

 when the temperature is more favourable to the growth 

 of the organisms, the purification is more complete than 

 in winter. In practice the layer of slime ultimately, at 

 periods varying with the nature of the water and rate of 

 filtration, becomes too dense to permit the passage of 

 sufficient water, and loses its capacity to prevent the 

 passage of organisms. The bed should before such periods 

 be put out of operation, and its surface should be scraped, 

 a fresh layer of slime being allowed to form before the 

 filtrate is again used. 



Nitrification of Ammonia. — The well-known and important 

 process of oxidation which takes place in the soil, whereby 

 the organic nitrogen and ammonia are converted into nitric 

 and nitrous acid, is the result of the activity of certain 

 micro-organisms. This process of ' nitrification,' as it is 

 called, was first studied by Miintz and Schloesing in 1877, 

 who found that nitrification did not take place in soil that 



