120 APPLIED BACTEKIOLOGY 



waterwork 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. 



The experiments of James Buchanan Young on the 

 soil of graveyards show that they are very rich in micro- 

 organisms, particularly those of a liquefying type, the 

 Proteus vulgaris being present in great numbers. Their 

 action is so effective that he found no notable quantities of 

 organic carbon and nitrogen in the upper layers, while 

 that in the lower layers was not so very much in excess 

 of that found in virgin soil. 



In addition to the two classes of oxidizing and putrefac- 

 tion-causing bacteria, there are the important organisms 

 discovered by P. F. Prankland and Winogradsky, which have 

 the power of abstracting nitrogen from the atmosphere and 

 fixing it for the use of plants. The absolute need for the 

 presence of these organisms in soil intended for crops has 

 been shown by experiments of Professor Nobbe on plants, 

 which have been made to grow on soil that has been 

 sterilised by heat, but which in all respects has its con- 

 stituents well fitted for their growth. Similar plants have 

 been grown in other portions of the same earth which had 

 not been sterilised, and while these afforded good luxuriant 



