2 4 o AIDS TO BACTERIOLOGY 



and by using a batter} 7 of filters the supply can be in- 

 creased at will. 



Sand Filtration. Filter-beds at waterworks are con- 

 structed of a layer of large stones, with unjointed pipes 

 placed at intervals at the bottom; smaller stones are 

 placed above these, then gravel and f ough sand, and lastly 

 a layer of sand from 2 to 4 feet deep. The fineness and 

 contour of the grains of sand, thejiepth of the filter, and 

 the rate of filtration, all affect the" working of the filter in 

 the removal of organisms. The fineness of the sand is 

 per se no criterion of filtering power, a comparatively 

 coarse sharp-angled sand acting better than a finer article 

 with rounded edges. Sea sand is for this reason unsuit- 

 able for filtration. Koch's coefficients for safe working 

 are filtration through a sand layer not less than 30 centi- 

 metres thick, at a rate not exceeding 100 millimetres per 

 hour, and giving a filtrate containing not more than 

 100 bacteria per c.c. growing on gelatin. These coeffi- 

 cients, however, take no account of the class of sand used 

 or character of water filtered, and they are no longer 

 regarded as trustworthy. When freshly constructed, 

 organisms are washed through a filter-bed with great 

 rapidity, but after a certain quantity of water has passed 

 through or the water has been allowed to stand upon it 

 for a certain time, a slimy coating of detritus, bacteria, 

 and other lowly organisms, is formed on the surface. 

 If water is slowly passed through the filter when sufficient 

 of this coating has formed, the majority of the bacteria 

 will be retained by this surface, either by sticking to it 

 or by being strained off. The increasing thickness of 

 this coating will reduce the velocity with which the water 

 passes, and at the same time some of the bacteria will 

 tend to grow downwards into the lower strata of the 

 filter, and, if the process were continued long enough, 

 would be washed through into the filtrate, and ultimately 

 become more numerous there than in the unfiltered water. 



This layer also serves as a culture ground for oxidation 

 bacteria, which to a large extent tend to prevent the multi- 

 plication of the other bacteria, and consequently their 

 growth through the filter, In summer, when the tem- 

 perature is more favourable to the growth of the organisms, 

 the purification is more complete than in winter. 



The indication for scraping usually adopted is that the 



