THE MICROBES OF WATER 299 



deposits the greater part of its suspended matter, 

 including a large proportion of the microbes. Then 

 a further diminution takes place through degenera- 

 tion and decay of the microbes, for the number of 

 microbes in the unfiltered river-waters diminishes 

 on keeping irrespectively of subsidence, probably 

 owing to the competition between different forms 

 hostile to each other, as well as by the production 

 of chemical compounds inimical to their further 

 multiplication, (b) That the thickness of the filter- 

 ing stratum should exercise an important influence 

 on the number of microbes passing through the filter 

 must be sufficiently obvious. In estimating the 

 thickness of such a sand filter the fine sand only 

 should be taken into consideration, as it is only this 

 portion of the filter which can have any effect in 

 the removal of microbes, (c) That the filtration is 

 the more perfect the slower the rate, (d) That the 

 complete removal of microbes from water, by filtra- 

 tion, is unattainable without frequent renewal of the 

 best filtering materials. 



' It is often urged that the bacteriological exami- 

 nation of water is of little practical importance, 

 inasmuch as the microbes found are not necessarily 

 prejudicial to health, and that the method of exami- 

 nation does not aim at the detection of harmful 

 forms. A little more mature consideration, how- 

 ever, will show that the actual detection of harm- 

 ful or pathogenic forms is a matter of very little 

 importance; and if methods of water purification 

 are successful in removing microbes in general, and 

 more especially those which find a suitable home in 



