PURIFICATION OF WATER FOR DPJXKIXG PURPOSES 181 



be adopted in testing the bacterial efficiency of such. 

 filters. 



Stone Filters. — These filters have for a long time 

 been regarded with favour, because they succeed in 

 producing a liquid bright and clear to the eye, removing 

 from visibly turbid water the coarser particles in 

 suspension. Some experiments as to their bacterial 

 efficiency have recently been made by Esmarch.^ 

 He investigated six stone filters coming from such 

 difierent parts of the world as the Canary Islands 

 and Mexico. In all cases he found them absolutely 

 unreliable, large quantities of bacteria being found in 

 the filtrate. 



Asbestos Filters. — Elaborate experiments have been 

 made by Gruber-Weichselbaum,^ and later by JoUes,^ 

 on the value of asbestos as a filtering material. The 

 result of these investigations shows that, although in 

 the first instance filters in which this material is used 

 may be regarded as satisfactory, they do not long con- 

 tinue to retain the micro-organisms. The passage of 

 the bacteria is attributed by these investigators to 

 irregularity in the pressure of the water supplied, 

 which gradually leads to the disruption of the layer of 

 asbestos, and by creating even the smallest rents facili- 

 tates the penetration of the micro-organisms into the 

 filtrate. 



STERILISATION OF WATER BY HEAT 



At the present time, when such grave suspicion has 

 fallen upon filters as a class, it is becoming not an 

 uncommon practice with many to boil susj^icious water 



^ ' Ueber Wasserfiltration durch Steinfilter,' Centralhlatt filr Bahte- 

 riologie, 1892, vol. xi. p. 525. 



~ ' Ueber die Wirksamkeit von Asbestiiltern znr G-e-winniing von 

 sterilem Wasser,' Oesterr. Sanitdtstoesen, 1891, No. 43, 



^ ' Untersuchung liber die Filtrationsfahigkeit des patentirten Wasser- 

 filters Pnritas,' Centralhlatt filr Bakteriologie, vol. sii. p. 596, 1892. 



