238 AIDS TO BACTERIOLOGY 



colonies to run together. The matter to be plated is 

 sufficiently diluted, and one or more drops are run on to 

 the surface, and spread by means of a sterile glass rod 

 bent at a right angle. 



China Green Agar (Werbitzki). An ordinary 3 per 

 cent, agar neutralised to + 13 on Eyre's scale, with 1-4 

 to 1-5 c.c. of a 0'2 per cent, solution of China 

 green added to each 100 c.c. contained in a flask. 

 McWeeney says that China green agar suppresses about 

 75 per cent, of the coli colonies, whilst the survivors 

 are much inhibited in their growth, remaining small, 

 opaque, and point-like. The typhoid colonies, on the 

 other hand, develop most luxuriantly, and are decidedly 

 more numerous than on any of the other media. Their 

 appearance is of a delicate transparent green, deeper 

 in the centre, with a thin, filmy, peripheral layer that 

 spreads out like a veil over the substratum, and presents 

 under a low power a typically sulcate appearance. The 

 motility of the typhoid bacilli is much diminished, and 

 they present a filamentous aspect. Subcultures in broth, 

 however, are typical by the next day, and can be sub- 

 jected to the usual agglutination and other tests. 



Filtration of Water. 



Domestic Filters. The primary object of a filter is to 

 remove suspended matter, including micro-organisms. 

 The filtering media include carbon (animal and vegetable 

 charcoal, silicated carbon, and manganous carbon), felt, 

 sponge, car feral (charcoal, clay, and iron), spongy iron 

 (Bishofs filter), magnetic iron, cellulose, earthenware, 

 and natural stone. Animal charcoal oxidises and absorbs 

 part of the organic matter, removes colour due to organic 

 impurities, and is credited with the power of removing 

 lead. Vegetable charcoal is less efficient. Iron may 

 reduce nitrates to ammonia. With the exception of 

 earthenware types, these filters only remove part of the 

 bacteria, and, unless the filtering medium be often re- 

 newed, may actually be sources of danger. If once 

 polluted with typhoid or cholera, they may subsequently 

 convey the bacilli to unpolluted water for long periods 

 after contamination. The initial efficiency of these filters 

 is due to the bacteria being temporarily arrested in the 

 filter media, where they multiply and are gradually 



