68 MICRO-ORGANISMS AND DISEASE [cHAP. 



is allowed to stand ; from the bottom layer a small quantity 

 is withdrawn and of this a drop or two are deposited in the 

 centre of a clean cover-glass and evaporated by heating. 

 This represents a film specimen, which is then stained, 

 washed, and mounted in the usual way. On microscopic 

 examination, according to the source from which the water 

 is derived, there will be found particles of amorphous debris, 

 cotton-wool threads, spores, and bits of mycelial threads and 

 stained bacteria in small or great numbers according to the 

 amount of pollution that the water has been exposed to. 

 In a so prepared specimen of the water that the different 

 London water companies, drawing their water from the 

 Thames and Lea, distribute to their consumers, as a rule 

 besides numerous bacteria, cotton fibres and amorphous 

 debris, there will be found various infusoria (see below). In 

 order to accurately study the number and character of the 

 bacteria, present in water, cultivations must be made. These 

 are of two kinds : 



A. — To Determine the Number and General 

 Character of the Bacteria. 



Plate-cultivations are used for this purpose, generally 

 gelatine plate-cultivations. Biit it must be remembered 

 that by determining the number of microbes in a given 

 small quantity of the water, added to the gelatine, by means 

 of gelatine plate-cultivation, we are determining only the 

 relative number of bacteria, that is to say, only those that 

 do and can grow at the temperature at which the gelatine 

 keeps solid, but there may be and sometimes are some 

 species present which only grow well at higher temperatures ; 

 in such cases their numbers must be determined by Agar 

 plates. But as a general rule in practice it is sufficient to 



