644 APPLICATION OF METHODS OF BACTERIOLOGY 



isolation of individual species which have been described 

 already, and of the means of studying these species when 

 isolated, is indispensable. 



For this analysis certain precautions essential to accuracy 

 are always to be observed. 



The sample is to be collected under the most rigid pre- 

 cautions that will exclude organisms from sources other 

 than that under consideration. If drawn from a spigot, 

 it should never be collected until the water has been flowing 

 for fifteen to twenty minutes in a full stream. If obtained 

 from a stream or a spring, it should be collected, not from 

 the surface, but rather from about one foot beneath the 

 surface. 



It should always be collected in vessels which have pre- 

 viously been thoroughly freed from all dirt and organic 

 particles, and then sterilized; and the plates should be 

 made as quickly as possible after collecting the sample. 



When circumstances permit, all water analyses should be 

 made on the spot where the sample is taken, as it is known 

 that during transportation, unless the samples are kept 

 packed in ice, a multiplication of the organisms contained 

 in it always occurs. 



For the purpose of qualitative analysis it is necessary 

 that a small portion of the water one, two, three, five 

 drops should first be employed for making the plates. 

 In this way one can form an idea as to the approximate 

 number of organisms in the water, and can, in consequence, 

 determine the amount of water best suited for the plates. 

 Duplicate plates are always to be made one set upon 

 agar-agar, which are to be kept in the incubator at body- 

 temperature, and one set upon gelatin, to be kept at from 

 18 to 20 C. 



