ANALYSIS OF WATER. 177 



For this purpose the methods for the isolation of 

 individual species, which have already been described, 

 and the means of studying these species when isolated, 

 are indispensable. 



For this analysis certain precautions essential to 

 accuracy are always to.be observed. 



The sample is to be collected under the most rigid 

 precautions 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 15 to 20 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 

 previously been thoroughly freed from all dirt and 

 organic particles, and then sterilized. And the plates 

 should be made as quickly after collecting the sample as 

 is possible. 



Where circqmstances permit, all water analyses should 

 be made on the spot at which 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. 



It is therefore advisable that where this work is to be 

 done, the Esmarch tubes or Petri plates should be pre- 

 pared on the spot. 



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 as the amounts from 

 which plates are to be made. In this way one forms 

 some idea as to the approximate number of organisms 



