BACTERIOLOGICAL STUDY OF WATER. 491 



should be made as quickly as possible after collecting 

 the sample. 



When circumstances 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 organ- 

 isms 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 as the amounts from 

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

 some idea as to the approximate number of organisms 

 in the water, and can, in consequence, determine the 

 amount of water necessary to use for each set of 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. 



As soon as the colonies have developed the plates are 

 to be carefully compared and studied. It is to be noted 

 if any difference in the appearance of the organisms on 

 corresponding plates exists, and if so, to what is it due ? 

 It is to be particularly noted which plates contain the 

 greater number of colonies, those kept at the higher or 

 those at the lower temperature. In this way the tem- 

 perature best suited for the growth of the majority of 

 these organisms may be determined. 



As a rule, the greater number of colonies appear upon 

 the gelatin plates that are kept at 18 to 20 C., and 

 from this it would seem that many of the normal water- 

 bacteria do not find the higher temperature so favorable 

 to their development as do the organisms not naturally 

 present in water, particularly the pathogenic varieties. 



