STUDY OF WATER. 425 



should be made as quickly as possible after collecting 

 the sample. 



Where 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 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 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 organ- 

 isms 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 temperature best suited for the growth of the 

 majority of these organisms may be determined. 



As a rule, the greater number of colonies appears 

 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 patho- 

 genic varieties, 



