200 BACTERIOLOGICAL EXAMINATION OF WATER. 



tube next below this in the series should be chosen. But the 

 character of the growth in the different tubes must be taken 

 into account. As a rule, a tube that has a thick scum on the 

 surface, or in which growth is only visible in the deeper layers 

 of the bouillon, or in which bubbles are seen in the liquid, 

 should be discarded. A tube should be preferred in which there 

 is a uniform turbidity. Usually the tube containing two 

 or three drops of Parietti's solution is the one chosen for 

 further use, although with the bouillon I employ, if infected 

 with dirty water, growths would occur in a tube containing ten 

 or twelve drops of the Parietti's fluid. Tubes containing the 

 larger quantity of Parietti should be employed if it is required 

 to isolate B. coli communis. The tube of bouillon chosen as 

 above is then used to inoculate a second series of bouillon tubes 

 to which successively increasing quantities of Parietti's solution 

 are added. The first tube of the second series has the same 

 number of drops of Parietti added to it as were present in the 

 tube taken for further inoculation from the first series< For 

 instance, supposing the tube taken from the first series was the 

 one containing three drops of Parietti, then the first tube of the 

 new series will also contain three drops, the next will contain 

 four drops, the next five drops, and so on. Two or three drops 

 of the bouillon in the tube from the first series are used to 

 inoculate each tube of the second series. These tubes are 

 covered with indiarubber caps and placed in the incubator as 

 before. On the following day choice of a tube has to be made 

 from the second series . . . the tube containing the highest 

 number of drops of Parietti's solution that is turbid is discarded, 

 and one of the lower tubes is taken for further use. The tube 

 of bouillon chosen must now be inoculated on to agar, having a 

 fairly dry surface in such a way as to produce isolated surface 

 colonies. A small quantity of the bouillon is taken up on the 

 end of an inoculating needle from the surface of the liquid. 

 The needle is introduced into the agar tube and rubbed on the 

 bottom of the slanting surface of the agar : then it is moved in 

 a zig-zag track over the remaining portion of the surface of the 

 agar. At least three agar tubes should be inoculated in this 

 way. After inoculation the agar is kept in the incubator. On 

 the following day the colonies that have developed must be 



