EXAMINATION OF WATER FOR MICRO-ORGANISMS 75 



turbid tubes the number of microbes in the volume of 

 water represented by the particular dilution-series can 

 be inferred. Thus the series of tubes A 1 3 , A 2 3 , A 3 3 , . . * . 

 A 10 3 , has received in all *1 c.c. of the original water, 

 and, to take a simple case, if three out of these ten 

 tubes became turbid, the other seven remaining clear, 

 it would be inferred that '1 c.c. of the water contained 

 three microbes, or thirty microbes in 1-0 c.c. 



It is obvious that this undoubtedly ingenious method 

 possesses a number of grave disadvantages ; thus it is 

 not only exceedingly laborious, but the possibility of 

 failure is very considerable, unless skill and judgment 

 are employed in arranging the necessary degree of dilu- 

 tion required by the particular sample of water under 

 examination. Moreover, it affords little or no imme- 

 diate indication as to the particular varieties of microbes 

 present in the water. On the other hand, some real 

 advantages attach to this method, viz. : First, that the 

 cultivation is made in a liquid medium in which some 

 bacteria will thrive which would not develop on gela- 

 tine-plates ; and second, that the incubation can be 

 effected at any desired temperature, whilst with gela- 

 tine-plates the temperature of incubation cannot exceed 

 about 22 C. ; on this account it is in exceptional cases 

 necessary to employ this dilution method for the exami- 

 nation of water. 



In actual practice Miquel has more recently adopted 

 what he calls the ' mixed process.' The water is diluted 

 to, say, 100, 1,000, 10,000, or 100,000, &c. times its 

 volume, according as the particular sample is supposed 

 to contain a smaller or larger number of bacteria. 

 After this has been done, from one to two drops are 

 introduced into a flask with a large flat bottom (about 

 9 centimetres in diameter) containing a layer, about 

 2 millimetres in thickness, of sterile and liquefied 



