856 THE BACTERIOLOGICAL EXAMINATION OF WATER 



exactly 50 drops to the cubic centimetre cannot always be obtained on 

 account of the difficulties in their manufacture but the actual number 48, 

 52, 54 drops or whatever the number may be is always marked on the stem. 



Sterilize a pipette (which it will be assumed gives 52 drops to the cubic 

 centimetre). Aspirate a little of the water under examination and introduce 

 one drop into a conical flask : add the gelatin and proceed with the enumera- 

 tion as above (from par. 5). 



Then by multiplying the number of colonies which have developed in the 

 flask by 52 the number of organisms per cubic centimetre can be ascertained. 

 If, for instance 96 colonies were counted 



96 x 52 = 4992 aerobic organisms per c.c. 



Two or three flasks should always be sown and the average of the results 

 recorded. 



Interpretation of the results of enumeration. 



The quantitative examination is not alone sufficient to allow an opinion 

 to be given on the purity of a water ; before drawing any conclusion a qualita- 

 tive examination a determination of the nature of the organisms present 

 must be undertaken. It is at once apparent, for instance, that a water con- 

 taining a large number of harmless saprophytes (B. subtilis, the white coccus 

 of water, etc.) would be infinitely preferable to another which contained a 

 few pathogenic organisms such as the typhoid bacillus. Still, from the point 

 of view of ordinary pollution the total number of organisms is of some import- 

 ance. Miquel has classified waters according to their content of organisms ; 

 this classification is convenient but it should not be utilized until the results 

 of the qualitative examination are known and have been taken into con- 

 sideration. 



MIQUEL'S TABLE. 



to 10 organisms per cubic centimetre, 



10 100 



100 1,000 



1,000 10,000 



10.000 100,000 



More than 100,000 



Extraordinarily pure. 



Very pure. 



Pure. 



Moderate. 



Impure. 



Very impure. 



Note. Any such classification as this is, of course, very arbitrary and of limited 

 value for the reasons given. It may be remarked here that most methods of enumera- 

 tion are liable to underestimate the numbers of organisms contained in a given 

 volume of water. For instance, there may be organisms present in the water which 

 fail to grow on the culture medium on account of the presence of other species acting 

 on them prejudicially as " inhibiting organisms." Moreover, pathogenic species of 

 organisms will hardly grow on gelatin plates, not only because they are held in check 

 by saprophytes, but also because the temperature of incubation is unsuited to their 

 multiplication. It will be obvious that Miquel' s dilution method in which an 

 attempt is made to sow each organism in a different tube is free from these sources 

 of error, but unfortunately by reason of its complexity it is of little practical value. 



QUALITATIVE EXAMINATION. 



2. Determination of the nature of the organisms present. 

 A. The isolation of saprophytic species. 



It is usual for the identification of the various species of micro-organisms 

 which may be present in a water to isolate the organisms on Petri dishes 

 (p. 78). Sow one drop of the water in a tube of melted gelatin, mix. sow 

 two or three loopsful of the mixture into a second tube of gelatin, mix 

 again, and sow two or three loopsful from the second tube into a third tube 

 and pour plates with the mixtures. The growths on the plates are carefully 



