Report on the Bacteriology of Water. 



201 



tine, kept fluid at about 35° C, and the mixture, after solidification 

 in a thin layer, is incubated generally at a temperature of 20 — 22° C. 

 in contact with air, but protected from danger of contamination : we 

 need not go into the particular methods of sterilisation, protection, 

 incubation, &c. : suffice it to say that in a few hours or days colonies 

 of bacteria appear on the culture plates, and the number of indi- 

 vidual bacteria in the measured sample of water is estimated from 

 these, on the assumption that each colony has sprung from one 

 germ. 



The comparison of numerous researches made in recent years, and 

 the experience gradually being gained in all branches of the technique 

 of the subject, have slowly led to the detection of numerous fallacies 

 in the almost established mode of procedure. 



In the first place, it was soon apparent that the mere numbers of 

 bacteria, per cubic centimetre of water, are in no sense a satisfactory 

 guide to the fitness of such water for domestic purposes ; it may be 

 quite true that one revolts from a water proved to yield large 

 numbers of colonies of bacteria, and one can understand that a water 

 yielding even 500 colonies per cubic centimetre should be preferred 

 to one yielding, say, 5000 colonies per cubic centimetre, but, so long 

 as this choice is based on the assumption that mere numbers decide 

 the safety or danger of the water, it is utterly fallacious. The 

 500 colonies may contain some which have been developed from 

 pathogenic germs, while the 5000 may have all arisen from harmless 

 forms. This consideration entirely invalidates all the older conclu- 

 sions, which were made in some quarters, as to a given water being 

 good or bad according as it yields few or many colonies per cubic 

 centimetre on plate cultures ; the only test is to determine what the 

 bacteria of the different colonies are, and the only general deduction 

 of any value to be drawn from mere quantitative bacteriological 

 determinations is, that a water obviously containing a number of 

 different species is, on the whole, more likely to have been subjected 

 to contamination than one which contains but few different kinds. 



A water should be suspected, therefore, and subjected to further 

 examination, if it yields several different kinds of colonies unknown 

 to the investigator. 



As a matter of practical experience, it is certainly impossible to 

 rapidly identify more than a few colonies in such cultivations ; if a 

 complete investigation of the life histories, &c, of all the forms were 

 attempted, the bacteriological examination of a single sample of water 

 might take years, and consequently this part of the subject is the one 

 which awaits and invites the attention of numerous and energetic, 

 properly equipped workers. 



Then as to the primary assumption which lies at the base of all the 

 older plate cultures. This was that each colony has taken origin 



P 2 



