228 Theobald Smith 



definite direction, as in the search for typhoid bacilli and 

 Asiatic cholera spirilla. For general purposes, however, the 

 bacteriologist had to fall back upon the numerical estimation 

 with no regard to any qualitative determination. The nu- 

 merical estimate, taken by itself, is not satisfactory. It is true 

 that in large surface waters, such as rivers, the number of bac- 

 teria is a very good index of the organic matter present, yet 

 here one remains in doubt whether the bacteria are in the 

 main from sewage or from decaying vegetable matter. Hence 

 in the few instances in which I have had occasion to determine 

 the hygienic character of a given water, I have endeavored to 

 get some idea of the fecal bacteria present, in other words, the 

 large group of colon bacteria which are such regular inhabi- 

 tants of the intestines of man and of the domesticated animals 

 and which are as good an index of sewage pollution as we 

 can desire. 



There are methods which enable us to isolate fecal bacteria 

 from water, but they either do not give us any information 

 concerning the number of such bacteria, or else this knoledge 

 is obtainable only after much labor. Passing by these meth- 

 ods as not bearing on our subject, I will briefl3' refer to one 

 which is an outgrowth of the observations on gas production 

 in the fermentation tube. 



If a series of such tubes containing glucose bouillon be in- 

 oculated, each with an equal but verj- small quantity of water 

 and placed at once in the thermostat at 37° C, it will be no- 

 ticed after one or more days, if the water is much polluted, 

 that some contain gas. If, for example, one ccm. of water is 

 distributed equally among ten tubes and of these, four subse- 

 quently contain gas, we may conclude that in one ccm. of this 

 water there were four gas-producing bacteria. All gas-produc- 

 ing bacteria are not intestinal species, however. Hence we must 

 try to eliminate those that are not fecal b}' the amount of gas 

 present. Bringing together all the information obtained by 

 cultivating a variety of bacteria in the fermentation tube, I 

 have come to the conclusion that all tubes containing less than 

 forty and more than seventy per cent, of gas are to be elimin- 

 ated. The lowest limit drawn excludes Proteus vulgaris, 



