224 Stephen DeM. Gage 



nitrates, and as nitrites; of the chlorine and of the oxygen consumed 

 from permanganate ; although determinations of odor, color, turbidity, 

 iron, hardness, etc., may enter into the consideration of the use of a 

 water for one purpose or another. In the bacterial analysis we have 

 the determination of the numbers of bacteria, usually determination 

 of the presence or absence of bacteria of the colon type, and occa- 

 sionally determinations of bacteria of the sporogenes or sewage 

 streptococcus type. The analyst having in his possession the above 

 data must know the significance of high or low values of each indi- 

 cating factor. Even with the number of factors at hand, many of 

 the data are often contradictory, and a correct interpretation of the 

 quality of the water is not always possible. 



The weakness in a sanitary survey is that, while it may show pollu- 

 tion, it does not always show whether that pollution is serious, 

 or whether the water has become purified; and, furthermore, it may 

 not reveal sources of pollution which can often be detected by analytical 

 means. The chemical examination can show us, when we know the 

 character of the source of the water, whether the water has or has not 

 been polluted, and the extent of the pollution. It cannot, however, 

 always reveal whether the water in its present state is safe or danger- 

 ous for domestic use ; and this remains for the bacteriologist to deter- 

 mine. The sanitary survey and the chemical analysis of water have 

 been sufficiently studied, so that our knowledge of the causes of varia- 

 tions and fluctuations in the various factors is well grounded. From 

 the side of the bacterial examination, however, much remains to be 

 investigated. We have a good working knowledge of the significance 

 and fluctuation in the numbers of bacteria in different waters when 

 determined by gelatin or agar plates after incubation at a tempera- 

 ture of 20° C. We are rapidly acquiring a good working knowledge 

 of the true significance of the appearance of B. coli, the sewage strep- 

 tococcus, and B. sporogenes in water of various classes. Many bac- 

 teriological factors, however, which may be of value have received 

 little or no attention. 



It is the purpose of the writer in the present paper to introduce data 

 bearing on the numbers of bacteria which develop on plates incubated 

 at different temperatures, and the ratios between such numbers, for 

 different classes of waters, in an endeavor to show that factors so 



