292 WATER HYGIENE 
changed. With the filtered water there was a less constant seasonal 
ration, which was much lower than that with the unfiltered water. This 
was not due to treatment of the water with calcium hypochlorite. The 
efficiencies in removal of B. colz correspond better to the counts on gela- 
tin, but Gaub does not state how these efficiencies were computed. 
Gaub forms no conclusion and asks, “‘ Which is the correct medium to 
“se In order to show the efficiency of a plant?” Prescott and Winslow 
(1915) state that the decomposition of organic matter may be measured 
either by the material decomposed or by the number of organisms en- 
gaged in carrying out the process of decomposition; and that the latter 
method has the advantage of far greater delicacy since bacteria respond 
by enormous multiplication to very slight increases in their food supply; 
thus the count on standard gelatin at 20° C. is roughly proportionate 
in not too heavily polluted waters to the free ammonia and oxygen 
consumed by chemical analysis. Savage asserts that many water bac- 
teria are unable to grow at 37° C. though most organisms associated 
with excreta grow readily at 37° C.; unfortunately, many harmless soil 
organisms also grow readily at 37° C. An experiment by Savage illus- 
trates the limitations of this enumeration. He plated on agar and gel- 
atin a pure tap water collected in a sterile bottle; he then added 1 gm. 
of soil to 500 ¢c.c. of the sample and plated on agar and gelatin and emul- 
sion thus obtained. The agar plates were incubated at 37° C. for forty 
hours and the gelatin plates at 21° C. for three days. The soil con- 
tained no B. colt in 3 gms. The water contained 3 bacteria per cubic 
centimeter by the agar count and 76‘by the gelatin count. The emul- 
sion contained 1360 bacteria per cubic centimeter by the agar count 
and 1970 by the gelatin count. Savage concludes that the count at 
37° C. is an index of the presence of bacteria other than those natural to 
pure water, but not necessarily harmful ones. 
Harrison expresses the following opinion: ‘‘ The number of organ- 
isms which develop on beef peptone agar ineubated at blood heat, com- 
monly termed the ‘agar’ or ‘blood-heat’ count, is perhaps more 
important than the gelatin count, as many water bacteria do not grow 
at blood heat, whereas sewage and soil organisms grow readily at this 
temperature. The agar count eliminates the water flora, but obscures 
the sanitary results by reason of the presence of soil bacteria.”? Muir 
and Ritchie state that the count on gelatin plates incubated at 20° C. 
gives an idea of the number of bacteria which grow at summer heat and 
that the count on agar plates “ incubated at 37° C. gives an idea of the 
number of those which grow at blood heat.” The following comments 
are added: “As the pathogenic and intestinal bacteria grow at this 
