The enumeration of bacteria by means of solid media depends upon the ability of the organism to reproduce at such a rate as to produce a visible colony within the period of incubation and any substance that prevents the growth of a visible colony is classified as a disinfectant; if on further incubation the bacterial count approximates that of the untreated sample the added substance has acted mainly as an antiseptic. In practice no substance acts entirely as an antiseptic as the organisms present have varying degrees of resistance and the less viable ones are killed by doses that are only antiseptic to the more resistant ones. An example of an antiseptic effect followed by a mild disinfectant action, caused by small doses of bleach is shown in Table XV. In this experiment the water designated as control was from the same source as the treated water. In order to make the bacterial count in this water approximately the same as in the treated water, the original count was reduced by diluting the sample with water from the same source, sterilised by boiling, and afterwards reaërated with sterile air. TABLE XV.[A]--ANTISEPTIC EFFECT OF CHLORINE Sample treated with 0.1 part per million of available chlorine. -------------+---------------------------------------+------------------ | | RATIO OF PLATED. | INCUBATION PERIOD, DAYS. | BACTERIAL | | COUNTS. --------+----+-------+-------+-------+-------+-------+-----+-----+------ Time. |Day.| 2 | 3 | 4 | 5 | 6 | 2:4 | 2:5 | 2:6 | | | | | | |Days.|Days.|Days. --------+----+-------+-------+-------+-------+-------+-----+-----+------ 11 a.m.| 1 | 520| 940| 1,350| 2,360| 2,780|1:2.6|1:4.5|1:5.3 12 noon| 1 | 390| 770| 1,080| 2,040| 2,320| 2.8| 5.2| 5.8 2 p.m.| 1 | 187| 260| 690| 1,840| 2,080| 3.7| 9.9| 16.4 4 p.m.| 1 | 91| 130| 280| 760| 840| 3.1| 8.3| 9.2 10 a.m.| 2 | 42| 120| 670| 920| ...| 15.9| 22. | ... 10 a.m.| 3 | 320| 1,210| 3,500| ...| ...| 10.9| ...| ... 10 a.m.| 4 | 8,700| 14,200| 26,000| ...| ...| 2.9| ...| ... --------+----+-------+-------+-------+-------+-------+-----+-----+------ CONTROL. NO CHLORINE ADDED -------------+---------------------------------------+------------------ | | RATIO OF PLATED. | INCUBATION PERIOD, DAYS. | BACTERIAL | | COUNTS. --------+----+-------+-------+-------+-------+-------+-----+-----+------ Time. |Day.| 2 | 3 | 4 | 5 | 6 | 2:4 | 2:5 | 2:6 | | | | | | |Days.|Days.|Days.