In some instances the rate of development of the organisms after chlorination is greater than in the same water stored under similar conditions. This is especially noticeable in the presence of organic matter and has been ascribed to the action of the chlorine on the organic matter with the production of other compounds that are available as food material for the organisms. Houston, during the treatment of prefiltered water Lincoln in 1905, found that although the removal of _B. coli_ and other organisms growing at 37° C. was satisfactory, there was almost invariably an increase in the bacteria growing on gelatine at 20° C. This was ascribed to the action mentioned above and the chemical results supported this view, more organic matter being found in the filter effluents than in the prefiltered water. Rideal's experiments with sewage at Guildford indicate that a similar action may occur in contact beds. The addition of bleach to the prefiltered water at Yonkers also resulted in an increased count and in these instances the aftergrowths are due to a disturbance of the equilibrium by the action of the chlorine on the zooglea and other organic matter invariably found in ripe filters. Similar results can be produced by the addition of chlorinated water to small experimental sand filters. This is shown by the results in Tables XX and XXI. TABLE XX.--AFTERGROWTHS IN SAND --------------+---------------+---------------+---------------- | Bacteria Per | | Available | Gram of |Typical B. coli| FREE CHLORINE Chlorine in | Sand After |After 24 Hours.| AFTER 24 HRS. Water p.p.m. +-------+-------+---+---+---+---+-------+-------- | | |100| 10| 1 |0.1|Without| After | 3 Hrs.|24 Hrs.|Gr.|Gr.|Gr.|Gr.| Acidification. --------------+-------+-------+---+---+---+---+-------+-------- Nil | 12,000| 21,000| + | + | + | - | - | - 3.0 | 80|114,000| - | - | - | - | - | - 5.0 | 50|150,000| - | - | - | - | - | - 7.0 | 25|214,000| - | - | - | - | - | - 10.0 | 26|500,000| - | - | - | - | - | - --------------+-------+-------+---+---+---+---+-------+-------- TABLE XXI.--AFTERGROWTHS IN SAND --------------+--------------------------------- | BACTERIA PER GRAM OF SAND AFTER Available in +----------+----------+----------- Water p.p.m. | 3 Hours. |24 Hours. |48 Hours. --------------+----------+----------+----------- Nil | 70,000 | ..... | ..... 0.1 | 7,200 | 20,400 | 12,800 0.3 | 5,240 | 6,400 | 11,200 0.5 | 5,120 | 4,700 | 10,800 1.0 | 1,100 | 8,800 | 20,400 --------------+----------+----------+-----------