Again, another example:
RIVER LEA WATER COLLECTED AT CHINGFORD
Number of Micro-organisms Obtained from 1 cc. of Water.
| Month. | 1886. | 1887. | 1888. |
| January | 39,300 | 37,700 | 31,000 |
| February | 20,600 | 7,900 | 26,000 |
| March | 9,025 | 24,000 | 63,000 |
| April | 7,300 | 1,330 | 84,000 |
| May | 2,950 | 2,200 | 1,124 |
| June | 4,700 | 12,200 | 7,000 |
| July | 5,400 | 12,300 | 2,190 |
| August | 4,300 | 5,300 | 2,000 |
| September | 3,700 | 9,200 | 1,670 |
| October | 6,400 | 7,600 | 2,310 |
| November | 12,700 | 27,000 | 57,500 |
| December | 121,000 | 11,000 | 4,400 |
"During the summer months these waters are purest as regards micro-organisms, this being due to the fact that during dry weather these rivers are mainly composed of spring water, whilst at other seasons they receive the washings of much cultivated land."—Frankland.
Prausnitz has shown that water differs, as would be expected, according to the locality in the stream at which examination is made. His investigations were made from the river Isar before and after it receives the drainage of Munich:
| No. of Colonies per cc. | |
| Above Munich | 531 |
| Near entrance of principal sewer | 227,369 |
| 13 kilometres from Munich | 9,111 |
| 22kilom"metres"muni" | 4,796 |
| 33kilom"metres"muni" | 2,378 |
Professor Percy Frankland also points out how the river Dee affords another example, even more perfect, of pollution and restoration repeated several times until the river becomes almost bacterially pure.
We cannot here enter more fully into the many conditions of a water which affect its bacterial content than to say that it varies considerably with its source, at different seasons, and under different climatic conditions. An enormous increase will occur if the sediment is disturbed, and conversely sedimentation and subsidence during storage will greatly diminish the numbers of bacteria. Sand filtration, plus a "nitrifying layer," will remove more than 90 per cent. of the bacteria. Sea-water contains comparatively few bacteria, and the deeper the water and the farther it is from shore so much less will be the bacterial pollution.