So convinced was Koch of the efficiency of sand filtration as protection against disease-producing germs that he advocated an adaptation of this plan in places where it was found that a well yielded infected water. Such pollution in a well may be due to various causes; surface-polluted water oozing into the well is probably the commonest, but decaying animal or vegetable matter might also raise the number of micro-organisms present almost indefinitely. Koch's proposal for such a polluted well was to fill it up with gravel to its highest water level, and above that, up to the surface of the ground, with fine sand. Before the well is filled up in this manner it must, of course, be fitted with a pipe passing to the bottom and connected with a pump. This simple procedure of filling up a well with gravel and sand interposes an effectual filter-bed between the subsoil water and any foul surface water percolating downwards. Such an arrangement yields as good, if not better, results than an ordinary filter-bed, on account of there being practically no disturbance of the bed nor injury done to it by frost. The effect of the remedies we have been discussing upon the number of bacteria is demonstrated in the results which Sir Edward Frankland arrived at in his investigation of London waters.[17] MEAN OF MONTHLY EXAMINATIONS FOR THE YEAR ------------------+---------+----------------------------+--------------- | | M.-O. PER CC. | Average per | +-------+--------+------- ---+ cent. of |Source of| | | |Micro-organisms NAME OF COMPANY. | Supply. | At | After | After | Removed | |Source.|Storage.|Filtration.| by Filtration. ------------------+---------+-------+--------+-----------+--------------- The Chelsea Co. |Thames at| | | | | Hampton |16,138 | 1067 | 34 | 98.96 West Middlesex Co.| " |16,138 | 1788 | 58 | 99.40 Southwark & | | | | | Vauxhall Co. | " |16,138 | .... | 80 | 97.72 | | | | { 623 | } Grand Junction Co.| " |16,138 | 2500 | { 100 | } 98.46 | | | | { 96 | } Lambeth Co. | " |16,138 | 7820 | 75 | 99.50 ------------------+---------+-------+--------+-----------+--------------- The teaching of these figures could, with great ease, be reproduced again and again if such was necessary; but these will suffice to show that sand filtration, when carefully carried out, offers a more or less absolute barrier to the passage of bacteria, whether non-pathogenic or pathogenic.