I. LONDON FILTERS. ----------------------+--------------+------------+------------------- | Effective | Uniformity |Albuminoid Ammonia. | Size; 10% |Coefficient.+------------+------ | Finer than | | Dirty Sand.|Washed |(Millimeters).| | | Sand. ----------------------+--------------+------------+------------+------ East London | 0.40 | 2.0 | 26.00 | 8.60 Grand Junction | 0.40 | 3.6 | 10.00 | 2.70 Southwark and Vauxhall| 0.34 | 2.5 | | 3.90 Lambeth | 0.36 | 2.4 | | 2.60 Chelsea | 0.36 | 2.4 | | 2.10 +--------------+------------+------------+------ Average | 0.37 | 2.6 | 18.00 | 3.98 ----------------------+--------------+------------+------------+------ II. GERMAN WORKS. ----------------------+--------------+------------+------------+------ Stralau | 0.34 | 1.7 | 12.20 | 4.00 Tegel | 0.37 | 1.6 | 11.00 | 3.00 Müggel | 0.34 | 2.0 | 10.80 | 0.80 Altona | 0.34 | 2.3 | 9.00 | 1.50 Hamburg | 0.31 | 2.3 | 8.20 | 1.07 +--------------+------------+------------+------ Average | 0.34 | 2.0 | 10.25 | 2.07 ----------------------+--------------+------------+------------+------ [Illustration: PLACING SAND IN A FILTER, HAMBURG. [_To face page 28._]] The averages show the effective size of the English sands to be slightly greater than that of the German sands—0.37 instead of 0.34 mm.—but the difference is very small. The entire range for the ten works is only from 0.31 to 0.40 mm., and these may be taken as the ordinary limits of effective size of the sands employed in the best European works. The average for the other sixteen works given above, including dune-sands, is 0.31 mm., or, omitting the dune-sands, 0.34 mm. [Illustration: FIG. 3_a_.—SAND ANALYSIS SHEET, WITH ANALYSES OF SEVERAL EUROPEAN FILTER SANDS.] It is important that filter sands should be free from lime. When water is filtered through such sands, no increase in hardness results. When, however, water is filtered through sand containing lime, some of it is usually dissolved and the water is made harder. The amount of lime taken up in this way depends both upon the character of the sand, and upon the solvent power of the water; and it does not necessarily follow that a sand containing lime cannot be used for filtration, but a sand nearly free from lime is to be preferred.