to nearly the normal, would be hardly appreciable. The results at Lawrence have shown that with very fine sands 0.09 and 0.14 mm., and 4 to 5 feet deep, with the quantity of water which can practically be made to pass through them, it is almost impossible to drive more than an insignificant fraction of the bacteria into the effluent. Even when the sands are entirely new, or have been scraped or disturbed in the most violent way, the first effluent passing, before the sediment layer could have been formed, is of good quality. Still finer materials, 0.04 to 0.06 mm., as far as could be determined, secured the absolute removal of all bacteria, but the rates of filtration which were possible were so low as to preclude their practical application. With coarser sands, as long as the filter is kept at a steady rate of filtration, without interruptions of any kind, entirely satisfactory results are often obtained, although never quite so good as with the finer sands. Thus at Lawrence the percentages of bacteria (_B. prodigiosus_) appearing in the effluents under comparable conditions were as follows: 1892 1893 With effective grain size 0.38 mm .... 0.16 With effective grain size 0.29 mm .... 0.16 With effective grain size 0.26 mm .... 0.10 With effective grain size 0.20 mm 0.13 0.01 With effective grain size 0.14 mm 0.04 0.03 With effective grain size 0.09 mm 0.02 0.02 We may thus conclude that fine sands give normally somewhat better effluents than coarser ones, and that they are much more likely to give at least a tolerably good purification under unusual or improper conditions. EFFECT OF GRAIN SIZE UPON FREQUENCY OF SCRAPING. The practical objection to the use of fine sand is that it becomes rapidly clogged, so that filters require to be scraped at shorter intervals, and the sand washing is much more difficult and expensive. The quantities of water filtered between successive scrapings at Lawrence in millions of gallons per acre under comparable conditions have been as follows: 1892 1893 Effective size of sand grain 0.38 mm .... 79 Effective size of sand grain 0.29 mm .... 70 Effective size of sand grain 0.26 mm .... 57 Effective size of sand grain 0.20 mm 58 .... Effective size of sand grain 0.14 mm 45 49 Effective size of sand grain 0.09 mm 24 14 The increase in the quantities passed between scrapings with increasing grain size is very marked.