This chapter is admirably written and covers the ground in a very satisfactory way. In general, it is above the writer's plane of criticism . There is a typographical error in the table giving the subsiding value of silt. In this, the time required for particles 0.01 mm. in diameter to settle one foot should be 330, not 33.0 minutes. In the bibliography, the reference to paper by the Sanitary Research Laboratory, M. I. T., 1917, should be to an "otherwise unpolluted," not "otherwise polluted" stream. Regarding oily films on water and their tendency to prevent aeration, Professor Gorham of Brown University has found in the laboratory that these films do not interfere with the absorption of oxygen, but as every one knows, they do prevent wave action, and wave action furthers absorption.