necessary oxygen, or just the parts which they play in the process of decomposition, are not completely known, although much progress has been made in their determination. It is positively known that their presence and that of uncombined oxygen are essential. Certain species of these bacteria will live and work only in the presence of sunlight and oxygen; these are known as aerobic bacteria. Other species, forming a class known as anaerobic bacteria, live and effect their operations in the absence of sunlight and oxygen in that offensive mode of decomposition which takes place in cesspools and other closed receptacles for sewage and waste matter. They play an essential part in what promises to be one of the most valuable methods of sewage-disposal in which the septic tank is a main feature. =219. Slow Filtration through Sand—Intermittent Filtration.=—In the slow sand filtration method of purifying the water of a water-supply the aerobic bacteria only act. In order that their operations may be completed, free oxygen and sunlight are essential requisites, and the first of these is found in every natural water which can be considered potable. Any water which does not contain sufficient free oxygen for this purpose is to be regarded with suspicion, and generally cannot be considered suitable for domestic purposes. The amount of uncombined oxygen contained in any potable natural water is greatly variable and changes much with the period of exposure in a quiet state, as well as with pressure and temperature. In the river Seine it has averaged nearly 11 parts in a million throughout the year, being lowest in July and August and highest in December and January. It has been found in the experimental work of the Massachusetts State Board of Health that free or dissolved oxygen in potable water may vary from 8.1 parts at 80° Fahr. to 14.7 parts by weight at 32° Fahr. in 1,000,000 at atmospheric pressure. [Illustration: FIG. 4.] [Illustration: No. 1. CROSS-SECTION AT NORTH END OF BED.] [Illustration: No. 2. CROSS-SECTION AT BEGINNING OF PIPE UNDERDRAIN.] [Illustration: No. 3. CROSS-SECTION AT SOUTH END OF BED.] [Illustration: No. 4. CROSS-SECTION AT END OF LOWEST GRAVEL UNDERDRAIN.] [Illustration: No. 5. LONGITUDINAL SECTION OF A BED, AT WESTERLY END OF FILTER. TYPICAL SECTIONS OF UNIT BEDS IN LAWRENCE CITY FILTER. APRIL, 1901. COPIED FROM PLAN FURNISHED BY A.D. MARBLE, CITY ENGINEER.]