160 MICRO-OKGAJS'ISMS IN WATER 



means of sand-filtration, but its application to sewage 

 has been made the subject of most exhaustive investiga- 

 tions in America. The experiments conducted by the 

 State Board of Health of Massachusetts ^ form a classi- 

 cal piece of work on the question of sewage purification 

 by intermittent filtration and chemical precipitation ; 

 but it is only possible here to indicate the lines upon 

 which these researches were undertaken, and to give a 

 brief summary of the results obtained. 



Experimental tanks were placed in the open and 

 under cover, the former were made of cypress, circular 

 in plan, sixteen feet eight inches in diameter inside at 

 the bottom, seventeen feet four inches at the top, and 

 six feet deep inside. The tanks were made completely 

 water-tight, and in each an under-drain fifteen feet in 

 length, of horse-shoe section of about two square inches 

 in area, was placed with open part downwards, and 

 half an inch above the bottom, resting on blocks six 

 inches apart ; the floor of the tank was covered with 

 one layer of coarse gravel stones about one inch by 

 two inches, this by another layer of smaller size, upon 

 which followed successive layers of gravel, the particles 

 of which diminished in size to one-eighth of an inch in 

 diameter, the thickness of the stratum of gravel being 

 three and a half inches. This fine gravel was covered 

 with a layer of very coarse mortar-sand, three and a 

 half inches deep in the middle of the tank. This sub- 

 stratum, as described above, was the same for all the 

 tanks, whilst to each tank was further added a special 

 layer at the top consisting respectively of different kinds 

 of sand, peat, river silt, brown soil, &c., the filtering 



^ Experimental Investigations hy the State Board of Health of 

 Massachusetts upon the Purification of Sewage, 1888-1890, Part II. 

 Boston, 1890. The Twenty-fourth Annual Eeport of this Board (1893) 

 also contains a very large number of further investigations on both the 

 purification of sewage and water by filtration. 



