52 , TKANSACTIONS OF THE [XOV. 29, 



up into nitrites, nitrates, carbon dioxide, and other partially or 

 wholly oxidized compounds. As a result, where the amount of 

 incoming sewage is large, the number of microbes is great, and 

 so likewise the quantities of the oxidized products of decomposi- 

 tion, the nitrates and carbon dioxide especially. At the same 

 time, the oxygen which should always be present in notable 

 quantity in good water, was shown to have undergone a process 

 of exhaustion, and to have fallen below its normal amount. 

 When we replace the oxygen so absorbed, we supply the bacteria 

 with something essential to their beneficent labors, and without 

 which a portion of the sewage remains unoxidized. After the 

 work of these oxygen-absorbing microbes is completed, they 

 either perish or remain as resting spores, and tlien tliey sJiould 

 be removed together with the 2}roducts of their labors by filtration. 



ASSISTED PRECIPITATION". 



Recourse is had to this process under the following conditions : 



1st. When the particles of clay and other suspended matters 

 are so minute that they pass through the pores of a filter with- 

 out being arrested. 



2d. When lime, magnesia, and other salts are present in 

 such large amounts that the water is hard, rendering it unfit for 

 laundry use, and making scale in boilers. 



3d. When the water is discolored, and more especially when 

 the discoloration is the dark yellow stain due to peaty matters. 



After trials upon a great number of substances, sulphate of 

 alumina has been found to be the best precipitant for substances 

 of the first and third class. It throws down both the suspended 

 matters and the peaty coloring substances by forming with them 

 an insoluble coagulum. The alumina salt is decomposed into a 

 basic aluminic sulphate which, being itself insoluble, is precipi- 

 tated at the same time, and is filtered out along with the sub- 

 stances it has united with and thrown down, no alumina remain- 

 ing in the water. The action of clay, which is the hydrated 

 silicate of alumina, in purifying drinking waters is well known, 

 the clay having a strong affinity for organic coloring matters 

 and the ammoniacal and albuminoid substances arising from the 

 putrescent decomi:)osition of animal and vegetable substances. 

 But precipitation and clarification by the hydrated silicate of 

 alumina is too slow for mechanical ])ur\Qcat\on, and the sulphate 

 must be employed, or the double sulphate of alumina and am- 

 monia, which is common alum. The amount of alum requisite 

 is extremely small, one grain to the gallon being ordinarily 

 sufficient. It communicates no taste to the water, nor does it 

 have any effect upon the digestion or health of the persons using 



