CHURNING AND WASHING BUTTER. 251 



sterile. It used to be thought by older German investigators 

 that these different filtering substances had almost miraculous 

 power of removing organisms from water. 



The factors which are to be considered in successful filtra- 

 tion are: 



(1) Storage capacity for unfiltered water. 



(2) Construction of filter-beds. 



(3) Rate of filtration. 



(4) Renewal of filter-beds. 



(1) Concerning the storage capacity, nearly all creameries 

 have storage- tanks overhead in the creamery; so far as that 

 is concerned, however, filtration can be successfully carried on 

 continuously as well as intermittently. 



(2) The construction of the filter-bed used in the experi- 

 ment carried on at the Iowa Experiment Station, Ames, Iowa, 

 is as shown in Fig. 153. The approximate proportionate 

 depth of each layer in the bed is as follows, beginning at 

 the bottom : 



Two inches small flint stones; 22 inches fine sand; 12 

 inches fine coke ; 9 inches charcoal ; 2 inches fine stone or coarse 

 gravel. The layer of fine sand should not be less than 15 inches. 

 It has been asserted that a few pieces of old iron mixed in 

 the filter-bed are beneficial. Alum, lime, and copperas have 

 been recommended for clarifying and deodorizing very impure 

 water. As these substances are soluble they should not be used 

 in filter-beds, which are intended for the filtration of water 

 for potable purposes. The filtering-can was made from 22 

 galvanized iron. The height of can is 48 inches; diameter, 

 18 inches. The bottom of the can is slanting towards the 

 faucet, or opening. Thus no water is permitted to stand on 

 the bottom and afford opportunities for germs to accunnilate. 

 On the inside are three plates. One lies horizontally, near the 

 bottom, and upon it the filtering-material rests. Another lies 

 on the top of the fine sand. Both of these plates were per- 

 forated with many small holes. Near the top is placed a 

 concave plate with a hole near the center. This plate directs 



