VARIETIES OF CHARCOAL FILTERS. 109 



should contain an aggregate of charcoal at least one half 

 the weight of the sugar they are intended to purify. Each 

 filter consists of a box, whose cubic contents may vary 

 from 300 to 600 lbs. weight. Three inches from the base 

 is a false bottom, consisting of a metallic sheet, perforated 

 like a sieve. Between it and the bottom are two orifices, 

 one opening externally, the exit pipe for the filtered liquor ; 

 and the other internally, being the end of a tube passing 

 up through the box; for the air, which is contained in 

 the interstices of the charcoal, is pressed down by the 

 descending syrup, till it finds its way out by the tube : 

 this latter terminates within the box, so that in case of 

 puking, which sometimes happens through carelessness, 

 the syrup is returned into the upper strainers. The air 

 tube is sometimes on the outside of the vessel, in which 

 case its end is carried inwards so as to hang over the syrup. 

 Although the manipulation is easy, it may be as well to 

 revert to a few points which unaccustomed boilermen 

 might overlook. The false metallic bottom, pierced with 

 holes, is covered with a fold of blanket, first wetted and 

 slightly wrung out. It is so placed as to touch the sides 

 of the filter all round. Upon this cloth is spread a layer of 

 charcoal three inches deep. This is levelled with some degree 

 of pressure, and successive layers are added till the whole 

 attain a depth of from fifteen to eighteen inches, when it is 

 covered with a cloth and metallic plate similar to the first. 

 Instead of the metallic plate covering the whole surface, 

 many planters use merely a piece of thin copper about five 



