DRINKING WATER FOR TRAVELLERS IN THE TROPICS 511 
surface and by reversing the current of water through it. It could be freed of 
organic matter if necessary by putting it in the fire, and it could readily be 
sterilized by boiling. 
B. If preferred, alum could be used to precipitate the mud. Either potassium 
or ammonium alum would serve. One grain of alum to the gallon of water 
might be sufficient for slightly turbid water. A muddy water may need 7 to 8 gr. 
A slight excess of alum can be tasted but does no great harm. The amount of 
alum required depends upon the turbidity and reaction of the water and must be 
determined by trial in each case. 
Peaty water may be slightly acid, and might require the addition of a little 
calcium hydroxide (lime) or sodium carbonate to render it alkaline in order to 
obtain precipitation by the alum. 
Mr. Melville C. Whipple, Assistant Professor of Sanitary Chemistry, of the 
Harvard Enginering School, told me that he believed a Berkefeld filter coarse 
enough to be of practical use in the field would still be so fine as to become 
clogged inside with fine silt which could not be easily removed from the pores of 
the filter. 
Mr. Whipple thought that some chemical method would be more practical 
and advised the use of alum. Aluminium sulphate, he said, has certain ad- 
vantages, but should not be chosen for use in the field because it tends to absorb 
moisture. Crystalline ' potassium or ammonium alum would serve. 
The amount of alum necessary would depend upon the turbidity of the water. 
The exact amount required in the special instance would have to be determined 
by trying it. The precipitate, fine at first, should be fully formed within an hour. 
Water having only a slight turbidity could be clarified by the addition of one 
grain of alum per gallon of water, but very turbid water might require six to eight 
grains per gallon. A very turbid, soft water might require the addition of so 
much alum as to render it slightly acid. It would then be necessary to alkalinize 
the water in order to obtain proper precipitation. For this purpose a small 
quantity (about one-third grain per gallon for each grain per gallon of alum) 
of sodium carbonate (washing soda) or of calcium hydroxide (lime) could be 
added. Hard waters are sufficiently alkaline and therefore would not require 
the addition of an alkali. 
Precipitation by alum goes on satisfactorily in cold water. The reaction can 
be speeded up by heat. The water may be brought to a boil, but, should it be 
kept boiling, the precipitate would be broken up. The process of precipitation 
in heated water can be completed in a few hours. In cold water precipitation 
would certainly be complete after standing over-night. 
The use of alum would not interfere with subsequent chlorination, or with the 
use of a Berkefeld filter, provided that the alum precipitate was first allowed to 
settle. 
Most if not all the turbid water which we encountered in Liberia and in the 
Belgian Congo was hard, and therefore would not have required the addition of 
an alkali had alum been used to clarify it. 
1 Powdered alum might cake, but its use would not be objectionable. 
