WITH KEGAED TO WATEE SUPPLY. 
25 
The rate of filtration is 2 gallons per hour.^ By comparing 
the chemical analysis of the filter-hed with those of the Millstone 
Grit and of the Pennant Grit, it will be seen that to all practical 
purposes the analyses are the same. There is nothing in the 
chemical composition of the filter which can oxidise the organic 
impurities of the Thames water passing through, but the oxida- 
tion is effected by air between the grains of sand with perfect 
effect, It is much the same with water percolating through the 
rocks of the earth ; it comes in contact with air colleg^ted in the 
interstices. "With such rocks as the Mountain Limestone, how- 
ever, where the water yielded comes through fissures and joints 
in the strata, and does not percolate, it is a question whether the 
purifying process would be always satisfactory. 
I Colonel Bolton’s Keport for February, 1882, for which I am 
indebted to the courtesy of Colonel Bolton. 
