WITH REGARD TO WATER SUPPLY. 
23 
between the size of the grains and the porosity. In the 
sandstones and grits there appears to be no connection 
whatever, but in the Magnesian Conglomerate the finer the 
material of which a bed is made up the more water there seems 
to be absorbed. In the case of the Oolites the more compact 
the rock the less porous it becomes. 
The Purity of the Water . — On account of shallow well-water 
being almost invariably contaminated with organic matter, the 
Royal Commissioners on Rivers Pollution have classed shallow’ 
well-water as dangerous. On the other hand, deep w^ell-water is 
classed as wholesome. It is, therefore, clear, that if shallow' 
well-water is dangerous and deep well-w'ater w’holesorne, there 
must be a purifying process going on during the percolation of 
W'ater through the strata. I have given the analyses of samples 
of Millstone Grit and of the Pennant (R’it, and on an examination 
of these it will be seen that the grits are practically composed 
of grains of silica. There is, therefore, nothing in the chemical 
composition of the rock which could purify the w’ater except 
mechanically, and in order to get rid of organic contamination 
there must be oxidation. We must, therefore, look to another 
source than the chemical composition of the rock for the oxidising 
agent, and I think it will be found in the air absorbed by the water 
and in the air contained by the rock. The water in the strata is 
constantly being drained by springs, wells, and outlets by which the 
w'ater-level is reduced. During a dry period, then, the interstices 
of the upper portions of porous rocks must be either occupied by 
air or there must be a vacuum. The former of these two con- 
ditions is the most probable, and it seems reasonable to assume 
that the oxygen of this air must oxidise any organic matter 
contained in water percolating through the earth. I have made 
observations with a view of ascertaining the volume of air 
absorbed by certain rocks. I have endeavoured to arrive at the 
result by displacing the air contained in given specimens by 
