24 
THE POEOSITY AND DENSITY OF BOOKS 
water. I have found the volume of water absorbed, and reduced 
it to the weight of air, assuming that water at 6'^° F. is 819*4 
times heavier than air at the same temperature. By this means 
I find that a cubic foot of Inferior Oolite, absorbing 1 gallon of 
water, would, in the event of complete drainage off of the water, 
absorb 0*16 of its volume of air. In short, we find in the rocks 
of the earth much the same process going on naturally as the 
London water companies are doing artificially for the filtration 
of the London water-supply. The following is a section of the 
filter-beds of the Chelsea Waterworks, for which I am indebted 
to the manager, Mr. Lott : — 
ft. in. 
Fine Sand 3 6 
Shells^ ... ... ... ... ... ... 0 4 
Shingle 2 0 
Coarse Shingle^ 2 2 
Totals 
8 0 
The chemical analysis of the top bed of the filter, which does 
the work, gave the following : — 
Silica 
90*05 
Alumina 
-40 
Oxide of Iron 
6*90 
Carbonaceous Matter ... 
-70 
Carbonic Acid 
1*50 
Magnesia 
*01 
Alkalies ... 
Trace 
Moisture 
*26 
99*97 
^ To keep the upper layer from mixing 
with the lower. 
* Gradually increases in coarseness towards the base to prevent 
the pipes which carry off the water from becoming clogged. 
3 The arenaceous material is taken from the Thames, and is well 
cleansed by a powerful hose playing upon it. 
