ON THE CIRCDLATION OF UNDERGROUND WATERS, 235 



up in the series. The first of these gave 0"146 of a gallon to the cubic 

 foot, or 13,000,000 gallons to the square mile 3 feet thick ; and the second 

 2'202 gallons to the cubic foot, or 184,000,000 gallons to a square mile 

 3 feet thick. We may, therefore, take it that the yield of water from 

 the Inferior Oolite varies between those limits. 



The Relation of Specific Gravity to Porosity. — In the report on Selec- 

 tion and Decay of Stone of the Houses of Parliament in 1839 ^ it is said 

 that the specimens of rock which had the greatest specific gravity absorb 

 the least quantity of water, though there are individual exceptions. I 

 cannot say that my observations bear out this rule, and I doubt whether 

 any such rule can be laid down. 



The Relation of the Size of Grains composing a Rock to the Porosity. — • 

 In the tabulated results I have given a column in which the size of the 

 grains composing the rocks is given. The object of this was to ascer- 

 tain whether any relation existed between the size of the grains and the 

 porosity. In the sandstones and grits there appears to be no connection 

 whatever, hut 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 ib 

 becomes. 



The Privity of the Water. — On account of shallow well-water being 

 almost invariably contaminated with organic matter, the Royal Commis- 

 sioners on Rivers Pollution have classed shallow well-water as dangerous. 

 On the other hand, deep well-water is classed as wholesome. It is, there- 

 fore, clear, that if shallow well-water is dangerous and deep well-water 

 wholesome, there must be a purifying process going on during the 

 percolation of water through the strata. I have given the analyses of 

 samples of Millstone Grit and of the Pennant Grit, and on an examination 

 of these it will be seen that the grits are pi-actically composed of grains of 

 silica. There is, therefore, nothing in the chemical composition of the 

 rock which could purify the water except mechanically, and in order 

 to get rid of organic contamination there mast 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 water-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 conditions 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 water. I 

 have found the volume of water absorbed, and reduced it to the weight of 

 air, assuming that water at 62° 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 



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