26 Mr. H. M. Witt on the Power possessed by Porous Media 



only remove bodies niechanieally suspended in water, but I am 

 not aware that this statement has been established by experi- 

 ment; in fact, I am not acquainted with any published ana- 

 lytical examination of the efifccts of sand filtration. 



These experiments supply the deficiency, and show moreover, 

 that these porous media are not only capable of removing sus- 

 pended matter (80 to 92 per cent.), but even of separating a 

 certain appreciable quantity of the salts from solution in water ! 

 viz. from 5 to 15 per cent, of the amount originally present, 

 9 to 19 per cent, of the common salt, 3 per cent, of the lime, 

 and 5 of the sulphuric acid. 



It is curious also -that the proportion of matter removed in 

 this way depends to a certain extent upon the degree of impurity 

 of the water; the greater the quantity of matter originally 

 present in the water, the larger the per-centage ratio of the 

 salts removed, e. g. : — 



In Sept. In Dec. In May. 



Total impurity of water . . . 65-527 31*467 55-90 

 Per-centage of salts removed . 15*69 5*42 15*04 



This point will, however, become more apparent by comparing 

 these experiments with those made at Kingston, where the 

 water is much purer, to which I shall have occasion hereafter 

 more particularly to refer. 



This fact, of the power possessed by sand and other porous 

 media, which rests for demonstration not only upon the three 

 preceding experiments, but also upon others to be presently 

 described, is one of great importance, not only in a hygienic 

 and oeconomic point of view as relating to the great question 

 of water supply, but also in its bearings upon agriculture and 

 geology. 



It is possible that soils may remove matters from solution in 

 water, not only by decompositions between the contained salts 

 and the aluminous silicates of the soil (as demonstrated by Mc. 

 Way), but also in virtue of this peculiar action. ! 



Again, water containing considerable quantities of saline 

 matter in solution may, by merely percolating through great 

 masses of porous strata during long periods, be gradually de- 

 prived of its salts to such an extent as probably to render even 

 sea-water fresh. 



This may in fact be one of the causes contributing towards 

 the production of freshwater springs which ebb and flow with 

 the tide in the vicinity of the sea : for instance, Darwin, in the 

 ' Voyage of the Beagle' (vol. iii. p. 545), mentions that on 

 Keeling Island, one of the coral reefs near the coast of Sumatra, 

 there are freshwater wells which ebb and flow with the tide. 

 Mr. Darwin, however, suggests another explanation of this 



