126
10. After filtration the water is of a very high standard of purity, notwithstanding its occasional milky appearance. In fact there are very few water supplies equal to this, and I know of none that rank above it. The organic constituents and Chlorine are exceptionally low, and even in its worst condition, the water is softer and contains less total solids than almost any public supply in Great Britain.
11. Cause of WHITISH APPEARANCE :· In my opinion the cause of the whitish appearance does not originate in the reservoirs or filtering beds as has been supposed, but must be referred to the source of the water.
12. The water falling as rain permeates the rocks, and yields up its dissolved Oxygen and Carbonic Acid to substances capable of combining with them, often penetrating great distances, and probably meeting with subterranean sources of Carbonic Acid which it absorbs, thereby increasing its solvent action. The rocks most acted upon are felspathic, consisting chiefly of Alumina, Iron, Calcium, Mag- nesia, Silica, and the Alkalis. The action of a soft aërated water on such rocks consists not merely in solution, but in the decomposition of the Silicates. The porosity of rocks which such a penetration presupposes is of a very general character, even as regards the most compact rocks. Calcium Silicate is readily decomposed by a solution of Carbonic Acid, Alkaline Carbonates, but Magnesium Silicate is unaffected by either. Of the substances present in spring water, some such as the Earthy Carbonates are no doubt derived from direct solution of the rocks, but there are others such as the Alkaline Car- bonates which do not exist in any rocks. Such substances can only be supposed to originate from the decomposition of the minerals contained within the rocks. It is such a decomposition of minerals contained in rocks that in my opinion causes the whitish appearance of the Taitam and Pokfulam waters. This is not peculiar to Hongkong, but is known in a few other localities, one of which is Worcester, England. The action may be shortly described as consisting in the abstraction of the whole of the Alkali, two thirds of the Silica, and the assumption of water as shewn in the following simple chemical equation :-
Soda felspar.
Na, Al, Oja Sig
16
H2 Al2 Og Si2
Kaolin.
Water.
Carbon dioxide.
Co, Na, Cos
+
H2O 4 Si 0,
Silica.
Sodium Carbonate.
An ultimate product being a hydrated silicate of aluminium, existing as a soft white unctuous powder known as Kaolin. This on account of its low specific gravity and extremely fine state of division remains suspended in the water for an indefinite period, and practically defies simple filtration, even in small quantities except through its own sediment.
13. The following is an extract from a report on the Worcester Water Supply, by the City Analyst, Mr. H. SWETE :--
"The water is taken from the Severn one mile above the city, passed through filter beds "of sand and gravel, (which are cleansed weekly) then pumped up to a reservoir on a hill, "and supplied by gravitation to dwelling houses; 1,600,000 gallons are pumped daily. The "water contains peat and kaolin (white-water) from the decomposition of the Felspathic "rocks in Montgomeryshire......White-water cannot be clarified by filtration."
Below are the results of an analysis of the Worcester Water, and for the sake of comparison I attach a copy of a recent analysis of the supplies here. I refrain from making any further comment beyond pointing out that if the Carbonate of Lime in the Worcester Water Supply was removed, the figures for practical purposes would be identical. The amount of Carbonate of Lime in the Worcester Water is indicated by the degree of hardness
Results expressed in grains per imperial gallon.
Appearance in a 2 feet tube,
Worcester.
Taitam.
Pokfulam.
.......................brown and opaque. slightly opalescent. clear and colourless.
Smell when heated to 100° F.
slight
none
none
Total solid matter dried at 212° F.
15.54
....
2.57
4.4
Chlorine,
1.83
.63
.84
Hardness,.
10.4
1.20
1.2
Ammonia,
none
none
none
Albumenoid Ammonia,
.0084
none
trace
Phosphoric Acid,
trace
none
none
Nitrogen in Nitrates,
.029
trace
trace
Oxygen absorbed at 80° F. in 4 hours,.........
..1680
.002
.0025