Walthamstow and Chingford, and from the river Thames at Sunbury. The latter is used only occasionally, when the other supplies are low. The company have nine stations, a large number of reservoirs with filtering beds, &c.

The general composition of the water is very similar to that supplied by the New River Company.

(99) 4. The Southwark and Vauxhall Company.

This company supply water to the following:

Barnes; Battersea ; Bermondsey; Clapham; East Sheen; Ham; Kew; Mortlake; Putney; Petersham; Part of Richmond; Roehampton; Sheen; Wandsworth; Part of Wimbledon; St. Giles, Camberwell; St. Mary, Newington; St. Mary, Wimbledon; St. Mary, Lambeth; St. Mary, Rotherhithe; St. Saviour's, Southwark ; St. Olave's, Southwark; St. Thomas, Southwark.

The company also, in conjunction with the Lambeth company, supply parts of the parishes of Lambeth, Newington, Camberwell, St. George, Christchurch, Bermondsey, and Clapham.

The water is derived from the Thames, from which river the company is authorized to take twenty million gallons per day.

There are two sets of works drawing supply from the Thames into subsiding reservoirs.

The upper works send water into the Battersea works, where the same is there filtered, and pumped into the London district. At the lower works the water is filtered at Hampton, and from the two sets of works the district is supplied by the company.

The total length of the mains and service pipes is about 711 miles.

The average composition of the water delivered by this company is, in grains per gallon :- chlorine, 10; total nitrogen, 16; organic carbon, 12; hardness, 140; total solids, 19.7.

(100) 5. The West Middlesex Company.

The West Middlesex Company supply the following places and parishes:

Hampstead, conjointly with the New River Company; St. Pancras, ditto; St. Ann's, Soho, ditto; St. James, Westminster, ditto; St. Marylebone, conjointly with the Grand Junction Company; Paddington, ditto; Chiswick, ditto; Hammersmith, ditto; Kensington, ditto and Chelsea Company; Chelsea, conjointly with the Chelsea Company; St. Margaret's, Westminster, ditto; Fulham, ditto; also the suburban parishes of Willesden and Hendon.

The water supplied by the company is wholly derived from the Thames, the intake being from the Thames, above that of the Southwark and Vauxhall and Grand Junction Waterworks. The water flows direct from the river, through fine screens into the engine wells. The water thus taken in at Hampton goes to subsiding reservoirs at

Barnes, is filtered through eight filter beds having a joint area of twelve acres and from thence passes to the chief distributing station at Hammersmith. There are also large reservoirs at Kensington, Barrow Hill, and Kidderpore.

The general composition of the water is the same as that of the Vauxhall Company.

(101) 6. The Grand Junction Company.

This company supply water to the following places :

:

Part of St. James, Westminster; part of St. George, Hanover Square; a small part of St. Marylebone; Paddington; north part of Kensington; part of Hammersmith; Chiswick; Acton; Ealing; New Brentford; Heston; Hounslow; Isleworth ; Hanwell; Twickenham ; Hampton; Hampton Wick; Teddington; Hampton Court; Bushey Park; Whitton; Hanworth.

The water is derived wholly from the Thames, the intake being at Hampton. The company have five stations for filtering, storing and pumping purposes at Hampton, Kew Bridge, Campden Hill, Kensington, at Kilburn, and at Ealing.

Of the 48,000 houses supplied by the company at least 30,000 are supplied by the constant water system.

(102) 7. The Lambeth Waterworks.

The Lambeth Waterworks supply the following places :

Thames Ditton; Esher; Long Ditton; Kingston-upon-Thames; Maldon ; Morden; Wimbledon; Merton; Mitcham; Tooting Graveney; Clapham; Wandsworth; Battersea; Streatham; Croydon; St. Mary, Newington; Camberwell; Bermondsey; St. Mary, Lambeth; St. Saviour; St. George the Martyr; Christchurch.

The intake is from the Thames three miles above Hampton Court Bridge; from this intake an underground conduit over four miles in length conveys the water through East and West Molesey and Thames Ditton to the existing works at Ditton. There is also an intake at Ditton. There are eight pumping, filtering, and storing stations, viz., at Molesey, Ditton, Kingston Hill, Brixton, Streatham, Norwood, and Rockhill.

(103) 8. The Chelsea Company.

The places and parishes in which the Chelsea Water Company supply water are as follows:

St. Luke, Chelsea; part of All Saints, Fulham; part of St. Mary Abbotts, Kensington; Kensington Palace and Precincts; part of St. George, Hanover Square; part of St. James, Westminster; part of St. Martin in the Fields; St. Margaret, Westminster; St. John the Evangelist, Westminster.

The water is derived from the Thames, the intake being at Walton, on the south bank of the Thames. There are also stations at Walton and West Molesey, Surbiton, and Putney Heath.

(104) Summary of the London Water Supply.

The relative proportion from the various sources of supply was for year 1884 as follows:

the

From the Thames and certain chalk springs in

the Thames valley about

50 parts of the whole.

From the Lee and certain chalk springs in the
Lee Valley.

From eighteen chalk wells (about

eight in the north and ten in the south)}

38

12

(105) Purification of London Water Supply. Filter Beds.1

The water supplied from the Thames to London is all submitted to the purification processes of subsidence and filtration, the filter beds of the different companies vary in detail, but are all constructed upon the same plan; for example the four filter beds of the West Middlesex Company (6 acres in total area) at Barnes have a total thickness of filtering medium of 5 feet 6 inches, consisting of 2 feet 3 inches of Thames sand, 1 foot of Barnes sand, and 2 feet 3 inches of gravel of various degrees of coarseness; beneath the filter beds there are collecting drains of 6 inches diameter; they are pierced and laid 20 feet apart.

Efficient filtration largely depends on the rate or flow of the water through the filtering medium. It is laid down as an accepted standard that the rate of filtration for the Metropolitan waters should not exceed 540 gallons per square yard of filter bed each 24 hours, or 2 gallons per square foot per hour. None of the London Companies at the present time infringe this rule, and the filtration rate in practice is slower than the above.

1 One of the most interesting experiments on the purification of water on a large scale was made at Antwerp in 1881. The water was first passed through a layer of fine sand, and then through a filter of spongy iron mixed with gravel. This filter was however found to clog at the top rather rapidly and the iron also required somewhat frequent renewal. It was found on further experiment that the water could be purified more economically by scattering cast iron borings in the water as it flowed through horizontal revolving cylinders these cylinders were furnished with projecting plates fastened at intervals round the inside, as the cylinder revolved the iron was scooped up and discharged on reaching the top. The iron was in this process first converted into ferrous oxide, by the oxygen in the water; the ferrous oxide was next in part dissolved by the carbonic acid gas as bicarbonate, a salt very easily decomposed into ferric oxide, which finally precipitates carrying organic matter down with it, the precipitate being very easily removed by filtration through sand.

Even this simple filtration does not act entirely mechanically but in some degree destroys organic matter. The oxygen dissolved in the water and the thin film of air coating the little floating particles tend to oxidize the various debris; added to this, minute micro-organisms are ever at work disintegrating the substances upon which they feed. Hence these filters purify by mechanical means, by the aid of bacteria, by other vital processes, and by a purely chemical process of oxidation.

The upper layer of the filter beds gets from time to time clogged up, and has to be broken up and renewed.

CHAPTER XIV.

THE SCIENTIFIC EXAMINATION OF WATER.

(106) Purity of Water.

ABSOLUTELY pure water, consisting of a chemical union with condensation of two volumes of hydrogen and one of oxygen, is a chemical curiosity-the purest distilled water always having some slight traces of foreign matters. Pure water is, indeed, only obtained by very refined methods. That which occurs in nature invariably contains some saline and some organic matters with certain gases in solution. in solution. Whether a water is pure or impure is entirely relative, for no ordinary water is pure. What, however, the chemist and the hygienist mean by "impure water" is water which there is reasonable ground for believing may injure health.

Water may be polluted injuriously in various ways. The most direct pollution is that in which excremental matters are passed straight into the water as, for example, occurs in a stream wherever there is a floating population; it is much the same thing when sewage is conveyed into a stream: there are also the various effluent waters from factories, from mines, the dead bodies of animals, insects, leaves, the droppings from birds, and various forms of personal pollution; all these are matters more or less appreciable to the sight, but there are also those insidious forms of secret underground pollution as, for instance, the contamination of a well from a far-off cesspool or midden-heap, or from an imperfect drain.

It will also be remembered that even when the water has left a reservoir and is being conveyed within the main, chances of pollution are not exhausted. (See p. 148.)