CHAMBERS'S INFORMATION FOR THE PEOPLE. 



decomposed ; and the carbon of the decomposed 

 acid gives rise to a green vegetation, which soon 

 acquires an offensive marshy smell. Water should 

 never be allowed to stand in tanks without roofs 

 to protect it from the effect of the sun's rays, as, 

 before many days, even though there may be a 

 current passing through it, a rank green vegetation 

 is certain to commence. This is more particularly 

 observable in the waters derived from the New 

 Red Sandstone, which is used in Liverpool, where 

 green plants commence to shew themselves in a 

 very short time. 



Organic matter in a state of putrefaction forms 

 one of the worst possible kinds of contamination 

 in water. Wells in proximity to graveyards are 

 very often found to be contaminated by the infil- 

 tration of decomposed organic particles, which 

 are in reality a deadly poison. 



It is a common notion that every drop of water 

 teems with life; but this is a mistake. Deep 

 wells, and spring-water in general, contain little or 

 no living organic matter. Consequently, it is 

 quite possible to obtain a liquid perfectly free from 

 animalcules and vegetation. The presence of 

 living creatures, vegetable or animal, discernible 

 either by the naked eye or by the microscope, is a 

 proof of organic taint in the water, and is one of 

 the tests of this kind of impurity. With respect 

 to rain-water, Dr Hassall slates, in his evidence 

 before the General Board of Health : ' I have 

 made several examinations of rain-water imme- 

 diately after its descent to the earth, obtained in 

 both town and country, and can confidently assert 

 that it does not, in general, contain any form of 

 living vegetable or animal matter.' The conditions 

 necessary for the development of vegetation and 

 animalcules over and above the presence of matter 

 for them to feed on, are air, light, and stillness. 

 With regard to the probable effects on health of 

 living creatures contained in water, Dr Hassall's 

 observations are worthy of attention : ' All living 

 matter contained in water used for drink, since it 

 is in no way necessary to it, and is not present in 

 the purest waters, is to be regarded as so much 

 contamination and impurity is therefore more or 

 less injurious, and is consequently to be avoided. 

 There is yet another view to be taken of the 

 presence of these creatures in water namely, that 

 where not injurious themselves, they are yet to be 

 regarded as tests of the impurity of the water in 

 which they are found.' 



In wholesome natural waters impounded in 

 reservoirs or lochs, the vegetable growth consists 

 mainly of fresh-water algae, which adhere to and 

 grow on stones in the quiescent parts where there 

 is little current, and where the stones do not roll 

 over each other, but are stationary : specimens of 

 them are to be found in every Highland loch, 

 wherever it is sufficiently sheltered to permit of 

 their growth. Water-fleas, of which the principal 

 kinds are the Cyclops quadricornis and the 

 Daphnia pulex, are found in most lochs, where 

 they feed on the fresh r water algae. They are very 

 delicate organisms, and a very little alcohol added 

 to the water kills them, and if the temperature of 

 the water rises even to 100, they die at once. 



Means of Purifying Water. 



In all water contaminated by inorganic matter, 

 especially in running water, a process of spontan- 



602 



eous purification is continually going on. Some 

 of the noxious matter is removed by fish and other 

 animal life, and a further quantity is absorbed 

 by the growth of aquatic vegetation ; but, in addi- 

 tion to these abstractions, important changes are 

 effected by chemical action. The organic com- 

 pounds dissolved in the water are of very un- 

 stable constitution, and very easily decomposed. 

 The great agent is oxygen, and the process is 

 considerably hastened by the motion of the water. 

 Now, as such waters always contain naturally 

 much oxygen dissolved in them, the decomposing 

 agent is ready at hand to exert its influence the 

 moment the matter is received into the water. 

 Were it not for this beneficent provision, a stream 

 once polluted would continue so through its whole 

 course. In the case of streams passing through 

 populous districts, the contamination goes on at a 

 rate far beyond the power of natural purification ; 

 but by means of it, streams that receive only a 

 moderate degree of contaminating matter in their 

 course are kept in a tolerable state of purity. It 

 is chiefly owing to this process that the present 

 water supply of London, which is taken from the 

 Thames at various points above Teddington Lock, 

 and thus washes a populous and cultivated area, 

 is not so excessively contaminated as is commonly 

 supposed, although it is still far from being a 

 desirable water. 



The mechanical impurities of water, or the solid 

 particles rendering it muddy or milky, may in 

 most cases be removed by mechanical means. 

 The two processes for this purpose are subsidence 

 and filtration. The effects of subsidence are 

 strikingly seen in the case of rivers that pass 

 through lakes. Take, for example, the Rhone, 

 which enters the Lake of Geneva of the colour of 

 pease-soup, while it issues from the other end clear 

 as glass and blue as the sky. 



In constructing an artificial filter on a large 

 scale, a basin is formed, having the floor nearly 

 level, but slightly inclining towards a centre line, 

 and made water-tight by puddling the bottom and 

 sides with clay. On the floor is laid a series of 

 layers of gravel, coarse at first, and getting gradu- 

 ally finer upwards ; next, a layer of slate-chips or 

 sea-shells ; then one of coarse sand, on which is 

 placed the actual filtering layer of fine sand. The 

 depth of this layer is from twelve to thirty inches, 

 that of the entire mass from four to six feet. The 

 water being admitted gently on the top of the 

 sand, sinks down, and is conducted by a series of 

 channels, generally of tile-pipes, into the main 

 drain. A filter in a clean state will pass from 

 twelve to eighteen vertical feet of water in twenty- 

 four hours. The solid matter intercepted does not 

 penetrate more than three-fourths of an inch into 

 the sand, so that, by removing a very thin film 

 from the surface, the filter is again clean. What 

 is scraped off the top, is capable of being washed 

 and put again to use. 



The cleansing power of sand can hardly be 

 accounted for on the theory of mere mechanical 

 interception. Though there is no chemical action, 

 strictly speaking, there is no doubt that the attrac- 

 tion of adhesion is at work a power that plays a 

 greater part in natural processes than has gener- 

 ally been assigned to it. Some substances mani- 

 fest this adhesive attraction more strongly than 

 sand, and have, therefore, still greater efficacy as 

 filters ; though practically, and on the large scale, 



