Chalk-water, which, as it issues from a spring, is perfectly free from organic matter, has a source of contamination within itself, When exposed to light and air, the dupli cate dose of carbonic acid that keeps the chalk dissolved, becomes decomposed; and the carbon of the decomposed acid gives rise to a green vegetation, which soon acquires an offensive marshy smell.
Organic matter in a putrefying state forms the worst kind of contamination that water can have. Though we may not know the precise effects of these impurities on the animal system, the single fact of their rendering the water repulsive to the taste and nauseous to the stomach would be sufficient to condemn their use. What is disagreeable to the senses, must be presumed to be unwhol&ome in addition, until the contrary is proved. Though no one has ever yet gone the length of maintaining, as a general ,truth, the wholesomeness of water abounding in vegetation, insects, and decaying matter, yet the water of the Thames, even within the influence of the tides, where it is contaminated by the whole sewage of the metropolis, found defenders until lately, on the plea that the amount of impurity was too small to do harm. This ground is at length given up; but Thames water above Teddington lock is still sanctioned as safe water for the companies to supply to the inhabitants of London, notwithstanding the sewage of the numerous populous towns that the river receives above that point. As to this plea of smallness of amount, the highest medical authorities hold that it is impossible to say how small a quantity of organic matter in a state of fermentation may not do harm. We are not, however, left merely to presume that organic impurity in water is prejudicial to health. During the cholera visitation of 1853-547 a gigantie experiment was undesignedly made on half a million of human beings. It so happened that a certain district of London was supplied by two rival water-companies, the two mains running often side by side, and some houses taking water from the one, and some from the other. The whole inhabi tants were living alike in all respects save one—viz., that•one company drew its water from high up the Thames, where it was of comparative excellence, while the other drew its water from low down the river, where it was profusely contaminated with town-drain age. Among this population, there were more than 4,000 deaths from cholera; and
When the epidemic had subsided, an inquiry was made, house by house, as to those deaths, and as to the water-supply of the several houses where they had occurred. The inquiry was conducted with every precaution, to avoid sources of fallacy; and the result was this: in the one set of houses, the mortality per 10,000 of the population was 37; in the other set of houses it was 130—that is to say, the cholera death-rate was 34 times as great in the one set as in the other.
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. Con sequently, it is quite possible to obtain a liquid perfectly free from animalcules and vege tation. 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 states, in his evidence before the general board of health: " I have made several examinations of rain water immediately after its dissent 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 creatures contained in water, Dr. IIassall'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 i be taken of the presence of these creatures in water—viz., that where not injurious them selves, they are yet to be regarded as tests of the impurity of the water in which they are found." Means of purifying Water.