Water in its most polluted form undergoes certain chemical and physical changes which have a distinct tendency to restore it to a wholesome condition. Under all circumstances water contains air, the oxygen of which acts with energy on septic bodies, causing them to undergo a new form of decomposition, resolving them into compounds of carbonic acid and ammonia. Further and even more destructive processes, the nature of which is not clearly understood, also take place under the influence of oxygen. It has been observed, for example, that water highly contaminated with sewage, so as to be quite turbid, if left entirely at rest for a long period becomes absolutely odourless and perfectly transparent; and what is more remarkable, this change is not the result of the subsidence of solid particles, but of the oxidation and conversion of solid material into soluble compounds. It does not, of course, follow that water under such circumstances will be fit for drinking purposes, but the instance is a remarkable illustration of the changes which are effected under the influence of oxygen.

A Sewage Fungus, Beggiatoa alba (sulphur bacterium).

Fig. 478. - A Sewage Fungus, Beggiatoa alba (sulphur bacterium).

a, In a medium rich in sulphuretted hydrogen, b, Almost depleted of sulphur granules by twenty-four hours' immersion in water free from sulphur, c, Sulphur disappeared; transverse walls now visible, after forty-eight hours' further immersion, d. Decaying through lack of sulphuretted hydrogen.

Stagnant pools undergo a considerable amount of purification owing to the presence of living vegetation, particularly when the plants belong to the flowering order. It can be observed, even in so small a space as that furnished by an ordinary aquarium, that bubbles of gas are constantly being emitted from growing water plants or weeds, as they are called. In addition to the action of plants in furnishing oxygen, aquatic animals also contribute very largely towards the destruction of organic bodies. Myriads of minute creatures belonging to the infusoria spend their lives in the assimilation of organic substances; in fact, the presence of these animaleulae and plants may be accepted as a proof that the water is capable of supporting animal and plant life. On the other hand, however, it has been observed by Bennett, Rafter, and other writers referred to by the author of Water in Relation to Health and Disease, that there are numerous living beings contained in water which tend rather to add to its pollution than to remove it. It would appear that nearly all the varieties of aquatic fungi derive their sustenance from decomposing substances, and their presence in water is proof in itself of the existence of septic material. Such organisms as live on decomposing bodies are described as saprophytes. The sewage fungus (fig. 478) is a notable instance of an organism of the class referred to, and it is most easily recognized in the description which is given of it as "a dirty-looking, jelly-like layer covering the bottom and sides of the water-course in which it occurs ". Its presence may always be taken as proof of the existence of sewage contamination.

Besides the various fungi which are found in streams, and water-courses, and drain-pipes, there are numerous algae, of which the blanket-weed (fig. 479) is a well-known example. Mr. Bennett describes algae in two distinct forms, the blue-green alga (fig. 480)and the chlorophyll-green alga (fig. 481). The first gives oft' a small amount of oxygen insufficient to exert any useful oxidizing function, while both excrete fetid gases during their decomposition; consequently, when found in water, they may be taken as an indication that it is unfit for use. Of the second class, one family (Conjugatae) is distinguished by the peculiarity of extreme sensitiveness to the influence of decomposing substances. They can only live in water which is charged with oxygen. Their presence, therefore, in a flourishing condition may be accepted as proof that the water is free from any large amount of objectionable organic constituent. It may be further noted of these plants that while they demand a large quantity of oxygen as a condition of their own life, they give off a considerable quantity of the same gas, to the manifest benefit of the water in which they reside.

Blanket weed.

Fig. 479. - Blanket-weed.

1, Anabaena flos-aquae. 2, Coelosphaerium Kutzingianum, with detached cells.

Blue green Algae.

Fig. 480. - Blue-green Algae.

1, Spirulina Jenneri. 2, Oscillaria insignis.

Chlorophyll green Alga (Vaucheria sessilis).

Fig. 481. - Chlorophyll-green Alga (Vaucheria sessilis).

1, Plant. 2, Portion of filament enlarged. 3, Swarm spore. 4, Portion of the same enlarged.

Volvos globator (colony).

Fig. 482. - Volvos globator (colony).

1, Antheroid. 2, Oosperm (mature). 3, Oogone. 4, Peripheral cells. 5, Antherozoids. 6, Mode of division of parent cell of a zoosporange.

An illustration of the purifying influence of plant-life on water is quoted as having occurred in India some years ago, when, by some accident, all the aquatic plants were removed from the water-tanks. The consequence was that the previously wholesome water quickly became unfit for consumption. According to Rafter, writing in the transactions of the American Society of Civil Engineers, the well-known and extremely beautiful alga, the Volvox globator (fig. 482), has on certain occasions appeared in enormous quantities in the reservoirs which supply Rochester in the State of New York, imparting to the water a fishy taste and odour, and apparently causing sickness and death among the cattle which drank it. The stoneworts (fig. 483), so called from becoming coated over with an earthy deposit, when existing in large quantities give off sulphuretted hydrogen, which is a highly poisonous gas. A variety of fresh-water sponge has been identified as giving a nauseous odour and taste to water, owing to the presence of ammonia. Its removal from the places in which it grew was followed by the restoration of the water to a wholesome condition.

Mr. Francis, of Adelaide, records that in 1878 the lakes which form the estuary of the Murray contained a plant which he believed to be allied to the Protococcus, which formed a thick scum like green paint, some 2 or 3 inches thick, on the surface of the water, and when swallowed by cattle, which drank of it, it rapidly caused death.

Numerous other instances might be referred to in proof of the fact that while certain plants, probably without exception all flowering plants, and to a large extent all the high order of green plants which flourish in water and give out oxygen, particularly during the daytime and when the sun is shining, exercise a purifying influence upon water, there are others, including a number of aquatic fungi, which either exercise an injurious influence or indicate by their presence that the water is unfit for use. It is, therefore, a matter of considerable importance that the very common procedure of weed-cutting should be exercised with discrimination. All the aquatic plants which are beneficial, as well as those which are injurious, to water, are distinguished by botanical characteristics which can be readily identified by an expert. There will be no real difficulty, therefore, in determining, at any rate within certain reasonable limits, what plants should be as far as possible extirpated and which of them should be allowed to flourish.