Two theories are advanced to explain the blue color of water when seen in large masses - one, held by Prof. Tyndall, being that small solid particles suspended in the water do not reflect the lower or red rays of the spectrum. According to the other theory, the color is due to the absorbent action of the water itself on the white light before and after reflection by these particles. The results of experiments made by Mr. John Aitken, and presented to the Royal Society, England, show that the latter theory is probably the more correct one. The greater number of white reflecting particles the greener the water appears to be, and hence the gradual deepening of the green to blue as the shore is left. The waters of Lake Como owe their darkness to the absence of reflecting particles, as Mr. Aiken ingeniously proved by scattering finely-divided chalk in the centre of that lake, thereby producing a very brilliant blue. The brilliancy depends on the color of the particles, and is greatest with white particles. Among coral reefs, which are generally strewn with white sand, the water also takes a very brilliant blue or green. The dull tinge of some river-waters is due to the dingy character of the suspended silt; but springs have often a bright blue color, owing to the whiteness of the chalk suspended in them.

We often talk of or read about the blue Danube and the green Rhine, but the latter at Cologne and the former at Vienna hardly justify the designation, and might more literally be described as of a muddy brown. Victor Meyer has, however, been occupying himself with an inquiry into the actual color of perfectly pure water, and he finds that it should be described as neither blue nor green, but a shade between the two. To demonstrate this he takes five wide but thin glass tubes, 40 mm. in diameter and about 1 1/2 metre in length; these are connected by means of caoutchouc tubing forming a tube about 7 1/2 metres long. Both ends of this tube are closed with even glass plates fitted in metal sockets. The latter are furnished with brass nozzles for filling the tube. The tube itself is placed in an exactly horizontal position and covered with a black cloth. Upon looking through the empty tube the field of vision appears perfectly colorless, the cloth and the metal sockets preventing the color of the glass from exerting any influence; directly, however, the tube is filled with distilled water, an intense bluish-green color is observed.

The characteristics of good water may be summed up as follows: It should be at all seasons clear, transparent, bright, and when seen in large bulk, pure blue, the natural color of uncontaminated water; it should be well aerated, holding in solution from seven to eight cubic inches of air per gallon, consisting of two or more cubic inches of oxygen and six of nitrogen; it should be free from living organisms, vegetable and animal, and from all dead, decomposing organic matter, and should not dissolve lead; it should hold only a moderate quantity of mineral matter in solution, and not deposit a coating of lime or magnesia when boiled.