In the first place, it is passed into a second division of the tank, where it is subjected to some violent electric action till every kind of organic germ it may contain is supposed to be completely destroyed.

It is then passed through several covered channels and mechanically or chemically cleansed from every kind of inorganic impurity, and finally oxygenated or aerated with air which has undergone a yet more elaborate purification.

At every stage in this process, a phial of water is taken out and examined in a dark chamber by means of a beam of light emanating from a powerful electric lamp and concentrated by a huge crystal lens.

If this beam detect any perceptible dust or matter capable of scattering the light, the water is pronounced impure and passed through further processes.

Only when the contents of the bottle remain absolutely dark, in the midst of an atmosphere whose floating dust renders the beam visible on either side, so that the phial, while perfectly transparent to the light, nevertheless interrupts the beam with a block of absolute darkness, is it considered fit for human consumption.