It is constructed of a glass or earthen jar (A), preferably square, to which is fitted a wooden top (B), this top being provided with a packing ring (C), so as to make it air-tight. Within is a vertical partition (D), the edges of which, below the cap, fit tightly against the inner walls of the jar. This partition extends down into the jar a sufficient distance so it will terminate below the water level. A pipe is fitted through the top on each side of the partition, and each pipe has a valve. An electrode, of any convenient metal, is secured at its upper end to the top of the cap, on each side of the partition. These electrodes extend down to the bottom of the jar, and an electric wire connects with each of them at the top. [Illustration: _Fig. 89._ DEVICE FOR MAKING HYDROGEN AND OXYGEN] If a current of electricity is passed through the wires and the electrodes, in the direction shown by the darts, hydrogen will form at the negative pole, and oxygen at the positive pole. These gases will escape upwardly, so that they will be trapped in their respective compartments, and may be drawn off by means of the pipes. PURIFYING WATER.--Advantage is taken of this electrolytic action, to purify water. Oxygen is the most wonderful chemical in nature. It is called the acid-maker of the universe. The name is derived from two words, _oxy_ and _gen_; one denoting oxydation, and the other that it generates. In other words, it is the _generator of oxides_. It is the element which, when united with any other element, produces an acid, an alkali or a neutral compound. RUST.--For instance, iron is largely composed of ferric acid. When oxygen, in a free or gaseous state, comes into contact with iron, it produces ferrous oxide, which is recognized as rust. OXYGEN AS A PURIFIER.--But oxygen is also a purifier. All low forms of animal life, like bacteria or germs in water, succumb to free oxygen. By _free oxygen_ is meant oxygen in the form of gas. COMPOSITION OF WATER.--Now, water, in which harmful germs live, is one-third oxygen. Nevertheless, the germs thrive in water, because the oxygen is in a compound state, and, therefore, not an active agent. But if oxygen, in the form of gas, can be forced through water, it will attack the germs, and destroy them. COMMON AIR NOT A GOOD PURIFIER.--Water may be purified, to a certain extent, by forcing common air through it, and the foulest water, if run over rocks, will be purified, in a measure, because air is intermingled with it. But common air is composed of four-fifths nitrogen, and only one-fifth oxygen, and, as nitrogen is the staple article of food for bacteria, the purifying method by air is not effectual. PURE OXYGEN.--When, however, oxygen is generated from water, by means of electrolysis, it is pure; hence is more active and is not tainted by a life-giving substance for germs, such as nitrogen. The mechanism usually employed for purifying water is shown in Fig. 90.