=To make or liberate Oxygen.=--If there is a chemistry class in school, one of its members will doubtless be glad to prepare some of the gas called _oxygen_, and furnish several glass jars filled with it to the biology class. If it is desired to make oxygen, the following method may be employed: Provide a dry glass flask of three to four ounces capacity. It should have a glass delivery tube, inserted through a one-holed rubber stopper, and so bent as to pass under the surface of water contained in a deep dish. Fill several pint fruit-jars with water, cover with pieces of stiff pasteboard, and turn mouth downwards in the dish of water. From one half to two thirds ounces of an equal mixture of potassium chlorate and manganese dioxid (procured at drug store) is put in the flask and heated by means of a gas or alcohol lamp. When the oxygen begins to form, collect some in jars by inserting the end of delivery tube under the jars as they stand in water. _Caution_: Remove delivery tube from water before cooling the flask, to prevent any water being drawn back. =Oxidation.=--That something besides wood or coal is necessary to a fire can be shown by shutting off entirely the draught of a stove. Fire and other forms of combustion depend on a process called _oxidation_. This consists in the uniting of oxygen with other substances. When wood decays, the carbon in it oxidizes (unites with oxygen) and _carbon dioxid gas_ is formed. When wood burns, the oxidation is more rapid. When iron oxidizes, _iron rust_ is formed. When hydrogen is oxidized, water is formed. Kerosene oil contains hydrogen, and water is formed when it is burned. Almost every one has noticed the cloud of moisture which collects on the chimney when the lamp is first lighted. By using a chimney which has been kept in a cold place, the moisture becomes apparent; soon the chimney becomes hot and the water no longer collects, but it continues to pass into the room as long as the lamp burns. Fats also contain hydrogen. Hold a piece of cold glass or an inverted tumbler above the flame of a tallow candle. Does water collect on it?