What is the nature of oxygen? What is its purpose in the body and how does it serve this purpose? How is the blood able to take it up at the lungs and give it off at the cells? What becomes of it after being used? These are questions touching the maintenance of life and they deserve careful consideration. *Nature of Oxygen.*—To understand the relation which oxygen sustains to the body we must acquaint ourselves with certain of its chemical properties. It is an element(44) of intense affinity, or combining power, and is one of the most active of all chemical agents. It is able to combine with most of the other elements to form chemical compounds. A familiar example of its combining action is found in ordinary combustion, or burning. On account of the part it plays in this process, oxygen is called the _supporter of combustion_; but it supports combustion by the simple method of uniting. The ashes that are left and the invisible gases that escape into the atmosphere are the compounds formed by the uniting process. It thus appears that oxygen, in common with the other elements, may exist in either of two forms: 1. That in which it is in a _free_, or uncombined, condition—the form in which it exists in the atmosphere. 2. That in which it is a part of compounds, such as the compounds formed in combustion. Oxygen manifests its activity to the best advantage when it is in a free state, or, more accurately speaking, when it is passing from the free state into one of combination. It is separated from its compounds and brought again into a free state by overcoming with heat, or some other force, the affinity which causes it to unite. *How Oxygen unites.*—The chemist believes oxygen, as well as all other substances, to be made up of exceedingly small particles, called _atoms_. The atoms do not exist singly in either elements or compounds, but are united with each other to form groups of atoms that are called _molecules_. In an element the molecules are made up of one kind of atoms, but in a compound the molecules are made up of as many kinds of atoms as there are elements in the compound. Changes in the composition of substances (called chemical changes) are due to rearrangements of the atoms and the formation of new molecules. The atoms, therefore, are the units of chemical combination. In the formation of new compounds they unite, and in the breaking up of existing compounds they separate.