PASSAGE OF OXYGEN THROUGH THE BODY 105 



1. That in which it is in a free, or uncombined, condi- 

 tion 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 combina- 

 tion. 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 sub- 

 stances (called chemical changes) are due to rearrange- 

 ments of the atoms and the formation of new molecules. 

 The atoms, therefore, are the units of chemical combina- 

 tion. In the formation of new compounds they unite, and 

 in the breaking up of existing compounds they separate. 



The uniting of oxygen is no exception to this general 

 law. All of its combinations are brought about by the 

 uniting of its atoms. In the burning of carbon, for .exam- 

 ple, the atoms of oxygen and the atoms of carbon unite, 

 forming molecules of the compound known as carbon 

 dioxide. The chemical formula of this compound, which 

 is CO 2 , shows the proportion in which the atoms unite 

 one atom of carbon uniting with two atoms of oxygen in 

 each of the molecules. The affinity of oxygen for other 



