Medicine has its specific symbol which has come down to us from ancient times. Today military doctors of many countries wear badges on their shoulder straps in the form of a snake coiled around a staff or the stem of a cup. 

    Now there is a similar symbol in chemistry. It is a snake with its tail in its mouth. 

    The ancients had a cult of all kinds of mystic signs, the meaning of which is often difficult to explain today.

    So much for mystic signs, but the "chemical snake" has a quite definite meaning. It symbolizes a reversible chemical reaction. 

    Two atoms of hydrogen and one of oxygen combine to form a molecule of water. Simultaneously another molecule of water decomposes into its component parts. Two opposite reactions take place in the same instant: the formation of water (the forward reaction) and its decomposition (the back reaction). A chemist would represent these two contradictory processes as follows: 

The arrow pointing to the right indicates the forward reaction, and that pointing to the left, the back reaction. 
 

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