This section is from the book "Applied Science For Metal Workers", by William H. Dooley. Also available from Amazon: Applied Science For Metal Workers.
When elements combine to form chemical compounds, they unite according to fixed proportions. To illustrate: When water is formed from the combination of hydrogen and oxygen there must be 2 parts of hydrogen to 1 part of oxygen. These gases must unite in this proportion or water will not be formed. Thus 2 atoms of hydrogen and 1 atom of oxygen unite to form 1 molecule of water.
 
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