Atoms tend to form groups known as molecules. The atoms in a molecule adhere with considerable force and some molecules can be broken up only with the greatest difficulty. These groups have a definite individuality and unless acted on from the outside they are apparently permanent. The same atoms may be grouped into quite different molecules just as the same bricks may be used to build a church or a jail, or the same letters used to form altogether different words. The individuality of a molecule is perhaps best appreciated by thinking of the individuality of a word. A word, though consisting solely of letters, has a definite unity of its own. A molecule made up of atoms is just as definite an aggregate.
Daniel F Comstock,
The nature of matter and electricity, an outline of modern views, p.5
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