Working with this order-producing instrument, they have regarded the properties of elements as properties of the atoms, or of groups of a few of the atoms, of these substances.

That they might think clearly and suggestively about the properties of elements, and connect these with other chemical facts, they have translated the language of sense-perceptions into the language of thought, and, for _properties of those substances which have not been decomposed_, have used the more fertile expression _atomic properties_.

When a chemist thinks of an atom, he thinks of the minutest particle of one of the substances which have the class-mark _have-not-been-decomposed_, and the class-name _element_.

The chemist does not call these substances elements because he has been forced to regard the minute particles of them as undivided, much less because he thinks of these particles as indivisible; his mental picture of their structure as an atomic structure formed itself from the fact that they had not been decomposed.

The formation of the class _element_ followed necessarily from observed facts, and has been justified by the usefulness of it as an instrument for forwarding accurate knowledge.