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Webster 1913 Edition

Noun.

State or quality of being solid; firmness; compactness; solidity, as of material bodies.

Soundness; strength; truth; validity, as of arguments, reasons, principles, and the like.

, n.

The quality of being firm, dense or compact; firmness; compactness; solidity; as of material bodies.

Soundness; strength; truth; validity; as of arguments, reasons, principles, &c.

solidness ‎(usually uncountable, plural solidnesses)

The state or quality of being solid.
When something melts it loses its solidness and becomes a liquid.