What is to make thin or less consistent; to render less viscid or dense; to rarefy. Specifically: To subtilize, as the humors of the body, or to break them into finer parts called?

attenuate — To make thin or less consistent; to render less viscid or dense; to rarefy. Specifically: To subtilize, as the humors of the body, or to break them into finer parts. (Webster's 1913.)