A powerful agent is the right word. Whenever we come upon one of those intensely right words . . . the resulting effect is physical as well as spiritual, and electrically prompt.
Source: In The Ultimate Success Quotations Library, 1997.
-- Mark Twain, (Nov 30 1835-1910), US novelist, journalist, river pilot; The Great American humorist was also known as The People's Author and creator of Tom Sawyer and Huck Finn.