FLINT, n. A substance much in use as a material for hearts. Its composition is silica, 98.00; oxide of iron, 0.25; alumina, 0.25; water, 1.50. When an editor's heart is made, the water is commonly left out; in a lawyer's more water is added — and frozen.
The Enlarged Devil's Dictionary (p. 96), Doubleday & Company, Inc.