In 1901 Russian scientist George Wulff stated (without proof) that the length of a vector drawn normal to a crystal face h j will be proportional to its surface energy γ γ j : h j = λ λ γ γ j . The vector h j is the "height" of the j th face, drawn from the center of the crystal to the face; for a spherical crystal this is simply the radius. This is known as the Gibbs-Wulff theorem.