 Hempel’s conditions had many virtues, not least of which was that they appeared to fit many familiar examples of scientific explanations, such as explaining why a coin expanded when it was heated by invoking the law that copper expands when heated and noting that the coin was copper. Hempel doesn’t specify the form laws may take, which could be simple or complex. Most of his examples were simple, such as “All ravens are black”, “All gold is malleable”, and so on. Others were quantitative, such as Archimedes’ principle (A body totally immersed in a fluid—a liquid or a gas—experiences an apparent loss in weight equal to the weight of the fluid it displaces), Snell’s law (During refraction of light, the ratio of the sines of the angles of incidence and of refraction is a constant equal to the refractive index of the medium), and Newton’s law of gravitation (Any two bodies attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them), which he discussed.