“Two clouds” in physics

The British physicist Lord Kelvin gave a speech entitled “Nineteenth-Century Clouds over the Dynamical Theory of Heat and Light”. The “clouds” to which Kelvin was referring were: Michelson-Morley experiment and the black-body radiation effect-known as the ultra violet catastrophe. The removal of these clouds led to the two great breakthroughs of modern physics: The theory of relativity and quantum mechanics.