  It was not long before Avogadro's number could be calculated by methods not closely tied to the kinetic theory of gases. Rayleigh speculated that the brightness of the sky is due to the scattering of light from the sun by molecules in the atmosphere. The theory predicted that light of shorter wavelength is scattered more effectively than that of longer wavelength, a prediction borne out by the blueness of the sky and the redness of sunsets. It also predicted that the scattered light be polarised, also in conformity with observation. It was possible to calculate Avogadro's number from the ratio of the intensity of skylight to that of light coming direct from the sun. Once the charge on the electron had been measured it was also possible to calculate Avogadro's number from the relation between current passed and weight of substance deposited in electrolysis. Radioactivity was to provide further access to the number. In all cases, the values for Avogadro's number agreed to a degree that could be reconciled with the accuracy of the experiments and the degree of approximation involved in the calculations.