  Emission of muonium ($\mu^{+}e^{-}$) atoms from silica aerogel into vacuum
was observed. Characteristics of muonium emission were established from silica
aerogel samples with densities in the range from 29 mg cm$^{-3}$ to 178 mg
cm$^{-3}$. Spectra of muonium decay times correlated with distances from the
aerogel surfaces, which are sensitive to the speed distributions, follow
general features expected from a diffusion process, while small deviations from
a simple room-temperature thermal diffusion model are identified. The
parameters of the diffusion process are deduced from the observed yields.
