  We observe thermal motions of ions, and molecules in water through light
extinction, at the individual particle level. The motions appear as time
dependent intensity variations, characterized through their averaged spectra.
Theoretical spectrum derived from random motions of one particle describes
these measured spectra. This theory is used to extract diffusion constants of
liquid mixtures and solutions, that correspond to binary diffusion, and thermal
diffusion, which are consistent with previous macroscopic measurements. We also
estimate the sizes of the particles.
