When a Hg+Tl gas mixture is excited by Hg 2537-\AA{} resonance radiation, microscopic energy-transfer processes occur which are manifested by the appearance of Tl emission lines. At constant thallium gas density, Swanson and McFarland have determined the intensity ratio ($R$) of the Tl 3776-\AA{} line to the Tl 5350-\AA{} line as the mercury density is varied. The functional form of the dependence of $R$ on mercury density has been successfully interpreted in terms of the self-absorption theory of Holstein. Specifically, the Tl 3776-\AA{} line is strongly self-absorbed at low Hg densities. As the Hg density increases, Hg-Tl collisions occur more frequently and the Tl 3776-\AA{} absorption line broadens, thus allowing more 3776-\AA{} radiation to escape. In this region, $R$ varies as the square root of the mercury density. At still higher mercury densities, the Tl 3776-\AA{} radiation readily escapes, and $R$ becomes a weak function of mercury density and finally levels off to a constant value. Similar considerations apply to the intensity ratio of the thallium 2580-\AA{} and 3230-\AA{} lines. These results constitute a semiquantitative verification of Holstein's self-absorption theory for the case where the shape of the absorption line is determined by impact broadening. In particular, this is the first experimental verification of Holstein's theory for the case of foreign-gas broadening and one may consider this as a new experimental technique for investigating self-absorption phenomena.