An experimental study of Marangoni flows in horizontal layers of tin based metallic alloys heated from the side is carried out, based on the observation of the motion of tracer particles at the surface associated with a direct measurement of temperatures in the melt. A key feature of the procedure is that the experiments are performed in a high vacuum environment to limit as much as possible the pollution of the liquid vapour interface. In pure tin, depending on the intensity of the driving force measured by the Reynolds-Marangoni number, either viscous or boundary layer flow regimes are found. In addition, the onset of temperature oscillations is found to be consistent with the hydrothermal wave instability mechanism proposed by Smith and Davis. In tin-bismuth alloys, the observed flow directions and velocities are interpreted in terms of a non linear dependence of the surface tension on temperature, and successfully compared with the dependence predicted by interpolating the available experimental surface tension data with the help of a simple thermodynamic model. During the directional solidification of the layer, the concentration gradient in the solute rich boundary layer ahead of the growth front is found to locally modify the flow pattern. For instance, when thermal and solutal driving forces act in the same direction, temperature oscillations are observed at low values of the Reynolds-Marangoni number. As for opposing driving forces, the situation is that a local reversal of the flow direction is observed in correlation with the morphological instability of the growth front.