Heat transfer from a circular tube to a contained gas in laminar forced convection was studied dimensionally and experimentally. The boundary condition of uniform heat flux was physically achieved, and the experiments were conducted in a statistically designed pattern. The dimensional analysis, based upon the principle of corresponding states, and more general than any hitherto proposed, indicated four independent variables in this experiment. Of these, only bulk velocity showed a statistically significant effect in the region studied: a reduction in heat transfer coefficient with decreasing bulk velocity. No existing theories could explain this effect.