Two theories, one by G. Hulett in 1903 and one by G. N. Lewis in 1908, describe how solutes alter water in an aqueous solution. Hulett recognized that the solutes alter the vapor pressure of water in a solution as would a negative pressure applied to pure liquid water at the same temperature, where the applied pressure equals the osmotic pressure of the water at a free surface of the solution. He proposed that the solutes exert an internal pressure so as to alter the internal tension of the water in the solution exactly as the applied negative pressure alters the internal tension in pure water and, thereby, alters all colligative properties of the water. Lewis defined a term, fugacity of a solvent, which equals the vapor pressure of an ideal solvent and which approaches the vapor pressure of the real solvent only when the real solvent vapor pressure is vanishingly small. Lewis also defined solvent activity as the ratio of fugacity of solvent in the solution to fugacity of pure liquid solvent, and he attributed the lower chemical potential of the solution solvent to its diminished activity. Internal tension is an intensive property of the solvent whereas fugacity and activity are not real properties of a real solvent in a real solution. Neither Hulett’s nor Lewis’s theory incorporates a kinetic theory to account for the altered state of the solvent in a solution. At that time, the existence of atoms and molecules was not well established. Moreover, an equation of state for liquid solvent was not known, and without an equation of state, no rigorous theory could be stated. In this article, I compare Hulett’s and Lewis’s theories and derive a kinetic theory compatible with Hulett’s theory. Although Lewis’s theory is almost universally taught to students of physical chemistry and chemical thermodynamics, Hulett’s theory deserves attention on its merit.