  The equilibrium relative humidity values for a number of the most commonly
used precipitants in biological macromolecule crystallisation have been
measured using a new humidity control device. A simple argument in statistical
mechanics demonstrates that the saturated vapour pressure of a solvent is
proportional to its mole fraction in an ideal solution (Raoult's Law). The same
argument can be extended to the case where solvent and solute molecules are of
different size.
