INTEREST has recently been revived in the subject of liquid viscosity, for many years a happy hunting-ground for framers of empirical equations. The increase of gas viscosity with increasing temperature was early accounted for on theoretical grounds, but the contrasted behaviour of liquids remained long unexplained, and theoretical bases were lacking for the equations proposed to represent the decrease of liquid viscosity with increasing temperature. Recently Prof. E. N. da C. Andrade has put forward a theory of liquid viscosity which envisages the transference of momentum between parallel-moving layers of the liquid as accomplished through the medium of temporary unions between contiguous molecules. The picture is that of “a transitory and fluctuating crystallisation”. The theory leads to a remarkably simple exponential formula for the variation of viscosity with tempera-ture, namely, η= Aexp(bT-1).