MANY attempts have been made to find a relation between permeability and other measurable properties of porous materials such as particle-size and porosity. The Kozeny equation, which depends on surface area of particles and porosity, has come into rather general use in fields concerned with flow of water, oil and gases and a considerable literature has developed around it1. Its weaknesses are that it is unsuited to material having a wide range of pore-sizes, and that it makes use of an empirical factor which departs seriously from its accepted value in consolidated materials. Attention has also been given to the possibility of using pore-size instead of particle-properties, and Childs and Collis George2 have developed a permeability equation on this basis. An empirical factor is involved and this has not been widely tested for constancy in different materials.