Aeration of particulate solids occurs by penetration of ambient air through the narrowest cross-section of the labyrinth of interstices as a presumed continuous succession of "virtual" bubbles. In the absence of inter-particle forces, these "virtual" bubbles cause voidage change of the packed bed similar to that of a rising bubble at the onset of bubbling fluidization. As long as the size of the "virtual" bubbles is smaller than the dimension of the narrow, voidage change is not possible and this criterion is taken as the benchmark for cohesiveness. The particulate mass is presumed to be mobile as soon as the diameter of the "virtual" bubbles of aeration equals the particle diameter. At a voidage of 0.383 these considerations result in a B-db-A(db)-C-db classification of hard spherical particulate solids. This classification indicates the degree of cohesiveness for particulate solids on the basis of a representative particle diameter and of the particle density at the dense bed porosity of 0.383 as validated with literature data. It differs appreciably from the classification of fluidized particles. (C) 2011 Elsevier B.V. All rights reserved.