Experiments conducted with a variety of powders, in high aspect ratio slugging fluidized beds, indicate that raining (or square-nosed) slugs are formed by materials which in lower aspect ratio vessels form bubbling or smoothly slugging beds. The largest of the three beds investigated was of small industrial size having dimensions of 0.21 m diameter and 6.9 m high. Measurements of rise velocity, slug length, slug frequency, and inter-slug spacing are reported. The rise velocities are in agreement with: USa = U - Umf + 0.35(gD))1/2. The inter-slug spacing, which is related to frequency, has assumed values up to 8 bed diameters. The angle of internal friction of a particulate solid is shown to be a significant factor for predicting which slug flow regime a solid will form when fluidized. Some experimental techniques are reported which attempt to identify and measure the forces which operate in a raining slug bed.