argument, the descending solids from A to B add gas to the leaving stream, Y. Since roughly the same amount of gas is gained at B as lost at C, a material balance will show that flow X agrees with flow Y. But this is not the flow through the experimental section.Since Professor Happel used a large volumetric flow rate of gas compared to that of solid, only a small error is made in assuming that flow X or Y is the flow through the bed (maximum error of about 3% for his run No. 104). However, if we would have used this procedure with our system of fine particles, our error would have been ridiculously large. This is why we used a tracer technique (tagging the flowing molecules in the experimental section) to measure flow rates.4. Dr Happel's final question asks what provision was made to counter gas leakage with the solids (the equivalent of gas entrainment from A to B, or from C to D). We did not make any such provision, and we do expect that significant amounts of gas were so entrained. Note that equalizing pressures up and dowpstream does not guard against gas leakage. It actually guarantees such leakage.We measured the actual gas flow within the experimental section by tracer methods.