Most of the many mooted problems that have confronted lime men as unsolvable, are readily explainable upon fairly simple physical grounds. The subject now before us is not overly complex and should be understandable to everyone with sufficient imagination to be able to visualize, that individual invisible particles can and do behave in elementary ways and collectively create gross characteristics. The greatest marvel of this entire subject of calcination is that limestone calcined at the mildest temperature for the shortest necessary time, retains as lime the original dimensions of the stone. It has lost 44 per cent of the initial weight but all the volume is still there. Although we well know that the true specific gravity of oxide is 3.4, the apparent gravity of this lime is only 1.46. Half of the volume is free space. The Ca atoms remain as a whole still approximately in their original positions, and the voids created by removal of CO result in a perfectly porous mass. It seems incredible that this delicate structure could be retained without some rearrangement and natural aggregation, but if there is change it must be inconsequential for otherwise the original volume would not persist. Lime so burned could be crudely compared to a city in which the buildings occupy half of the space and the interlacing boulevards the other half, all widely open for rapid traffic, only in fact so minute as to be atomic in dimensions. There is traffic of CO2 molecules rushing at high speed outwards and, during hydration, of water molecules inward.