I suspect that in this volume my reader will feel that I have given him a stone when he asked for bread, and his feeling in this respect will need no apology. I fear there is more of the matter of hard science and of scientific speculation in this collection than of spiritual and aesthetic nutriment; but I do hope the volume is not entirely destitute of the latter. If I have not in some degree succeeded in transmuting my rocks into a kind of wholesome literary bread, or, to vary the figure, in turning them into a soil in which some green thing or flower of human interest and emotion may take root and grow, then, indeed, have I come short of the end I had in view.