Take another and quite different example of how variations in time bring about wide contrasts of result: a rubber ball thrown in play at a wall rebounds; send it forth from a cannon, with a hundred-fold this velocity, and it pierces the wall as might a shot of steel. CHAPTER XV PROPERTIES--_Continued._ RADIO-ACTIVITY Properties most evident are studied first . . . Then those hidden from cursory view . . . Radio-activity revealed by the electrician . . . A property which may be universal and of the highest import . . . Its study brings us near to ultimate explanations . . . Faraday’s prophetic views. Properties age after age have become more and more intimately known. At first the savage took account solely of the obvious strength of an oak, the sharpness of a flint, the pliability of a sinew. With the first kindling of fire he discovered a new round of properties in things long familiar. All kinds of wood, especially when dry, were found combustible, so were straw and twigs, as well as the fat of birds, the oil of fish. Then it was noticed that the ground beneath a fire remained unburnt and grew firm and hard, so that its clay or mud might be used for rude furnaces and ovens. Soon come experiments as to the coverings which maintain coals at red heat, ashes proving the readiest and best. A century ago the mastery of electricity began to unfold a new knowledge of properties, so wide and intimate as to recall the immense expansion of such knowledge that long before had followed upon the kindling of fire. The successors of Volta, as they reproduced his crown of cups, asked, What metals dissolved in what liquids will give us an electric current at least outlay? Then followed the further question, What metals drawn into wire will bear currents afar with least loss? With the invention of the electro-magnet came another query, What kinds of iron are most swiftly and largely magnetized by a current; and when the current ceases, which of them loses its magnetism in the shortest time? Plainly enough the electrician regards copper, zinc, iron, steel, acids, alkalis from a new point of view; he discovers in them properties which until his advent had been utterly ignored.