um--the parent of radium--and the whole
radioactive series, ending with the end-product lead."

He hung the plate by two fine wires fastened to its corners, and
adjusted a coil of wire opposite its centre, while within the coil he
slipped a small black capsule.

"This is the best we can do now," he said. "The capsule is made of
zircorundum, and we shall get only a trace of the disintegrating rays
before it blows up. But you'll see 'em, or, rather, you'll see the
lavender phosphorescence of the air through which they pass."

He arranged a thick slab of plate glass between Thornton and the thermic
transformer, and stepping to the wall closed a switch. An oscillatory
spark discharge started off with a roar in a closed box, and the coil of
wire became white hot.

"Watch the plate!" shouted Bennie.

And Thornton watched.

For ten or fifteen seconds nothing happened, and then a faint beam of
pale lavender light shot out from the capsule, and the metal plate swung
away from the incandescent coil as if blown by a gentle breeze.

Almost instantly there was a loud report and a blinding flash of yellow
light so brilliant that for the next instant or two to Thornton's eyes
the room seemed dark. Slowly the afternoon light regained its normal
quality. Bennie relit his pipe unconcernedly.

"That's the germ of the idea," he said between puffs. "That capsule
contains a mixture of vapours that give out disintegrating rays when the
temperature is raised by thermic induction above six thousand. Most of
'em are stopped by the zirconium atoms in the capsule, which break down
and liberate helium; and the temperature rises in the capsule until it
explodes, as you saw just now, with a flash of yellow helium light. The
rays that get out strike the uranium plate and cause the surface layer
of molecules to disintegrate, their products being driven off by the
atomic explosions with a velocity about equal to that of light, and it's
the recoil that deflects and swings the plate. The amount of uranium
decomposed in this experiment couldn't be detected by the most delicate
balance--small mass, but enormous velocity. See?"

"Yes, I understand," answered Thornton. "It's the old, 'momentum equals
mass times velocity,' business we had in mechanics."

"Of course this is only a toy experiment," Bennie continued. "It is what
the dancing pithballs of Franklin's time were to the multipolar,
high-frequency dynamo. But if we could control this force an