Note 3 — VIII FOUNDATIONS OF THE UNIVERSE
Reading edition
The "Alpha rays" were presently recognised as atoms of helium gas, shot out at the rate of 12,000 miles a second.
The "Gamma rays" are waves, like the X-rays, not material particles. They appear to be a type of X-rays. They possess the remarkable power of penetrating opaque substances; they will pass through a foot of solid iron, for example.