On the occurrence of helium in beryls

When Lord Rayleigh first discovered the occurrence, in samples of beryl, of helium greatly in excess of the amount attributable to the traces of uranium and thorium contained therein, he suggested as the most plausible hypothesis that an unknown element present in beryl may emitα -particles with less than the critical velocity (Strutt 1908). Later, on the suggestion of B. B. Boltwood, he discussed another possibility, viz. that in crystallizing from the rock magma the beryls had occluded one of the shorter livedα -radioactive elements such as radium or ionium, which later decayed, leaving nothing but the helium as evidence of its former presence (Strutt 1910). In view of the subsequent discovery of isotopy both hypotheses were reconsidered in the light of this new knowledge. It has, for example, been suggested that the “unknown element”, decaying without detectable rays, might well be the isotope8 Be; and further, that this isotope might have already completely decayed (Atkinson and Houtermans 1929; Rayleigh 1929).