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Many years ago I found that the mineral beryl (composition Al2 Be3 (SiO3 )6 ) contains much more helium than can be accounted for by the small traces of radioactive matter of the uranium and thorium series present in it. The same ground has been partly gone over again by other workers but it cannot be said that their papers contain any further advance towards determining definitely the origin of the helium found. Recently, on reconsidering the subject, it appeared that there was one outstanding point at least which was reasonably accessible to investigation, namely the relation of helium content to the geological age of the specimen. If helium has accumulated in the mineral as the result of atomic disintegration continued through long geological periods, we should expect large helium content to be found only in beryls coming from ancient formations. If, on the other hand, it had been trapped in some way when the mineral was formed, or if its presence were due to the rapid disintegration of a comparatively short-lived parent such as radium or ionium originally present in the mineral, we should expect to find more helium in the beryls of younger formations, since the opportunities for its escape have been less.

Beryllium and helium. I.—The helium contained in beryls of varied geological age