RECENT evidence1, 2 of neutral particles of atomic mass and great penetrating power suggests the possibility that terrestial matter may contain similar particles of low (thermal) energies, undetected hitherto by reason of their small interaction with matter. While it has been argued that neutrons of mass 1 would rapidly escape from the earth, one would hesitate to deny the possible existence of similar bodies of greater mass, and it seems pertinent to remark that experimental evidence already exists which, if we knew the laws of collision of neutrons with matter, would at once fix an upper limit to their concentration in the atmosphere.