MR. G. W. WALKER tells me that he has sent to NATURE his interesting problem of the mutual attraction between two uniform concentric hemispherical shells, bounded by a common diametral plane. The ...

Beveridge and Denbigh 1 have directed attention to the development of a concentric shell of zinc oxide when a compressed sphere of zinc sulphide powder was oxidized in air at 1,250° C. This shell was ...