1922, Joseph William Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longmans, Green and Co.; Volume I, Chapter V, § 17, page 226:For example, V. Meyer and E. Riecke (1888) assumed that the carbon atom is surrounded by an æthereal envelope which, in the case of isolated atoms, has a spherical shape like that supposed to be possessed by the atoms themselves. The atom in the core carries the specific affinities; the æthereal envelope is the seat of the valencies. Each valency is determined by the presence of two opposite electrical poles—called double or diâ€poles—situated at the ends of a straight line which is small in comparison with the diameter of the æthereal shell. The four valencies of carbon are represented by four such diâ€poles each of which is able to move freely within the æthereal shell, and to turn freely about its middle point.
