  A continuum limit treatment of planar spin chains with arbitrary S is
presented. The difference between integer and half-integer spins is emphasised.
While isotropic half-integer spin chains are gapless,and have power-law decay
of correlations at T = 0 with exponent eta = 1, integer spin systems have a
singlet ground state with a gap for S=1 excitations and exponential decay of
correlations. The easy-plane to easy-axis transition is described.
  Note: this is a verbatim transcription of ILL preprint SP-81/95, which is
cited in a number of places, but was rejected for publication in 1981, and
remained unpublished in this original form, which gives somewhat different
arguments for the central result as compared to the later paper finally
published in Phys. Lett. 93A, 464 (1983); many thanks to Jeno Solyom for
preserving this historical document.
