  We present high S/N echelle spectra of the recently discovered ultraluminous
QSO APM 08279+5255 and use these data to re-examine the abundance of Carbon in
Ly alpha forest clouds. In agreement with previous work, we find that
approximately half of the Ly alpha clouds whose hydrogen column densities log
N(HI) > 14.5 have associated weak CIV absorption with log N(CIV) > 12, and we
derive a median N(CIV)/N(H I) = 1.4 x 10^-3. We then investigate whether there
is any CIV absorption in weaker Ly alpha clouds with 13.5 < log N(HI) < 14.0.
Two contrasting techniques are compared - firstly by stacking individual CIV
lines to produce a high S/N (=580) composite spectrum and secondly by
constructing pixel-by-pixel optical depth distributions, a technique recently
developed by Cowie & Songaila (1998). By simulating stacks of synthetic
absorption lines, the composite spectrum technique is found to be severely
hampered by random redshift offsets between CIV lines and their corresponding
HI. Whilst the optical depth method showed evidence for CIV asborption even at
HI optical depths as small as 0.5 - 2, simulations showed that even higher S/N
ratios are required in order to determine whether the CIV/HI ratio remains
constant at these low column densities.
