  We present an analysis of the Lyman alpha forests of five quasar spectra in
the near UV. Properties of the intergalactic medium (IGM) at an intermediate
redshift interval (0.9 < z < 1.9) are studied. The amount of baryons in the
diffuse photoionised IGM and the warm-hot intergalactic medium (WHIM) are
traced to get constraints on the redshift evolution of the different phases of
the intergalactic gas. The baryon density of the diffuse IGM is determined with
photoionisation calculations under the assumption of local hydrostatic
equilibrium. We assume that the gas is ionised by a metagalactic background
radiation with a Haardt & Madau (2001) spectrum. The WHIM is traced with broad
Lyman alpha (BLA) absorption. The properties of a number of BLA detections are
studied. Under the assumption of collisional ionisation equilibrium a lower
limit to the baryon density could be estimated. It is found that the diffuse
photoionised IGM contains at least 25% of the total baryonic matter at
redshifts 1 < z < 2. For the WHIM a lower limit of 2.4% could be determined.
Furthermore the data indicates that the intergalactic gas is in a state of
evolution at z=1.5. We confirm that a considerable part of the WHIM is created
between z=1 and z=2.
