  We present the analysis of the atmospheres of 70 gaseous extrasolar planets
via transit spectroscopy with Hubble's Wide Field Camera 3 (WFC3). For over
half of these, we statistically detect spectral modulation which our retrievals
attribute to molecular species. Among these, we use Bayesian Hierarchical
Modelling to search for chemical trends with bulk parameters. We use the
extracted water abundance to infer the atmospheric metallicity and compare it
to the planet's mass. We also run chemical equilibrium retrievals, fitting for
the atmospheric metallicity directly. However, although previous studies have
found evidence of a mass-metallicity trend, we find no such relation within our
data. For the hotter planets within our sample, we find evidence for thermal
dissociation of dihydrogen and water via the H$^-$ opacity. We suggest that the
general lack of trends seen across this population study could be due to i) the
insufficient spectral coverage offered by HST WFC3 G141, ii) the lack of a
simple trend across the whole population, iii) the essentially random nature of
the target selection for this study or iv) a combination of all the above. We
set out how we can learn from this vast dataset going forward in an attempt to
ensure comparative planetology can be undertaken in the future with facilities
such as JWST, Twinkle and Ariel. We conclude that a wider simultaneous spectral
coverage is required as well as a more structured approach to target selection.
