  Horizontal Convection (HC) at large Rayleigh and Prandtl numbers, is studied
experimentally in a regime up to seven orders of magnitude larger in terms of
Rayleigh numbers than previously achieved. To reach Rayleigh up to $10^{17}$,
the horizontal density gradient is generated using differential solutal
convection by a differential input of salt and fresh water controlled by
diffusion in a novel experiment where the zero-net mass flux of water is
ensured through permeable membranes. This setup allows for measuring accurately
the Nusselt number in solutal convection by carefully controlling the amount of
salt water exchanged through the membranes. Combined measurements of density
and velocity across more than five orders of magnitude in Rayleigh numbers show
that the flow transitions from the Beardsley & Festa regime to the Chiu-Webster
et al. regime and frames the present results within the scope of Shishkina et
al., and the companion paper theory. In particular, we show that even for large
Prandtl numbers, the circulation eventually clusters underneath the forcing
horizontal boundary leaving a stratified core without motion. Finally, previous
regime diagrams are extended combining the present results at high Prandtl
numbers, the results at low Prandtl numbers of the companion paper, together
with previous results from the literature. This work sets a new picture of the
transition landscape of horizontal convection across six orders of magnitude in
Prandtl number and sixteen orders of magnitude in Rayleigh numbers.
