Milt,
we tried it in the UK in the 1950s and 1960s, but with very mixed and inconclusive results. Many papers published on the experiments. The largest experiment was carried out by Imperial College London in 1953, called operation CUMULUS, flying out of Cranfield airfield.
Of late, several commercial companies in the USA have made quite spectacular claims, that do not seem to stand up to independent peer reviewed examination, apart from one experiment carried out in the late 1990s over Mexico.
The Russians and Isrealis have also tried of late, again with mixed results.
Search some of the Meteorological litrature and you will turn up all sorts of refrences.