1. IN A PROCESS FOR LEACHING SOLID MATERIALS AMENABLE TO PERCOLATION LEACHING WHEREIN A BED OF THE MATERIAL CONTAINING A DESIRED SOLVENT SOLUBLE CONSTITUENT IS PERCOLATION LEACHED WITH A SOLVENT LIQUID AT ELEVATED TEMPERATURE TO PRODUCE AN EFFLUENT CONTAINING THE DESIRED SOLUBLE CONSTITUENT, THE DESIRED SOLVENT SOLUBLE CONSTITUENT HAVING A SINGIFICANT HEAT OF SOLUTION AND THE BED EXHIBITING A SHARP TEMPERATURE CHANGE IN A ZONE OF ACTIVE LEACHING, WHICH ZONE PROGRESSES DOWNWARDLY IN THE SAID BED OF MATERIAL DURING THE PERCOLATION LEACHING THEREOF, THE IMPROVEMENT COMPRISING DETERMINING WHEN ACTIVE LEACHING IS OCCURRING AT A GIVEN DEPTH IN THE BED OF MATERIAL BY MEASURING THE TEMPERATURE OF THE SOLVENT LIQUID AT THE DESIRED DEPTH OVER A PERIOD OF TIME TO OBTAIN A SERIES OF TEMPERATURE READINGS, THE TEMPERATURE BEING MEASURED PRIOR TO THE OCCURRENCE OF ACTIVE LEACHING AT THE SAID DEPTH AND THE TEMPERATURE MEASUREMENT BEING CONTINUED UNTIL A SHARP CHANGE IN THE TEMPERATURE IS NOTED, THE SOL-