Professor JimeÂnez-Salas offered me the opportunity of\ud giving further details about my work. If I understand\ud him correctly, Professor JimeÂnez-Salas holds\ud that, in the tests I have conducted, exposure of the\ud Ponza bentonite to a saturated KCl solution might\ud have caused a partially irreversible potassium ®xation.\ud On the other hand, I have put forward the\ud hypothesis that such exposure caused a cation\ud exchange that might be reversed by imposing\ud appropriate test conditions, for instance by exposing\ud the clay to a concentrated NaCl solution.\ud Actually, I stressed that potassium effects are permanent\ud under the given test conditions, and that\ud they may be permanent under the conditions that\ud normally occur in nature, but they are not irreversible\ud under all conditions.\ud Some experimental results that I have obtained\ud in the meantime seem to conferm that potassium\ud ions can readily be replaced by sodium ions by\ud mass action. The phenomenon is observable, from\ud a macroscopic point of view, under low values of\ud axial stres