Mary could do two types of experiments
<em>Type 1</em>. She could <em>hold the cation constant and vary the anion</em>.
She could try solutions of NaCl, NaBr, NaI, Na₂SO₄, Na₃PO₄, etc.
These experiments would tell her which <em>anions</em> are the best conductors.
<em>Type </em>2. She could <em>hold the anion constant and vary the cation</em>.
She could try solutions of NaNO₃, KNO₃, Mg(NO₃)₂, Ca(NO₃)₂, Al(NO₃)₃, etc.
These experiments would tell her which <em>cations</em> are the best conductors.