CONTENTS I. Introduction 90 II. Sub-Compounds 91 III. Colour centres in ionic crystals 94 IV. Solubility of metals in pure salts 95 V. Influence of foreign cations on the solubility of metals 98 VI. Cryoscopic studies of solutions of metals in fused salts 99 VII. Influence of the dissolution of metals on the vapour pressure of fused salts 100 VIII. Volume effects accompanying the dissolution of metals in fused salts 100 IX. Potentiometric studies of solutions of metals 101 X. Electrical conductivity of solutions of metals in fused salts 102 XI. Magnetic and spectroscopic studies of solutions of metals in fused salts 103 XII. Dissolution of metals in electrochemical processes 103 XIII. Conclusion 104