“…Ultrasonic velocity measurements are convenient for measuring hydration numbers from ion compressibilities (Padova 1964 The crystals of the first series of the transition metals are colored; for example, Ti(III), V(II), V(III), Mn(II), Fe(II), Fe(III), Co(II), Ni(II). The hexaaquo-salts of these metals dissolve in water without changing the color.…”