The author conducted a study on the behaviours of foreign positive ions toward the crystal form and transparency in case these ions were contained in sodium chloride crystals. The results obtained were as follows: 1. It was found that when foreign positive ions were contained, Ca2+ ions indicated no conspicuous changes in the form and transparency of crystals, but Mn2+, Cd2+ and Pb2+ ions brought about outstanding changes. In the latter case, moreover, a series of changes in the form and transp arency when the amount of these foreign ions was gradually increased were similar and could be roughly classified into the three stages. 2. The order of the influential force of the above-mentioned ions was Cd2+>pb2+>mn2+>Hg2+. 3. When Cd2+ions were contained, the crystal habit (111) was remarkably developed. 4. It was made clear by calculating the bond energy on each lattice point of crystals that the distribution of the bond energies on lattice points was susceptibly changed by the lattice defects caused by the foreign ions. 5. It was possible to presume the form of crystals by tracing the changes in the distribution of bond energies during the growth of crystals. 6. It was also presumed that the crystal habit (111) developed by Cd2+ ion was chiefly resulted from the combination of crystal nucleous. 7. The three stages of crystal changes owing to the amount of foreign ions could be explained from the changes in the distribution of bond energies caused by the lattice defects.