The rapid world-wide growth of chromium plating since its commercialization in 1924 is ascribed to a combination of outstanding properties of chromium, including high resistance to atmospheric corrosion at ordinary and elevated temperatures; high degree of hardness and wear resistance; low coefficient of friction and good reflectivity. Full details are presented for the preparation of the chromium plating baths ; the cleaning of the articles preliminary to plating; control of optimum plating conditions; tests of deposits; etc. An exhaustive bibliography is included.

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The nature of the diffusion of anode and cathode polarization products through thin sheets of platinum and palladium has been investigated and it was found that no diffusion of electromotively active material occurs through solid metal, even though very thin. When a change in the potential of the back of a thin electrode occurs, it is found that the metal is not perfectly continuous and that a small solution connection has been established in the discontinuities through which a small amount of current flows and charges the back. The previous work in this field is critically reviewed and it is found that the results of earlier workers confirm the present findings. Inconsistencies in the previous work are satisfactorily explained.A general theory is developed to explain the results obtained in a number of different fields of investigation. A satisfactory theory of the blistering of metals is proposed. The value of the present research in the clarification of the theory of polarization and overvoltage is discussed. INTRODUCTION. It has been shown in the previous papers of this series 4 that a "transfer resistance" component of overvoltage does not exist under the conditions studied. It was erroneously stated in one of the earlier articles5 that the concept of transfer resistance was introduced by 'Manuscript

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