It has long been known that the rotative power of certain optically active salts in aqueous solution is greatly, and in some cases enormously, changed by the addition of certain non-active salts. Of late years a good deal of work has been done on this subject, and a considerable amount of data has been collected; but, generally speaking, little explanation has been obtained of the precise causes of the observed phenomena. In some cases at least the reason for this no doubt lies in the fact that in such solutions there exist complicated molecular combinations, about which it may not be easy to obtain ordinary chemical evidence. This, however, only makes it the more important that we should utilise tothe fullest extent such means of investigation as are readily available, and of these none is so good as that of optical activity.