IN order to assist in elucidating the question of the relationship between the chemical composition of solid substances and the nature of the crystals which they are observed to form, both as regards the exterior geometrical configuration and the interior physical character of such crystals, a series of researches were commenced by the author six years ago, having for their immediate object the exact determination of the differences presented by certain well-defined series of isomorphous salts. The differences in question, due to the different nature of the interchangeable chemical elements, belonging to the same family group, which by their mutual replacement give rise to the series, are so small in the case of the morphological constants, that extremely refined methods of investigation are requisite in order to detect and determine them. A large amount of detached data had previously been accumulated in crystallographic literature, but a very small proportion was characterised by the requisite degree of accuracy, and no organised attempt had hitherto been made to investigate any definitely related series of crystallised compounds in a sufficiently detailed and accurate manner. The care and precision demanded will be at once apparent when it is pointed out that the use of slightly impure or imperfect crystals, or the occurrence of slight errors of orientation in grinding out of the crystals the section-plates or prisms requisite for the optical portion of the work, would be sufficient to render the results valueless for the purpose in view. In fact such sources of error have in certain cases been shown by the author during the progress of the work to have led previous observers to conclusions diametrically opposed to the truth.