It is a common chemical fact, that when a solution containing an alkaline silicate is acted upon by any reagent capable of separating the silica from the base, the silica is in the majority of cases thrown down in a solid form. The fact of this reaction is all that, as a rule, the chemist would look at: but in addition to the mere chemical fact of the precipitation of the silica, another most important element from the mineralogist's point of view is that of the form in which this precipitate of solid matter appears. This form naturally varies with the various conditions of the experiment, and it is by the study of these varying forms that the authors have sought to elucidate, experimentally , the wondrous and beautiful varieties of structure which are so familiar to us in the endless forms of silica occurring in nature.