The practical side of a subject must always be considered, the question of utility being a very important one. What claim has this subject for attention and what has it accomplished? It has thrown light on the mode of the natural formation of minerals and rocks. Thus even down to late time water was thought to play an important part in the formation of a great number of volcanic rocks and to be indispensable in the formation of the great group of rocks termed basalt. Yet basalt and all the modern volcanic rocks have been formed by purely igneous fusion. Again, certain minerals—as chiastolite, garnet, staurolite, and a large number of metamorphic minerals—are always found impure in Nature, and their exact composition was unknown until reproduced artificially. The majority of natural minerals are complex combinations in which many bodies are introduced by isomorphous agency. Synthesis has furnished the theoretical types and given forms which could be accurately measured and show the true physical properties.
Mineral synthesis determines the individuals belonging to a family and distinguishes the true isomorphism of the series in question. The artificial reproduction of the feldspar series proved that the two members, albite and anorthite, were isomorphous and could be united in all proportions, some new forms being found which were unknown in Nature. Other mineral types which are suggested by, but are absent in Nature have been formed artificially, thus completing a mineral series, making the limits of isomorphism more clear. This was accomplished by Ebelmen in the spinel family, showing the relation of ferrites, chromates, and aluminates to each other; also by Foque and Levy in the feldspar family, who formed new feldspars with bases of lithia, barytes, strontium, and lead. This work has also been of great assistance to geology, a science which has been encumbered by theories and hypotheses, where observation was in very many cases insufficient to settle definitely the doubts. Synthesis, when applied, enlarged the field of observation and so often furnished definite solutions. Thus the origin of granite was one of the great problems confronting geologists. The opinion that it was purely igneous prevailed in the science for the first part of our cycle, replacing the Neptunist or aqueous theory of Werner, but the difficulties were increased a little later when, by means of the microscope, it was found the quartz was consolidated after the other minerals; this was against the idea of a purely igneous fusion of the granite. The upholders of this theory then argued for an extra fusion of the quartz analogous to sulphur. Élie de Beaumont in 1849 modified the theory by admitting the intervention of water. For proof he called attention to the number and frequency of the minerals sublimed on the pe-
