retarded them and made the progress very slow down to the middle of the present century.
At the beginning of the cycle there existed the two opposing geological camps, the one attributing everything to fire, the other all to water; after long years of wrangling their union was accomplished through the efforts of Lyell and his followers. In addition to this, the accumulating observations overthrew two old ideas—namely, that a mineral can only originate in one way characteristic to it, and a single homogeneous magma can give rise to only one mineral. It was found that a mineral may originate under different conditions which are determinable, and that the homogeneous magma may at the same time give rise to different minerals. The various mineralogists appeared to take pleasure in throwing an envelope of mystery around the origin of minerals, and they were regarded, even by Zirkel, as the work of a kind of vital force.
Practical difficulties deterred the progress of the study; the crystals formed were sometimes imperfect and usually microscopic. So it was almost impossible to study them before the development of mineralogical micrography and the advent of the mineralogical microscope. Then it was found that these minute imperfect crystals were of more value and led to greater results than the more beautiful cabinet specimens, for they settled the problems of origin. Natural crystals were found to contain small inclusions which are indices to the origin. If these are vitreous, then the origin is vitreous, and the action of volatile agents is wholly excluded; if these be aqueous, the intervention of water is indisputable. In certain minerals—as quartz, beryl, topaz—liquid carbonic acid appears as an inclusion, giving evidence of their formation under great pressures.
From this brief survey we see the strong prejudices of the ancients are disappearing; observation and the processes of investigation have acquired a remarkable precision; materials and apparatus in the laboratories have been perfected to a remarkable degree.
Under the head of artificial minerals we exclude those accidentally formed in the industrial works, as graphite on the walls of iron furnaces, for such do not answer the question of their origin, since the reagents and conditions remain unknown. Nevertheless, the recorded observations of such products have aided reproduction in the laboratory, and it is of interest that these observations have been noted especially by German workers, while the home of active laboratory investigation is in France. The Germans collected the facts, while the French co-ordinated them, forming hypotheses and then experimenting to prove them. The Russians followed with almost equal success; also much important work has been done in the laboratory by the Germans.
