workmen, to recall such structure in any case. He finds the substance of glass always presenting the same vitreous, amorphous appearance, except in cases of devitrification, and, in the absence of any proof of such condition, cannot bring himself to believe in glass of a fibrous structure.
He finds in "a cohesive polarity, which dictates to the particles of glass a certain regularity in their arrangement, but which requires some time for its development," as laid down in the "Brittanica," a theory which is far from satisfying or giving him any useful aid, and he requires some proof (which he cannot find) of such "polarity" before absolutely adopting this theory.
He looks in vain for the fissured character of the interior substance of the Rupert drop, mentioned in the article of The Popular Science Monthly, and in the "Encyclopædia Britannica," but finding, even under the microscope, that the substance of the interior, as well as the exterior, of the drop is apparently solid and undisturbed, gives up his attempt to understand the authorities, and even Dr. Ure's explanation in the "American Cyclopedia," of the Rupert-drop phenomena, fails to satisfy him.
He now feels that, to pursue this subject further, he must put together the facts in his possession, and ascertain if their combination will not suggest a more satisfactory theory than those laid down in the books.
He begins, of course, upon the foundation which his twenty years' experience in the glass-house has strongly impressed on him, viz., the fact that in passing from a fluid to a solid state glass shrinks.
His next fact is that glass is a poor conductor of heat, as he has often noticed in the manipulation of heated glass, during its process of manufacture, that in the same piece of glass, and close together, are portions, the one solid and the other fluid.
To these facts he puts the third fact, that the surface of fluid or semi-fluid glass chills very quickly upon exposure to the air, and very quickly becomes solid.
Here are all the facts necessary by which to construct a theory for the explanation of the phenomena of fracture in unannealed glass and in the Rupert drops.
Watching a thick mass of glass cool, he notes the color: by an oblique look, he perceives that the surface has a green tint; while through this transparent tinted medium a direct look shows the centre still of a glowing red color. He knows by experience that the green tint in cooling crystal glass indicates solidification, while the red glow tells that such glass is yet soft. But, not depending upon his experience of color, he tests the surface with an iron tool and finds it absolutely rigid; then with a hammer breaks this rigid surface, and finds, as its color indicates, the centre still semi-fluid.
Here is proved, the condition of an outer skin or shell of rigid
