PRINCE RUPERT'S DROPS. 319 



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 Avhich requires 

 some time for its development," as laid down in the " Brittanica," a 

 theory wliich is far from satisfying or giving him any useful aid, and 

 he requires some proof (which he cannot find) of such " polarity " be- 

 fore absolutely adopting this theory. 



He looks in vain for the fissured character of the interior sub- 

 stance of the Rupert drop, mentioned in the article of The Popular 

 Science Monthly, and in the " Encyclopedia Britannica," but find- 

 ing, 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. 

 TTre'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 to- 

 gether 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 Avhich 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 pro- 

 cess of manufacture, tliat in the same piece of glass, and close to- 

 gether, 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 

 tlirough this transparent tinted medium a direct look shows the cen- 

 tre 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 



