﻿480 Intelligence and Miscellaneous Articles, 



In the preparation of a Rupert's drop the sudden cooling solidi- 

 fies the external coating of the glass while the interior is still liquid. 

 This layer surrounds, therefore, a volume greater than the volume of 

 the cooled glass will occupy. During cooling, the internal mass is 

 connected by adherence to the outside layer already formed. The 

 glass cannot, therefore, undergo its normal contraction ; it undergoes 

 a traction which tends to keep its volume larger, and its condition 

 must greatly resemble that of a metallic bar elongated by an ex- 

 ternal effort. A Rupert's drop may be regarded as a rigid en- 

 velope, inside which bars raised to a high temperature have been 

 fixed to the sides, these bars being in considerable numbers, inter- 

 lacing one another, and connected by countless solderings. During 

 cooling, all these bars in contracting would undergo a traction on the 

 part of the rigid envelope, they would be drawn out ; and the whole 

 system would be in a condition of unstable equilibrium. In a Rupert's 

 drop the infinitely small and infinitely numerous particles of the 

 glass play the part of the bars in question, and they contract at the 

 moment of breaking. The contraction of these particles, like that 

 of the metal wires in the experiments of Messrs. Joule and Edlund, 

 is accompanied by very small molecular displacements, to which a 

 state of repose quickly succeeds. Vis viva disappears, and it is to be 

 expected that at the same time a certain quantity of heat will ap- 

 pear. This is doubtless the origin of the heat observed in the expe- 

 riments above described. 



But the return to the condition of stable equilibrium in a Rupert's 

 drop is accompanied by this remarkable and sudden projection of the 

 particles of glass, which gives the phenomenon the appearance of an 

 explosion. This is a very curious phenomenon, and one difficult to 

 explain in a satisfactory manner. It may perhaps arise from parti- 

 cles of glass only attaining their volume and their form of stable 

 equilibrium after some oscillations resembling those which a spring 

 performs when suddenly let go. When an elastic body is in vibra- 

 tion, it impels with more or less velocity foreign bodies in contact 

 with it. If the spring itself is moveable and it strikes against fixed 

 bodies, the reaction will send it in the opposite direction. If a 

 considerable number of small elastic fragments undergo vibrations 

 and are, moreover, in contact, they strike against and mutually 

 repel each other. In a Rupert's drop it may be supposed that at 

 the time of rupture the particles of glass, till then drawn out, vi- 

 brate also for a short time before attaining their position of stable 

 equilibrium. The mutual shocks which are thus produced are per- 

 haps the cause of the remarkable projection suffered by the debris 

 of an exploded drop. 



i Moreover, whatever may be the immediate cause of this projection, 

 it is probable that the motion thus produced in the glass consumes a 

 portion of the internal heat, a fraction of which is partially regenerated 

 in the fragments when they are stopped by external resistances. — 

 Comptes Rendus, February 15, 1S69. 



