Intelligence and Miscellaneous Articles. 465 



sistanee is destroyed by breaking off the thin end or another 

 portion of the drop, the state of equilibrium ceases and bursting 

 takes place. 



The experiments whose results I have the honour to present to 

 the Academy seem to prove, on the contrary, that the effects in 

 question are chiefly due to the peculiar condition of the exterior 

 layers, and that the interior play no part, or only a secondary 

 part, in the phenomenon. 



The mechanical actions by means of which the drops are ordi- 

 narily broken, necessarily produce vibrations in the glass, the effect 

 of which it is impossible to appreciate. That is why, in this in- 

 vestigation, I have preferred to make use of fluorhydric acid, the 

 action of which can be moderated at pleasure, and which permits 

 us to destroy at will, and without any shock, any portion we wish 

 to attack. 



On suspending a Kupert's drop by a thread over a platinum 

 vessel containing fluorhydric acid, in such a manner that the extre- 

 mity of the thin end dips into the liquid, we find that we can always 

 dissolve the whole of the thin end without destroying the drop ; 

 but when the acid touches the origin of the neck (that is, the point 

 of divergence of the pear), equilibrium is always broken ; the drop 

 then separates into a great number of fragments, and in most in- 

 stances without explosion. 



Eeciprocally, the swollen part may be immersed in the acid, the 

 origin of the neck and the whole of the thin end being kept out of 

 the liquid ; in this case the drop is completely dissolved without 

 rupture, and the thin end remains intact. If with different drops 

 the experiment is arrested at different stages of the dissolution, it 

 is found that the nucleus which remains presents no longer the pro- 

 perties of the original drop ; it no longer breaks up when the thin 

 end is broken off — which shows clearly that the interior mass of 

 the glass does not intervene in the phenomenon. 



These two experiments prove at once that the stability of the 

 drop is bound up with the existence of the origin of its neck, since 

 whenever it is preserved no disaggregation of the drop takes place. 



Now it is known that chilled glass remains more expanded than 

 if it had been cooled slowly ; the exterior layers of the drop, more 

 strongly chilled, are more expanded than the interior layers, which 

 have occupied more time in cooling. We may therefore regard the 

 drop as formed by the superposition of layers of glass unequally 

 chilled and expanded, cemented to one another. The exterior 

 layers, kept by the resistance of the interior ones, can only yield to 

 the force of elasticity which solicits them if, through any cause 

 whatever, they are all at the same time set free to return to their 

 normal state of expansion. 



It results, moreover, from the form of the drop, that all these 

 layers, unequally stretched, meet together at the origin of the neck ; 

 so that on destroying this the common point of resistance vanishes. 

 and these layers, the actions of elasticity of which are added to- 



PhiL Mag. S. 4. Vol. 45. No. 302. June 1873. 2 H 



