BOLOGNIAN FLASKS. 775 



is dropped into it, or the flask is slightly shaken while 

 the piece of flint is in it, it falls to pieces. 



The thick end of a Prince Rupert's drop may be held in the left 

 hand while the tail is broken with the forefinger and thumb of the 

 right hand; a small explosion takes place, and a slight blow is 

 felt in the left hand. There is no real danger in making the 

 experiment, which may be rendered still more safe by making a 

 small hole in a piece of writing-paper, pushing the tail partly through 

 the hole, wrapping the paper round the thick portion, and holding 

 the covered drop in the left hand while the point is broken off. 

 The small fragments have obtuse edges, but if a Bolognian flask is 

 shivered the fragments are rather sharp, and care must be taken 

 when they are removed. 



When the stopper of a bottle or decanter becomes firmly fixed, it 

 may often, but not always, be loosened by moderate continued knocks 

 applied with a piece of wood sideways against the handle of the 

 stopper. If it cannot be moved by this means the neck is heated 

 over a spirit or gas flame, the bottle being constantly turned, and 

 the neck heated not more than the hand can bear. Since the neck 

 becomes hot before the stopper, it expands first, and the stopper is 

 freed from its hold. After the stopper has been removed it must 

 not be put into the neck again until the latter has become quite 

 cold ; otherwise the stopper sinks deeper into the expanded neck, 

 and when contraction takes place the neck either cracks or closes so 

 firmly around the stopper that its removal afterwards can scarcely 

 be accomplished by heating it again. 



It has been already shown that liquids, when heated, 

 expand more than solid bodies. This fact may be further 

 demonstrated by means of a small ' float/ of glass, (fig. 

 385), partly filled with water, and so adjusted that it 

 floats in cold water and projects a few millimetres above 

 the surface of the liquid. When placed in boiling or at 

 any rate very hot water it sinks to the bottom of the liquid. 

 Since the volume of a body which is heated increases, 

 while its absolute weight remains unaltered, the specific 

 gravity of a body must clearly decrease when it is 

 heated, and become the less the more the body expands. 



