774 



PEINCE RUPERT'S DROPS. 



hammer without breaking. It requires rather consider- 

 able force to break a piece l cm long off the tail ; fre- 

 quently it cannot be done with the fingers, but the flat 



FIG. 384 (A, real size ; B, C, f real size}. 



pliers must be used. No sooner, however, is the 

 extremity of the tail broken off, when in an instant 

 the entire mass cracks and is reduced to innumerable 

 extremely small pieces of glass, like grains of salt, and 

 even partly to a fine powder. 



' Bolognian flasks ' are simply small glass bottles, 

 which, after being formed, have been suddenly cooled 

 in the air; they have usually one of the forms shown 

 at B and C, fig. 384. The flask is sufficiently strong to 

 bear a smart blow upon the outside ; for example, it 

 may be held within the closed hand, and the thick end, 

 which is allowed to project, may be struck with con- 

 siderable force against a wooden table, or it may be 

 let fall upon a stone from a height of one metre. It 

 will not usually break in these cases, or at most a small 

 splinter of glass will be broken off the convex larger 

 portions of the flask. But when a bit of glass or flint, 

 measuring about 6 mm or 8 mm each way, with sharp angles, 



