Peculiar Properties of Glass. 47 



//. — On Prince Rupert's Drops. 



In the seventeenth century Prince Rupert astonished and 

 amused the people of the English Court by producing drops 

 of glass with long tails attached, which burst into small 

 pieces the moment the tail was broken. Since his time 

 Robert Hooke and others have made experiments upon 

 them. It is believed that the explosive power of these 

 drops depends on an internal tension in the glass of the 

 drop due to the red hot, and consequently expanded, glass 

 being suddenly cooled and solidified, whilst the internal 

 contents have to adapt themselves to the rigid and ex- 

 panded envelope. These drops are produced by allowing 

 drops of molten glass to fall into cold water, a long tail 

 being left as the highly viscid molten glass falls. As a rule, 

 Rupert's drops contain a number of bubbles, which are due 

 to vacuous spaces, but there are some drops which are free 

 from such bubbles, and when the tail of one of these is 

 broken it bursts with greater force than a drop containing 

 bubbles. 



That these bubbles are vacuous I proved by heating the 

 drop to redness, when the bubbles disappeared, and after 

 cooling the drop of glass appeared quite solid and trans- 

 parent. 



To determine whether the Rupert's drop was less dense 

 than the drop after annealing, I took a large Rupert's drop 

 quite solid and transparent (free from bubbles) which 

 weighed in air I70"30 grains, and in water 102-66 grains. 

 It was laid on a piece of platinum, placed in a muffle 

 furnace, heated to redness, and allowed to cool gradually. 

 It then weighed in air 1 70*36 grains, and in water 102-960 

 grains. The specific gravity of the Rupert's drop was, there- 

 fore, 2-5177, whereas the specific gravity of the drop, after 

 the strain had presumably been removed by annealing, 

 was 2-5276, in other words, 100 volumes of ordinary glass 



