PRINCE RUPERT'S DROPS. 321 



traction. One arch, falling, brings down another, and, once started, 

 they go in such rapid succession that the ear detects but one sound, 

 one explosive burst, in which the imp of contraction exults in the ruin 

 he has wrought. 



The peculiarities of the Rupert drops are toughness, elasticity, and 

 the property of breaking into small fragments when any fractui-e, 

 however slight, is made ; their strength to resist such fracture is, how- 

 ever, greater than that of annealed or unannealed glass. 



Wlien we consider that these same peculiarities are the character- 

 istics of the so-called "toughened glass" of M. de la Bastie, and that 

 the method of treating his "toughened " glass, in the cooling process, 

 is at least analogous to that of the Rupert drops, we are forced to be- 

 lieve in a certain relationship between them. 



The Rupert drop falls into a water-bath ; M. de la Bastie's glass 

 into an oleaginous bath, the exact composition of which has not been 

 made public. 



M. de la Bastie's glass is not malleable, is not unbreakable, but 

 simply tougher, harder to break than the ordinary annealed glass ; so 

 also is the Rupert drop. 



As the characteristic distinction between annealed glass and the 

 Rupert drop is the excessive strain upon the molecules of the latter 

 contraction versus cohesion it is fair to infer that the superior 

 strength, toughness, and elasticity, of the drop are due to such strain. 

 As it is harder to displace the key-stone of a loaded arch than of 

 an unloaded one, the simile may hold good in this case, and the 

 strain of contraction upon the molecules of the glass of a Rupert's 

 drop may help resist any outside force tending to disturb cohesion. 

 If an outside force could be so exerted as to act exactly in the same 

 direction as the power of contraction acts, undoubtedly such force 

 would be aided by contraction to destroy cohesion ; but, acting in 

 any other direction, contraction would aid cohesion to resist it. As 

 the molecules of glass are exceedingly small, and as, in the cooling 

 process, they one after another individually become rigid, the lines of 

 their contractive strain become so complicated that it is very unlikely 

 any outside force can be exerted in such direction as to unite its im- 

 pulse wdth theirs against cohesion. 



As the toughened glass of M. de la Bastie flies into many pieces 

 when fracture is effected, in a manner analogous to the breaking of 

 the Rupert drop, it is probable, at least, considering the process of 

 the oil-bath, that such flying into fragments is due to a strain of con- 

 traction exerted by the molecules of its substance. And if such a 

 strain exists, as the flying seems to prove, it is also reasonable to sup- 

 pose that, exactly as in the case of the Rupert drop, this strain of 

 contraction among the molecules of its mass produces the superior 

 toughness, strength, and elasticity, which are claimed for this newly- 

 invented glass. 



VOL. Tin. 21 



