l.jS 1'HOVKRTIES OF GLASS. 



glass (lies, and sometimes at flic very moment of the shock: nay, 

 a bit of Hint no larger than a grain dropped into several glasses 

 successively, though it did not immediately break the' i. ycr wln-n 

 set by, they all (lew in less than three quarters of an hour Some 

 other bodies produce this effect as well as flint as sapphire, dia- 

 mond, porcelain, hard tempered steel, also marbles MI ii as boys 

 play with, and l'kewi->e pearls. Those experiments were made be. 

 fore the Royal Society, and succeeded equally when the glasses 

 were held in the hand, when they were rested on a pillow, put in 

 water, or filled with water. It is also remarkable, Hut the glasses 

 broke upon having im-ir bottoms slightly rubbed with the finger, 

 though some of them did not fly till halt' an hour afUr the rubbing. 

 If the glas-es are every where extremely thin, they do not break 

 in these circumstances. 



Some have pret n-ied to account for these phaenomen,a, by say. 

 ing, that the bodies dropped into the vessels cause a concussion 

 which is stronger than the cohesive force of the iilass, and conse. 

 quently that a rupture must ensue. But why does not a ball of 

 iron, gold, silver, or copper, which are perhaps a thousand times 

 heavier than flint, produce the same effect ? It is because they are 

 not elastic. But surely iron is more elastic than the end of one's 

 finger. Mr. Euler has endeavoured to account for these appear- 

 ances from his principles of percussion. He thinks that this ex. 

 periment entirely overthrows the opinion of those who measure 

 the force of percussion by the. vis viva, or absolute apparent 

 strength of the stroke. According to his principles, the great 

 hardness and angular figure of the flint, which makes the space of 

 contact with the glass extremely small, ought to cause an impres- 

 sion on the glass vastly greater than lead, or any other metal ; 

 and this may account for the flint's breaking the vessel, though the 

 bullet, even falling from a considerable height, does no damage. 

 Hollow cups made of green bottle-glass, some of them three inches 

 thick at the bottom, were instantly broken by a shiver of flint, 

 weighing about two grains, though they bad resisted the shock of 

 a musket-ball from the height of three feet. 



That Mr. Luler's theory cannot be conclusive any more than 

 the other, must appear evident from a very slight consideration. 

 It is not by angular bodies alone that the glasses are broken. The 

 marbles with which children play are round, and yet they have the 

 came effect with the angular flint. Besides, if it was the mere 



