VOL. XLVI.] PHILOSOPHICAL TRANSACTIONS. 677 



inside of this mass of glass: there is none in the liquid red-hot matter of a glass 

 furnace. It was purely out of complaisance for a generally received opinion, 

 that Mr. Mariotte allowed the exterior fluid any share in the phenomenon ; and 

 Mr. Homberg put the finishing hand to its exclusion. But the kind of temper 

 given to the drop by plunging it red-hot into cold water, and its comparison with 

 tempered steel, is not so much a cause as a comparison : and is this comparison 

 very just? can there be any, between a long thin sword- blade, which breaks into 

 2 or 3 pieces, and a thick inflexible mass of glass, which flies into powder? 

 The tail alone of the drop might seem to favour this parallel , but an experiment 

 which he made entirely destroys this opinion, and proves that it is not the spring, 

 or the vibrations of the parts of the drop, that occasion its bursting. 



He put about half the tail of a glass drop into a vice, between two bits of 

 deal-board of about a finger's breadth. He screwed the vice, till he saw this 

 small cylinder or thread of glass make impressions in the wood on each side, for 

 its lodgment, in order to be sure that it could not be susceptible of vibrations. 

 In this condition he broke the end of the tail, supporting it on his nail, to pre- 

 vent forcing any part but the end which he intended to break; and in order to 

 be the more certain of giving no shock to the part that was squeezed in the vice. 

 The drop flew into powder as usual: and the portion secured between the two 

 bits of wood, perfectly retained its figure in the impressions wherein it was 

 lodged. But when he touched this little cylinder, it was reduced to powder, 

 much in the same manner as is said to have happened to some men who had been 

 struck with lightning. Now it was not possible for this glass to receive, or 

 convey to the body of the drop, any vibrations; or if any, they must be infinitely 

 small; and yet the effect was precisely the same as usual. Therefore the system 

 of vibrations is not happier than those invented before it. 



It is among the glass-workers, and in their art, that the secret of the glass 

 drop is to be sought; and there it is that he discovered it. Those who have seen 

 glass-houses know, that when a piece fails in the hands of a workman, Jie throws 

 it aside; and this piece is not long exposed to the air, before it breaks in pieces: 

 and when the same workman has succeeded in making a piece, and is willing to 

 preserve it, he takes great care not to let it cool in the air; but carries it hot 

 into another oven of a moderate heat, where he leaves it. for a certain time. 

 And this last operation is called annealing the glass. 



A natural philosopher, who is witness to this management, ought to inquire 

 into the reasons and necessity of it. How comes it that the glass which cools in 

 the air breaks, and when it has been nealed, it does not break? this is the reason. 

 A bit of melted glass, red-hot and liquid at the same time, is in that state, purely 

 because its particles are divided by so great a quantity of particles of fire, or 

 subtile matter so violently agitated, that these component parts of the glass hardly 



