VOL. XLVI.] PHILOSOPHICAL TRANSACTIONS. 6fQ 



produces no effect while close shut up, but takes fire, as soon as a free commu- 

 nication with the outward air is given it. 



On the union of these causes depends the phenomena of the glass drop. It is 

 of a hardness that resists the strokes of a hammer, because the violent conden- 

 sation, given to its surface by the cold water, into which it was thrown when in 

 a soft state, rendered its texture very close, compact, and consequently hard. 

 It bursts with great noise; and in so doing it retains the character of all the 

 effects produced by the explosion of the igneous matter. Its dust flies 2 or 3 

 feet all around, because it is pushed forward by the action of a fluid contained 

 in its centre: which would not happen, if it had been the effect of an exterior 

 fluid. This same dust of the glass drop darts forward with greater force in the 

 air-pump than in the air, because the air is an obstacle, from which it is freed in 

 the receiver of the air-pump; hence it sometimes breaks the receiver; and for 

 the same reason its dust is finer, that is, more minutely broken, as being done 

 by a stronger power, and less counterbalanced. 



This violent explosion produces light, because the property of shining light- 

 ning is always the effect of such an explosion of the matter of fire; hence this 

 fact affords another proof, tiiat this matter is the principle of the phenomenon 

 of the drop. If the surface of the drop be ground with fine powder of emery, 

 imbibed with oil, it frequently happens that it does not burst; because the kind 

 of oily mastic that results from this mixture, stops the pores of the drop, and 

 prevents the sudden communication of the exterior fluids with the imprisoned 

 igneous matter; and as glass cannot be ground with very fine emery and oil, but 

 by long rubbing; such rubbing heats the drop, and graduidly opens the pores so 

 as to grant an insensible passage to the igneous matter, by which the drop be- 

 comes at last in the same case with nealed glass; and in the case in which itself 

 is, when it is put into the oven to be nealed. When a glass drop is made, by 

 suspending it in the air only, it does not break sooner than nealed glass; because 

 as this small mass of glass retains its heat a long while in the air, the heat serves 

 as a nealing oven, and keeps its pores dilated long enough for the igneous par- 

 ticles to find a free passage. 



The principles, by which he has accounted for the effects of the glass drop, 

 are not confined to this phenomenon alone: they are more general than is com- 

 monly imagined. Thus steel, like the glass drop, acquires its hardness by being 

 plunged into water; and if Messrs. Mariotte and Romberg had compared tnem 

 together in this circumstance alone, they had been in the right. 



The most celebrated natural philosophers, in order to account for the temper- 

 ing of steel, have had recourse to different arrangements of its parts produced 

 by the fire, and fixed, by the cold*of the water, in the new state, in which the 

 violent heat had put them. 



