C l8 3 ] 
fliut up, but takes Fire, as foon as a free Commu- 
nication with the outward Air is given it. 
On the Union of thefe Caufes depend the Phoe- 
nomena of the Glafs-Drop. It is of a Hardncfs that 
refills the Strokes of a Hammer, becaufe the violent 
Condenlation, given to its Surface by the cold Wa- 
ter, into which it was thrown when in a foft State, 
render’d its Texture very clofe, compad, and con- 
fequently hard. 
It burfts with great Noife; and in fo doing it re- 
tains the Charader of all the EfFeds produced by 
the Explofion of the igneous Matter. 
Its Dull flies two or three Feet all around, becaufe 
it is pufh’d forward by the Adion of a Fluid con- 
tained in its Centre ; which would not happen, if 
it had been the Effed of an exterior Fluid. This 
fame Dull of the Glafs-Drop darts forward with 
greater Force in the Air-Pump than in the Air, be- 
caufe the Air is an Obftacle, of which it is freed in 
the Receiver of the Air-Pump : Wherefore it fome- 
times breaks the Receiver j and for the fame Reafon 
its Dull is finer, that is, more minutely broken, as 
being done by a ftronger Power, and lefs counter- 
balanced. 
This violent Explofion produces Light, becaufe the 
Property of fhining Lightning is always the EfFed of 
fuch an Explofion of the Matter of Fire : Wherefore 
this Fad affords another Proof, that this Matter is 
the Principle of the Phoenomenon 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 burft ; becaufe the Sort of 
oily Maftic that refults from this Mixture, flops the 
Pores 
