58 Pbilofophical Conjectures. [Book VI. 



part, which is necefiarily cooled in a more gradual 

 manner j if, therefore, by a crack or rafure, a folution 

 of the continuity takes place in the external furface, the 

 fudden action of the parts which remained in a ftate of 

 tenfion, to recover that of perfect: cohefion, is fuppofed 

 to effect the deftruction of the mafs. 



This explanation I confefs has not appeared to me 

 fatisfactory, and I have been inclined to fufpect thac 

 the phenomenon arifes either from a quantity of air 

 being included in the fubftance of the glafs, which 

 rufhes fuddenly out, on the furface which includes it 

 being broken ; or that by the fudden cooling the pores 

 of the glafs are fealed up by the fine fmooth furface, 

 and contain little or no air, fo that on the continuity 

 of that furface being interrupted, the air fuddenly 

 ruftiing into all the pores of the glals may effect its 

 diflblution. That the whole effect depends on de- 

 ftroying the continuity of the furface, I have fufficiently 

 proved; for unlefs the flint or pebble which is let 

 fall into the philofophical phial is large and angular 

 enough to fcratch the furface of the glafs, it will not 

 break. To afcertain whether the fracture of thefe 

 toys depends or not upon the air, a few experiments 

 mi-ght be made on Prince Rupert's drops in a va- 

 cuum j when, if it proceeds from air included in the 

 drop, the explofion will be more violent, or the drop 

 would perhaps fpontaneoufly burft; and if from the con- 

 trary caufe, it will not break at all in vacuo. 



