071 Sal Nitrum and Nitro-Aerial Spirit 63 



it moves. Meanwhile the subtle matter is reflected 

 from the parts which it strikes, and dashes instantly 

 against the particles opposite, to be repelled anew. 

 And so the subtle matter strikes incessantly the parts 

 of the rigid body, and its particles which are almost 

 infinite in number and distributed through the whole 

 mass of the rigid body all co-operate to restore to their 

 original shape the pores of the rigid body and conse- 

 quently the rigid body itself. And in this efi'ort the 

 explanation of elasticity seems to lie. 



We remark here in passing that the force with 

 which a string that has been violently stretched con- 

 tracts to its original length arises from the same cause. 

 For although the string itself may be a loose body, yet 

 its minute fibres are somewhat rigid and their shape 

 alters with the extension of the string. But the move- 

 ment of the contracting string is caused by the effort 

 of its fibres to revert to their original form. 



From what has been said, we may conjecture why 

 the glass globules with a sharp beak attached to them, 

 commonly called glass drops, which are formed by 

 dropping a little molten glass into cold water, burst 

 with remarkable and almost explosive violence into 

 the minutest fragments if the fine end of the beak is 

 broken. Some imagine that the violence with which 

 the drops burst is caused by the bending and tension 

 of the parts of the glass. But it is hard to conceive 

 how the parts of the said glass can be in a state of 

 tension, since to produce the tension from which 

 elastic force results it is an indispensable requisite that 

 the rigid body after it has cooled should be bent by 

 some force or, what amounts to the same thing, 

 should be stretched. For in whatever way the heated 

 rigid body or its parts may be bent or distended 

 while cooling, they will never be under tension unless 



