64 Mayow 



their form be altered by some force after they have 

 cooled. 



But although I do not think that the parts of the 

 rigid body are under tension, still it is probable that 

 that violence with which the glass drops burst is truly 

 elastic, and that it results as elastic force does from the 

 impulse of the subtle matter. And it is probable that 

 this elastic force arises in the following way. When 

 the small portion of glass glows and is in a sense fused, 

 its structure is opened to such an extent by the rapid 

 movement of nitro-aerial and fiery particles, that space 

 enough exists in it for the nitro-aerial particles and 

 besides for the subtle matter to execute their move- 

 ments. But when the molten glass is dropped into 

 cold water the fiery particles crowded at its surface are 

 immediately arrested in their motion when they meet 

 with the water particles, and those in the interior also 

 soon desist from moving. But it should now be 

 noticed that when the glass is cooled in this manner 

 its parts settle down and it becomes itself contracted, 

 not because these parts of the glass spontaneously 

 approach each other (for the parts not yet cooled are 

 not under tension and therefore do not possess a power 

 of recoil or a motion of restitution, as we have shown 

 above), but it is rather to be supposed that the sub- 

 sidence of the vitreous parts arises from this, that the 

 nitro-aerial particles occupy less space when they 

 cease from their fiery motion and no longer push the 

 parts of the glass away from each other, so that the 

 particles of the glass are forced towards each other by 

 the pressure of the atmosphere. 



But, now, since the outer surface of the said glass 

 immediately becomes rigid from being rapidly cooled 

 by the water, it becomes so solid by reason of the 

 nitro-aerial particles infixed in it, that the nitro-aerial 



