On Sal Nitriun and Nitro- Aerial Spirit 55 



ruptured. Indeed rigid bodies seem to acquire a 

 nature of this kind because their pores are crammed 

 and wholly filled up with certain particles fixed in 

 them, so that the shape of their pores can by no force 

 be altered, for in order that anything may be elongated 

 by extension or shortened by compression its pores 

 must change to a more oblong shape. For example, 

 let us put before our eyes a row of parts or branchlets 

 linked together such as is delineated in Plate I., Fig. i. 

 If this is to be lengthened or shortened it will be 

 necessary for the pores of the said branchlets to become 

 more oblong, as is manifest in Plate I., Figs. 2 and 3, 

 the first of which represents the pores of the elongated 

 and the second of the shortened branchlets changed in 

 both cases, though in a different manner, into a more 

 oblong shape. If these pores, however, be completely 

 filled with any solid particles, then in this case neither 

 the branchlets nor their pores could have their length 

 extended. And indeed it is probable that nitro-aerial 

 particles are fixed like little pegs in the pores of rigid 

 bodies and fill them up, since it is by these being 

 fixed in the pores of bodies that rigidity is produced, 

 as was previously shown. And hence it comes about 

 that neither the pores of rigid bodies nor consequently 

 the rigid bodies themselves can be either elongated or 

 shortened. And from this we may infer that when 

 perfectly rigid bodies are bent they remain on every 

 side of the same length as before. 



These things being assumed, it follows that the 

 convex surface of a perfectly rigid body will in bending 

 be drawn towards the concave surface as is the case 

 when a rigid body is bent in the second manner. For 

 if it were bent in any other way some one of its 

 surfaces would have to be either elongated or con- 

 tracted, as is evident from what has been said. But 



