i04 Thickness^ Cement and Materials of JFalLs. 



otherwise ; as has been observed in the instance (though 

 it is not singular) of the stone ^vo^k of the Schuylkill 

 bridge; which will long afford, under our own view, 

 facts, proofs and examples, for most of the operations, 

 and component parts of strong and masi^ive masonry. 

 It has been the over richness of the moitar, that has 

 suggested this mistaken opinion.* The cement will 

 solidif}-, if properly composed, wherever there is czr.- 

 and it is well known, that this subtile fluid pervades ail 

 matter. If it were not other\vise proved, as it often has 

 been, both philosophically and experimentally ; the in- 

 stances of toads and frogs^ founcl in perfect animation, 

 in the midst of solid blocks of marble, and granite, 

 would be sufficient. Air must be inhaled for respira- 

 tion, and must exist (as it is essential to life) in the stone; 

 and communicate with, and circulate through, the com- 

 mon atmosphere, so as to bring fresh supplies, and car- 

 ry off the mephitic; or these animals, thus isolated, 

 would soon perish; if they could begin their existence; 

 though they are said to require a less portion of «/r, 

 than others. f There is no recess so retired, as to es- 



* A decided proof of this, is mentioned in one of the re- 

 ports of the committee, who superintended the building of 

 this bridge ; in the account of the masonry intended for a 

 pier ; but abandoned, as to its use, in that capacity. See 

 Statistical Account. Page 41. 



■j- Like all reptiles and amphibious animals, they can live 

 with less air, than is usually required by others ; yet it does 

 not follow that they can exist without it. Some have doubt- 

 ed their being furnished with the organs of respiration. But 

 I have, while attending experiments with the air pump^ seen 

 them die in an exhausted receiver. 



