6. YORKSHIRE. 119 



century of time it has been cxpofed to the 

 influence of the atmofpherc. 



It is therefore probable that the atmo- 

 fphere imparts nothing, voluniarily, of a co- 

 hefive nature to the niortar of walls which 

 are expofed to it. 



But it is more than probable that cement, 

 containing a portion of lime, imbibes from 

 the atmolphere fomething which gives it a 

 degree of tenacity fuperior to that which it 

 had on its fiiTi becoming dry in the wall ; 

 and it is a fad: well eflablifiied, that lime 

 begins to imbibe, the moment it Jirows cool 

 from the kiln, tiat which the fire has d^^- 

 jirived it of •, namely, fixed air ; which fixed 

 air being imbibed, after the cem.ent is depo- 

 fited in the walls, is, frobably, a princip:ii 

 caufe of tenacity. 



This being admitted it mav feemto follow, 

 that the niore qu"ckly it is transferred from 

 the kiln to the building, the greater portion 

 of air will be imbibed afrer it is laid in the 

 walls, and of courfe the greater effcvft will 

 lime have on the tenacity or cohefion of the 

 cement : and hence we might be led to in- 

 fer, that if the ancients had any fuperior fkill 

 I 4 irj 



