OFNATURE. 53 



ft was cut tranfverfly by a vein of clay, was 

 changed into a puvQjiher. The number there- 

 fore of fpecies, and varieties of foj/ils^ each 

 ferving for different purpofes, according to their 

 different natures, will be in proportion, as the 

 different kinds of earths and J^ortes are vari- 

 oufly combined. 



§. 4- 



Prefervation. 



As foj/ils are deftitute of life, and organifa- 

 tion, are hard, and not obnoxious to putre- 

 fadlion ; fo they lafl longer, than any other 

 kind of bodies. How far the air contributes 

 to this duration it is eafy to perceive, fmce air 

 hardens n\2Lnyftones upon the fuperficies of the 

 earth, and makes them more folid, compadt, 

 and able to refift the injuries of time. Thus 

 it is known from vulgar obfervation that limey 

 that has been long expofed to the air, becomes 

 hardened. The chalky marl^ which they ufe 

 in Flanders for building houfes, as long as it 

 continues in the quarry, is friable *, but when 

 dug up and expofed to the air, it grows gra- 

 dually harder. In the fame way our old walls, 

 and towers gain a firmnefs in procefs of time, 

 E 3 and 



