50 FIELD, FOREST AND FARM 



the part played by mortar in masonry. Look at the 

 water that for several days has covered a bed of lime 

 slaked by the masons. You will see floating on the 

 surface small transparent particles resembling ice. 

 Well, these tiny fragments of crust are nothing 

 but stone like that from which the lime was obtained; 

 in a word, they are limestone or carbonate of lime. 

 To make stone of that kind two substances are neces- 

 sary, as I have just told you : lime and carbonic acid 

 gas. The lime is furnished by the water, in which 

 it must be present in solution, since the water covers 

 a thick bed of this material ; and as to the carbonic 

 acid gas, it is furnished by the air, where it is always 

 to be found, though in small quantities. Lime, then, 

 has this peculiarity, that it slowly incorporates the 

 small amount of carbonic acid gas present in the at- 

 mosphere, and so once more becomes the limestone 

 that it was before. 



"A similar process goes on in mortar: the lime 

 takes back from the atmosphere the gas that it had 

 lost in the heat of the lime-kiln, and little by little 

 becomes stone again. The sand mixed with it serves 

 to disintegrate the lime, which thus more easily ab- 

 sorbs the air necessary for its conversion into lime- 

 stone. When the mortar has fully resumed the form 

 of limestone the courses of masonry are so strongly 

 bound one to another that the stones themselves 

 sometimes break rather than give way. 



"What is known as fat lime is lime that develops 

 great heat when brought into contact with water, and 

 also increases considerably in volume, forming with 



