C 



The Romans, according to Pliny, made their 

 best mortar a year before it was used; so that it was 

 partially combined with carbonic acid gas before it 

 was employed. 



In burning lime there are some particular pre- 

 cautions required for the different kinds of limestones. 

 In general, one bushel of coal is sufficient to make 

 four or five bushels of lime. The magnesian lime- 

 stone requires less fuel than the common limestone. 

 In all cases in which a limestone containing much alu- 

 minous or siliceous earth is burnt, great care should 

 be taken to prevent the fire from becoming too intense; 

 for such lime easily virtrifies, in consequence of the 

 affinity of lime for silica and alumina. And as in some 

 places there are no other limestones than such as con- 

 tain other earths, it is important to attend to this cir- 

 cumstance. A moderately good lime may be made at 

 a low red heat; but it will melt into a glass at a white 

 heat. In limekilns for burning such lime, there 

 should be always a damper. 



In general, when limestones are not magnesian 

 their purity will be indicated by their loss of weight 

 in burning; the more they lose the larger is the quan- 

 tity of calcareous matter they contain. The magne- 

 sian limestones contain more carbonic acid than the 

 common limestones; and I have found all of them lose 

 more than half their weight by calcination. 



Besides being used in the forms of lime and carbon- 

 ate of lime, calcareous matter is applied for the pur- 

 poses of agriculture in other combinations. One of 

 these bodies is gypsum or sulphate of lime. This sub- 



