calcium oxide 56%, carbon dioxide, 44%. In the magnesian 

 limestones part of this calcium carbonate is replaced by magne- 

 sium carbonate, the resulting rock therefore having a formula of 

 the type X CaCC>3, Y MgCC)3. This replacement may reach 

 the point at which the rock has the composition of the mineral 

 dolomite an equal mixture of the two carbonates, with the 

 formula CaCO3, MgCO3, corresponding to the composition 

 calcium oxide 30.43%, magnesium oxide, 21.74%, car- 

 bon dioxide, '47.83%. Limestones may therefore occur 

 with any intermediate amount of magnesium carbon- 

 ate, and the lime which they produce on calcination 

 will carry corresponding percentages of magnesium ox- 

 ide, from o% to 21.74%. Commercially those limes which 

 carry less than 10% of magnesium oxide are, for building pur- 

 poses, marketable as "pure limes"; while those carrying more 

 than that percentage will show sufficiently different properties 

 to necessitate being marketed as "magnesian limes." 



Aside from the question of magnesia, a limestone may con- 

 tain a greater or lesser amount of impurities. Of these the most 

 important are silica (SiO2), alumina (A12O3), and iron oxide 

 (Fe2O3). These impurities, if present in sufficient quantity, 

 will materially affect the properties of the lime produced, as 

 w r ill be noted under the heads of Hydraulic Limes and Natural 

 Cements. 



The Carbonate Cements may be divided into tw r o classes : 



1 i ) High calcium limes ; 



(2) Magnesian limes. 



High Calcium Limes, On heating a relatively pure carbon- 

 ate of lime to a sufficiently high degree, its carbon dioxide is 

 driven off, leaving calcium oxide (CaO) or "quicklime." 

 Under ordinary conditions, the expulsion of the carbon dioxide 

 is not perfectly effected until a temperature of 925 C. is 

 reached. The process is greatly facilitated by blowing air 

 through the kiln, or by the injection of steam. On treating 

 quicklime with water, "slacking" occurs, heat being given off, 

 and the hydrated calcium oxide (CaH2O2) being formed. The 

 hydrated oxide will, upon exposure to the atmosphere, slowly 

 reabsorb sufficient carbon dioxide to reassume its original com- 

 position as lime carbonate. As this reabsorption can take place 



