i] 
150 The American Geologist. March, 190. 
importance compared with that occurring in consequence of 
the reabsorption of carbon dioxide from the air. 
Cements of this class are prepared at a higher temperature 
than are those of the preceding group, carbon dioxide requiring 
a greater degree of heat for its dissociation from . limestone 
and dolomyte than is necessary for driving off the water from 
gypsum. Both pure lime and (to a somewhat less degree) the 
magnesian limes are of great importance as building cements. 
A note at the end of the section on magnesian limes calls 
attention to the fact that, under certain conditions, they de- 
velop hydraulic properties ; but in this case their set is not due 
to recarbonation, but to the formation of magnesium hydroxide. 
Lime. On heating a relatively pure carbonate of lime to a 
sufficiently high degree, its carbon dioxide is driven off, leay- 
ing calcium oxide (CaO) or “‘quicklime.” Under ordinary con- 
ditions, dissociation is perfect 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, “slaking” occurs, heat being given oft 
and the hydrated calcium oxide (Ca H,O,) being formed. 
This hydrated oxide, will, upon exposure to the atmosphere, 
slowly reabsorb sufficient carbon dioxide to reassume its orig- 
inal composition as a lime carbonate. In order to counteract 
the shrinkage which takes place during this process, sand 
is invariably added in the structural use of the material; and 
it is probable that certain reactions take place between the lime 
and the silica of the sand. These, however, though doubtless 
contributing to the rapidity of set and final hardness of the 
resulting mortar, are only incidental to the principal cause— 
recarbonation. The presence of impurities in the original 
limestone affects the character of the lime produced. Of these 
impurities, the presence of silica in certain quantities gives the 
product hydraulic properties; these silica-limes will be dis- 
cussed in the next group as Hydraulic Limes. 
Magnesian Limes. The presence of any considerable 
amount of magnesium carbonate in the stone from which a 
lime is obtained has a somewhat noticeable effect upon the 
character of the product. If burned at the temperature usual 
for pure limestone, magnesium limestones give a lime which 
slakes without evolving much heat, expands less in slaking, 
