568 SOSMAN, HOSTETTER, MERWIN : CALCIUM CARBONATE 



rate appreciably. At 400° this mixture gave a pressure of 0.0026 

 mm., in 18 minutes, and 0.0039 in 35 minutes; aragonite gave 

 0.0019 mm. in 18 minutes. 



The carbonate formed by absorption of COo by porous lime 

 dissociates readily. After the initial pumping out of residual 

 gas it gave at 400° a falling pressure of 0.0129 in 25 minutes, 

 and after a second pumping, a constant pressure of 0.0117 mm. 

 in 42 minutes. 



Mixtures were made containing 30 per cent CaO and 70 per 

 cent calcite or aragonite, in the expectation that the presence of 

 the second phase would hasten the establishment of equilibrium. 

 This seemed at first to be the case, as pressures were quickly 

 attained which then rose only slowly. But repeated pumping 

 out of the gas caused a progressive lowering of the pressures 

 attained and of the rates of increase, until the condition of pure 

 slow-dissociating calcite or aragonite was approached. It seems 

 probable, therefore, that the gases came chiefly from the CaCOs 

 formed by absorption of small amounts of CO2 from the air by 

 the CaO. This CaCOa dissociates readily, as just shown, and, 

 after the exhaustion of the CO 2, the CaO remains simply as a 

 neutral substance in the presence of the slowly dissociating 

 crystals of original calcite or aragonite. 



Whether the compound formed by the combination of the 

 porous CaO with CO2 is calcite, aragonite, or amorphous CaCOs 

 we are unable to say. The high dissociation pressure suggests 

 that it is amorphous. 



Sumin'ary. Pure lime, CaO, is obtainable in two forms. The 

 first, which is probably amorphous, results from the dissociation 

 of CaCOs at low red temperatures. On heating for a consider- 

 able time at higher temperatures, it changes gradually into the 

 cubic crystalline lime of refractive index 1.83. The latter forms 

 directly from silicate melts or from fused calcium nitrate, and is 

 the stable form at high temperatures. There are indications 

 that it has an inversion point (perhaps similar to the inversion 

 between high- and low-temperature quartz) between 400° and 

 430°. 



