SOSMAN, HOSTETTER, MERWIN : CALCIUM CARBONATE 567 



Transition point in CaO. Lastchenko^ has shown by specific 

 heat measurem,ents that a transition point probably exists in 

 crystalline lim.e at about 400-415°, having a heat effect of 280 

 calories per molecular weight, or 5 calories per gram. This heat 

 effect is just about on the border of recognizability by a direct 

 time-temperature heating curve. Nevertheless we tried heating 

 curves on a sample of the crystalline lime from calcium nitrate, 

 using the vacuum furnace in order to avoid any heat effect 

 due to hydration or carbonation. At 11° per minute no break 

 was detected. At 5° per minute there was a distinct absorption 

 of heat at about 425-430°, but the corresponding evolution was 

 not distinguishable on the cooling curve. From the forms of the 

 crystals obtained from melts at higher temperatures it is evi- 

 dent that both the high- and low-temperature forms must be 

 cubic; the transformation is probably a crystallographic change 

 similar to the inversion of quartz at 575°. 



Dissociation pressures of calcite and aragonite. Although not 

 sufficiently definite to decide the question as to which is the more 

 stable form, the CO2 pressures measured were not without cer- 

 tain regularities. All of the pressures measured at 400° lie 

 between the limits 0.0019 mm. and 0.0168 mm.^ The greater 

 part lie between 0.0030 and 0.0090 mm. The dissociation pres- 

 sure of calcite at 400° calculated from Johnston's curve is 0.0020 

 mm. ; the pressures found are of this order of magnitude, which 

 is all that might reasonably be expected of an extrapolation to 

 a pressure only about yVoo^ of the lowest accurately measured 

 pressure on the curve referred to. 



Both calcite and aragonite, when heated alone at 400°, were 

 found to dissociate so slowly that it seemed hopeless to wait 

 for the pressure to reach an equilibrium, since the vacuum fur- 

 nace could be operated only during the working hours of the day. 

 Even at 600° calcite dissociated quite slowly. The addition of 

 5 per cent Fe203 to calcite did not seem to affect its dissociation 



'J. Russ. Phys.-Chem. Soc, 42:1604-1614. 1911. Bull. Inst. Polyt. Don. 

 1913, II, 9-46. 



8 The relatively high pressure of 0.0168 mm. was obtained from the inclusion- 

 bearing artificial calcite. 



