564 SOSMAN, HOSTETTER, MERWIN : CALCIUM CARBONATE 



The downward extrapolation of Johnston's curve,- obtained 

 from measurements between the temperatures 587° and 894° 

 and the pressures 1 mm. and 716 mm., indicates the follow- 

 ing carbon dioxide pressures at lower temperatures: 450°, 0.017 

 mm. mercury; 400°, 0.0020 mm. A "blank" run in the fur- 

 nace up to 400° showed that the expected pressure from calcite 

 at 400° could be measured definitely and without ambiguity; 

 the equilibrium pressure decreases so rapidly with temperature, 

 however, that at 350°, or even 375°, it would be obscured by the 

 evolution of gases from the walls of the furnace and the glass 

 connections, if measurements were necessary over a period of an 

 hour or m.ore. 



Measurem.ents were accordingly begun at 425° on aragonite. 

 The material used consisted of clear crystals (U. S. National 

 Museum No. 17692) ground to pass 65 mesh. 350 mg. of this 

 powder was mixed with 150 mg. CaO made by heating the ara- 

 gonite powder in platinum over a Meker burner. The measure- 

 ments at 425° lasted 90 minutes. When cold, the aragonite was 

 found to have been completely converted into calcite. 



At 400°, on the other hand, 68 minutes heating converted 

 none of the mixture into calcite. The few calcite grains visible 

 were not more numerous than the calcite grains in the original 

 aragonite. It is evident, therefore, that 400° is practically the 

 only temperature at which calcite and aragonite can be com- 

 pared as regards their dissociation pressures. 



Measurements were therefore m.ade on aragonite and calcite 

 at 400°. Two varieties of calcite were used: (1) clear crystals 

 of Iceland spar (Kahlbaum, 1913);'' (2) artificial precipitated 

 CaCOs (J. T. 'Baker, lot No. 8212)." The latter is in small 

 well-formed rhombs, each with an inclusion at its center. Mix- 

 tures were made containing 350 mg. calcite, and 150 mg. CaO 

 made from the calcite by ignition. Experiments were also made 

 on the combination of CaO with CO2. 



2 Jour. Am. Chem. Soc, 32: 938-946. 1910. 



3 Containing 0.098 per cent MgCOs (Hostetter, J. Ind. and Eng. Chem., 6:392. 

 1914). 



* Containing 0.081 per cent MgCOs. 



