566 SOSMAN, HOSTETTER, MERWINI CALCIUM CARBONATE 



4 mg. remained unabsorbed. Practically all of this was absorbed 

 on reheating to 460°, the small residue being probably nitrogen. 

 An interesting feature of this reheating was the increase of pres- 

 sure observed between 400° and 460°, in which interval the pres- 

 sure nearly doubled, although already at least 10 times the cal- 

 culated dissociation pressure of CaCOa at these temperatures. 

 On continued heating the pressure again continued to fall. The 

 phenomenon suggests a condensation or absorption of the gas 

 in the porous solid before true chemical union occurs. 



A second variety of lime is obtained by carefully dehydrating 

 calcium nitrate, melting it, and gradually raising the temperature 

 until all nitric oxide is driven off.'^ The resulting crystals are 

 isotropic cubes and octahedra, of refractive index 1.83. They 

 are identical with the CaO found in the lime-alumina-silica melts 

 of Rankin and Wright.*' The porous lime obtained from cal- 

 cium carbonate seems to gradually go over to the cubical form 

 on heating; calcite heated in platinum over the blast lamp for 

 1 hour gave cubic lime of index 1.83, and impure commercial 

 building-lime also had the same properties. 



Some lime which had been fused in a graphite vacuum furnace 

 in 1913 by C. W. Kanolt of the Bureau of Standards was also 

 examined. The greater part of it consisted of rounded grains 

 of isometric lim.e of index 1.83, being similar to the rounded lime 

 grains formed in silicate melts. 



The much lower reactivity of the cubical crystalline lime is 

 shown by the following experiment: 501 mg. of 100-mesh crystal- 

 line CaO from calcium nitrate was heated in the vacuum furnace 

 with quantities of from 5.0 to 12.5 mg. CO2, and at temperatures 

 between 400° and 600°. The pressures were from 1.65 to 4.10 

 mm. Although the dissociation pressure at 600° is only 2.35 

 mm. according to Johnston, the total amount of COo absorbed 

 in all these heatings was not oVer 0.86 mg. The porous lime 

 referred to above, on the other hand, absorbed 11 mg. in a few 

 minutes at 110° and lower. 



^ By special precautions clear crystals of CaO up to 10 mm. in length can be 

 obtained by this method. Brtigelmann, Zs. anorg. Chem., 10: 415-433. 1895; 

 69:248-270. 1908. 



« Am. Jour. Sci., 39: 1-79. 1915. 



