SOSMAN, HOSTETTER, MERWINI CALCIUM CARBONATE 563 



If then adjustments are made so that no current flows through 

 the galvanometer, the position of the sliding contact, the value 

 of the test current, and the voltage across the slide wire are read, 

 we have data from which the resistance R is readily calculated. 



From the measured resistance, and the depth and distance 

 between electrodes a value may be obtained for the effective 

 resistivity as explained above. The value depends mainly 

 upon the resistivity in the neighborhood of and between the 

 potential electrodes and very little upon the resistivity at dis- 

 tances from either of these electrodes equal to half the distance 

 between the current electrodes. 



So far the method has been used only for determining resis- 

 tivities in a region very close to the surface, a few meters or less 

 in radius. To measure the effective resistance of a much larger 

 portion of earth extending to a considerable depth, the elec- 

 trodes would be placed much farther apart. Such a measure- 

 ment might be of assistance in locating deposits of ore of high 

 conductivity. 



CHEMISTRY. — The dissociation of calcium carbonate below 

 500° C. R. B. SosMAN, J. C. HosTETTER, and H. E. Mer- 

 wiN, Geophysical Laboratory. 



In connection with investigations in progress in this Labora- 

 tory it appeared possible to apply a vacuum furnace, developed 

 by the authors for the measurement of low dissociation pressures 

 of oxides,^ to the measurement of the carbon dioxide pressures 

 of calcite and aragonite. If aragonite is a metastable form at 

 all temperatures, its dissociation pressure should be measurably 

 higher than that of calcite at temperatures below that at which 

 aragonite is rapidly transformed into calcite. 



This transformation takes place in a few minutes in air at 470°, 

 although in contact with solutions it will go on as low as 350°, 

 according to results obtained by Williamson in this Laboratory. 

 The problem was therefore to measure the dissociation pressures 

 at several temperatures below 470°. 



' This Journal, 5: 277-285. 1915. 



