26 



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



Heating 

 coil -,$ 



Fig. 



3. The furnace, showing ther- 

 moelement and charge. 



The coil, which was obtained from Dr. Heraeus, was of platin- 

 iridium wire (90 parts Pt., 10 parts Ir.), 1.5 mm. in diameter, and 



required about 3000 watts to 

 maintain a constant temperature 

 of 1600 C. The furnace was 

 carried at times on a 1 10-volt 

 direct-current street main, but 

 accurately constant temperatures 

 could not be depended on without 

 the storage battery. 



The insulation in these furnaces 

 was so perfect that shutting off or 

 reversing the heating current at 

 the highest temperatures did not 

 produce a quiver in the galvanom- 

 eter to which the thermo-element 

 was connected, although the sen- 

 sitiveness of the system was such 

 that a leakage amounting to a 

 single micro-volt (corresponding to 

 less than o.i) at 1600 would have caused a displacement of more 

 than two millimeters on the scale. 



STANDARDS. 



The thermo-electrical potential was measured upon a potentiom- 

 eter (Wolff, Berlin, Reichsanstalt calibration) in terms of a standard 

 cadmium cell (saturated) prepared by ourselves. Two of these 

 cells were used interchangeably during the earlier measurements. 

 Toward the close of the series four fresh cells were prepared for com- 

 parison with the earlier ones and were found to agree with them within 

 0.0001. One of these later cells (the readings of the four were iden- 

 tical to the fifth significant figure) was verified bv Dr. Wolff, of the 

 Bureau of Standards, by comparison with the standard Clark cells of 

 that institution and found to be 1.0195 V at 20 C, assuming the legal 

 value (United States) of the Clark cell, 1.434 V, at 15 C. Substitut- 

 ing the Reichsanstalt value, Clarke = 1.4328,* our cells would give 

 a normal potential difference of 1.0186 at 20 . The temperature 

 determinations which follow are, therefore, calculated in terms of this 

 number. 



With the apparatus here described, the authors were enabled to 

 command any furnace temperature up to 1600 conveniently, to regu- 



* Jaeger u. Kahle, Wied. Ann., 65, p. 926, 1898. 



