20 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



mineral to a liquid must of course be defined by an absorption of heat. 

 Whether the appearance of the mineral charge in the furnace will 

 offer a trustworthy index through which to locate this absorption 

 may well be expected to differ with different substances. Nearly all 

 observers have recorded the fact that many substances of this class 

 remain very viscous after melting, and that the transition is not well 

 marked in the appearance of the material. 



We therefore planned an apparatus which should be as sensitive as 

 possible to heat changes over a long range of temperatures, and then 

 prepared to examine the thermal behavior of simple minerals of 

 natural or artificial composition when gradually heated or cooled. 

 Changes of crystalline form (Umwandlungen) or of state (melting and 

 solidifying) must involve a more or less sharply marked absorption 

 or release of heat, and be recorded as breaks in a smooth curve in the 

 same way as in the determination of metal melting points or the 

 singularities of any of the well-known chemical compounds at lower 

 temperatures. 



APPARATUS. 



The apparatus used in these determinations may be assumed to be 

 fairly well known. It is the same in all essential particulars as that 

 used by Holborn and Day* in establishing the high-temperature scale 

 with the gas thermometer at the Reichsanstalt. And yet it is plain 

 that such a scale requires some care in the transplanting, particularly 

 as the authors were without a gas thermometer and were, therefore, 

 not in position to make direct comparisons with the gas scale. 



THERMO-ELEMENTS. 



The temperatures were measured with thermo-elements exclusively. 

 We obtained from Dr. Heraeus (Hanau, Germany) a set of four ele- 

 ments cut successively from the same roll of wire, which, when joined 

 together, proved to be identically alike in their readings over the 

 range of temperatures covered by the gas scale of the Reichsanstalt 

 (250 to 1150 C.) within the limits of observation error. Through 

 the courtesy of Prof. Holborn these were taken to the Reichsanstalt 

 and measured in the original melting-point furnace with the same 

 elements in terms of which the gas-thermometer scale had been ex- 

 pressed, and five careful comparisons made. These were the melting 

 points of the pure metals, cadmium (in air), zinc (in air), antimony 

 (reducing atmosphere), silver (reducing atmosphere) and copper (in 



* Ludwig Holborn and Arthur L,. Day, Am. Journ. Sci (4), 8, p. 165, 1S99 



