Wright and Lar sen — Quartz as a Geologic Thermometer : 435 



20'4 sq. mm. ; plate 3 was *22tt mm thick and covered an area of 

 6'S sq. mm., or one-third that of plate 1. This difference in 

 size of the two plates might account, in part at least, for the 

 differences in inversion temperature recorded. 



The temperature determinations in the thermal microscope 

 were furthermore checked by means of heating and cooling 

 curves after the method of Frankenheim,* on a large mass of 

 pulverized pure quartz. Although the most sensitive experi- 

 mental conditions (temperature change of '01° visible) were 

 adopted, the thermal effect of the inversion was exceedingly 

 slight and was furthermore distributed over a temperature 

 interval of 20° or more. Nevertheless, on both the heating 

 and cooling curves a slight absorption and corresponding 

 release of heat could be observed in the inversion temperature 

 region. To designate any particular portion of these temper- 

 ature intervals for especial interpretation is little more than 

 arbitrary where the total energy involved is so small, but the 

 temperatures at which the rate of absorption (or release) was 

 greatest are approximate^ as follows : On heating, 560°, 559% 

 569°, 564°, average 563° ; on cooling, 560°, 562°, average 561°. 

 It is not even safe to say that these numbers represent the 

 average inversion temperature for a great number of quartz 

 fragments, for finely pulverized quartz is such a poor con- 

 ductor for heat that the temperature of the charge cannot be 

 assumed to be uniform from surface to center during such a 

 measurement. The thermoelectric record accordingly lags 

 and the temperatures given are necessarily low. The deter- 

 mination by this means is therefore only approximate and the 

 method merely confirms in a general way an absorption and 

 release of a small quantity of energy in this region, but is not 

 competent to locate so small a quantity accurately. 



Very recent and still incomplete measurements by Dr. W. 

 P. White, of the Geophysical Laboratory, on the specific and 

 latent heats of quartz indicate an abrupt change in the thermal 

 capacity of quartz in the region of the inversion temperature. 

 At about 575° the specific heat measures '282=b*017, where 

 the (large) probable error ( + "017) includes the energy change 

 in passing the inversion point. The latent heat at 575° is 4 - 3 

 ±1 calories. There are some uncertainties in both the latent 

 and specific heat values due to the fact that a sufficient number 

 of observations has not yet been taken to determine, with 

 sufficient accuracy, the character of the function near the dis- 

 continuity. 



The above measurements and data prove definitely that 

 quartz undergoes a small energy change at about 575° and 

 that the change is reversible or enantiotropic. The amount of 



* Measurements by Dr. A. L. Day. 



