Constants of Igneous Rock. . . 17 



above ratios and for the individual quantities, but the results 

 cannot be reproduced here. After a minute and careful com- 

 parison of all of this (see Bulletin, No. 54, pp. 227-233), I 

 concluded that the values of high temperature obtained in 

 case of the two reentrant bulbs, Nos. 1 and 2, not glazed 

 internally, were identical ; that by using the revolving muffle- 

 furnace described above (§ 8) temperature (increasing or 

 decreasing) changes slowly enough to permit the use ol* the 

 above metallic capillary connexions (§ 6) between bulb and 

 manometer with impunity ; that the platinum - iridium 

 (20 per cent.) thermocouple is free from serious anomalies ; 

 that in using the furnace and the reentrant bulb, a virtual 

 identity of environment for the thermocouple and the air- 

 thermometer had actually been secured ; and that, finally, the 

 methods preliminarily tested in the above pages are sufficient 

 for the rigorous solution of the calibration problem up to an 

 accuracy of 1° in 1000°. 



18. Conclusion. — For some years I have had the parts of a 

 standard constant-pressure high-temperature air-thermometer 

 in the laboratory, but have been prevented, by duties of a 

 more immediate geological bearing, from putting them 

 together. I will briefly refer to these parts here. 



In the manometer, volumes are to be read to *05 cub. cm., 

 temperatures to *07° C, and pressures to *01 cm. of mercury. 

 Hence I surround both branches of the U-tube with a tabular 

 cistern, through which w r ater at constant temperature con- 

 tinually circulates. Moreover, both branches are now made 

 of glass and communicate below, by means of a suitable stop- 

 cock, with an external flexible tube surmounted by a mercury 

 reservoir, by which pressure may be changed at pleasure. A 

 sufficient length of the metallic capillary between bulb and 

 manometer is also submerged in the water, so that the fine 

 current of air within may enter the manometer, already 

 reduced to the temperature of the bath. Finally, instead of 

 employing a cylindrical volume-tube in the manometer, as 

 was done above, whereby the accuracy of measurement 

 becomes rapidly insufficient in proportion as the air-thermo- 

 meter temperature rises (see distribution of points on the 

 chart, PI. 1.), the first 100 cub. cm. of the tube are blown out 

 into a spherical bulb, and the remaining length of 140 cm. of 

 the tube utilized to measure the remaining 150 cub. cm. Thus 

 even *01 cub. cm. is appreciable, and the tube is now available 

 for accurate measurements between about 200° and over 1500°. 



Again, since the normal volume of the gas actually expand- 

 ing must be known to '01 cub. cm., it is necessary to reduce the 

 stem volume of variable temperature to the smallest limits 



Phil. Mag. 18 5. Vol. 34. JNo. 206. July 1892. 



