142 Mr. Carl Barus on the Pressure- Variations of 



wide ranges of temperature, with the hope of stimulating 

 speculations on the subject, somewhat more rigorous than that 

 of Groshans (§16). It is clear that if a law can be found by 

 which the normal (76 cm.) boiling-point of a substance can 

 be predicted from an observed low-pressure boiling-point, 

 then a more complete knowledge of high temperature boiling- 

 points can be arrived at than is now available. More than 

 this : by varying pressure, boiling-points of different metals 

 may be made to overlap each other. Hence a thermocouple 

 calibrated as far as the boiling-point of zinc, for instance, 

 may be used to measure a low-pressure boiling-point of bis- 

 muth (say) ; and the couple then may be further calibrated 

 by the normal boiling-point of bismuth, predicted by aid of 

 the law in question. The process may obviously be repeated. 

 The couple whose calibration interval has been enlarged in 

 the manner given may now be used to fix the low-pressure 

 boiling-point of some other suitable metal, and then in turn 

 be further calibrated by aid of its boiling-point. The limit of 

 such a method is the fusibility of platinum. The fact that at 

 high temperatures the vapours to be used will be either mon- 

 atomic or diatomic enhances the interest of the project (cf. 

 Biltz and V. Meyer, § 15, below). 



2. The platinum /iridio-platinum thermocouple used in 

 this work had on another occasion * been tested for poly- 

 merization anomalies, by minute comparison with the porce- 

 lain air-thermometer, and between about 400° and 1300° 

 none were found. The alloy contained about 20 per cent, of 

 iridium. Unfortunately 1 could not avail myself of the former 

 air-thermometer comparisons, nor was it expedient to repeat 

 the work. M. Le Chatelier f rejects the iridio-platinum 

 couple because of irregularities of the kind referred to. Dr. 

 Hallock and I have also found them pronounced in other 

 iridio-platinum couples J. In the present couple slight irre- 

 gularities between 350° and 450°, equivalent to about 5°, had 

 to be allowed for. When the range of temperatures is very 

 large, the Avenarius-Tait equation is insufficient ; but this 

 equation subserves a good purpose when interpolations be- 

 tween two fixed temperature-data are called for, the data lying 

 anywhere on the scale, but not too far apart. 



Apparatus. 



3. Boiling-points below 500° may be studied in a closed 

 glass tube, a a a a, fig. 1, within which a thin- walled tube, d d d, 

 open at both ends, passes coaxially quite through. In the 



* Bull. U.S. Geo! Survey, no. 54, p. 208 et seq., 1839. 



t Le Chatelier, /. c. 



X Bull. U.S. G. S. no. 54, pp. 80, 114 et seq., 1889. 





