Correction of the Gas- Thermometer, 71 



pressure^ or -043° for /? = 56 cms. He finds, as the result 

 of some experiments between 0° and 100°, that the scale- 

 difference of the constant-pressure thermometer is about 

 twice that of the constant-volume instrument. This agrees 

 very fairly with the result deduced from the Joule-Thomson 

 equation and exhibited in Table V. Assuming the ratio 2, 

 the correction for the constant-pressure thermometer at 

 445° would be about +*115° at 76 cms. initial pressure, 

 which would give a difference of '072° between our instru- 

 ments at the boiling-point of sulphur. Chree has since 

 detected a small error in ChappmV calculation which would 

 increase this estimate in the proportion of *023 to '017. 

 A direct comparison between the constant- volume and con- 

 stant-pressure scales at this temperature would certainly be 

 desirable and feasible ; but in the meantime the estimate 

 derived from the Joule-Thomson equation appears to be more 

 probable than that of Chnppuis, and more in accordance with 

 the known behaviour of gases deduced by other methods of 

 investigation. 



12. Thermodynamical Correction of C0 2 Thermometer, 



The case of C0 2 is of particular interest because the pro- 

 perties of this gas have been so widely studied, and because the 

 deviations from the thermodynamical scale and the values of the 

 Joule-Thomson effect are so much larger and more easily 

 measured. A study of the properties of this gas might bo 

 expected to throw light on the effects to be expected with 

 other gases more perfect and better suited to thermometry ; 

 but it must not be forgotten that the type of the molecule is 

 different, and that for this 'reason it might be expected to 

 behave in a different manner to the diatomic or monatomic 

 gases. The gas actually employed by Joule and Thomson was 

 not quite pure, but they corrected their results empirically to 

 the case of pure gas, and their value of the cooling-effect at 

 17° t C. has been independently confirmed by Natanson (Wied. 

 Ann. xxxi. p. 502, 1887) employing gas from cylinders of 

 liquid C0 2 . The A*alues given by Table V. for the difference 

 from the hydrogen scale in the case of the constant-volume 

 thermometer at ^o=100 cms. are compared with the obser- 

 vations of Chappuis at -10°, +10°, 20°, 30°, 40°, and 60° C, 

 which are represented by the crosses ( + ) in fig. 1. The full 

 curve, as before, indicates the differences calculated by the 

 Joule-Thomson equation, the broken curve indicates the 

 formula of Chappuis. Three of Chappuis' observation- at 

 30°, 40°, and 60° were taken with a thermometer tilled with 



