542 Prof. H. L. Callendar on a 



Travers, in a compensated apparatus at constant volume as 

 well as at constant pressure. 



In the case of nitrogen, the calculated difference between the 

 scales of the constant-volume and constant-pressure thermo- 

 meters is dependent on the extrapolation of purely empirical 

 formulae, and must be regarded as still somewhat uncertain. 

 The only direct comparison hitherto made between the two 

 scales at high temperatures on the same mass of gas in the same 

 bulb, is that which I made in 1886, employing a platinum- 

 thermometer enclosed in the bulb as the medium of comparison. 

 According to these experiments, the constant-volume air-ther- 

 mometer appeared to read three or four tenths of a degree 

 higher than the constant-pressure in the neighbourhood of 

 450° C. I did not attach much weight at the time to this 

 result because the apparatus for the difficult pressure mea- 

 surements was not sufficiently perfect for the purpose, although 

 it is probable that good results might be obtained with suitable 

 apparatus by this method. The theoretical difference between 

 the scales at this point, as calculated by Rowland and myself 

 from the experiments of Joule and Thomson, by Rankine's 

 formula, is about two-tenths of a degree in the same direction. 

 According to the formula of Van der Waals it would be nearly 

 half a degree. From a comparison of the constant-pressure 

 nitrogen scale at 20° and 40° C. with the hydrogen scale, 

 M. Chappuis has been led to the conclusion that the differ- 

 ence between the scales would amount to about four-tenths 

 of a degree at the S. B.P. in the case of nitrogen, but might 

 be greater in the case of air. We may conclude from these 

 considerations that about half the difference between our re- 

 sults for the S. B.P. may probably be attributed to the 

 difference between the scales, the exact value of which is still 

 uncertain. 



On the Correction for Expansion of the Bulb. 



Another correction of some difficulty is that for the 

 expansion of the bulb. I made a number of experiments in 

 1893, in conjunction with Mr. Griffiths, with a pair of compen- 

 sated thermometers, one of which contained hydrogen and 

 the other air or nitrogen. The primary object of these ex- 

 periments was to verify the parabolic difference formula for 

 the platinum thermometer with the greatest possible accuracy 

 between 0° and 100° 0. The hydrogen-thermometer could 

 not be used at higher temperatures owing to the reduction of 

 the glass ; but the nitrogen-thermometer was incidentally 

 employed for a determination of the S. B.P. The expansion 

 of the bulbs between 0° and 100° was very carefully determined 



