538 Prof. H. L. Callendar on a 



to a certain extent on the value assumed for this point. Since 

 the proposed scale is a practical and not an ultimate or theo- 

 retical standard, it has been suggested as a matter of practical 

 convenience to retain the value which has hitherto been 

 assumed, and to which so much work has already been 

 referred. A change of 1° in the value assumed for the S.B.P. 

 would mean a change of 6° at 1000° C. It is possible that a 

 change of this nature might lead to more accurate results at 

 the higher points, but the evidence at present is rather the 

 reverse. It may be desirable in special cases to make sub- 

 sidiary tests at other points, such as the B.P. of oxygen, or 

 the F.P. of silver ; but the simplicity of the process is in 

 marked contrast to the elaborate calibration that is necessary 

 in the case of a mercury-thermometer which is to be used for 

 accurate work. 



The order of accuracy in reading the B.P. of sulphur 

 with a platinum-thermometer is about a hundredth of a 

 degree, if all the conditions of the experiment are changed. 

 This has been verified by observing the B.P. with the same 

 thermometers, but with different resistance-boxes and other 

 apparatus, in Cambridge, Montreal, and Chicago, and also 

 recently at Paris and Kew. The remaining uncertainty in 

 the absolute value of the S.B.P. is purely a question of gas- 

 thermometry. The recent experiments * of Messrs. Harker 

 and Chappuis at the International Bureau at Sevres, while 

 verifying the parabolic difference-formula with remarkable 

 precision, have given the value 445 0, 27 C. for the S.B.P., 

 which exceeds the value previously assumed by O- 74. This 

 order of agreement is really very satisfactory, considering 

 the difficulty of the measurements, and the entirely different 

 conditions under which the experiments were conducted. 

 The discrepancy, in fact, is so far within the possible limits 

 of uncertainty of gas-thermometer measurements, that it is 

 quite possible that the two results may be perfectly reconciled 

 when the corrections for the difference between the scales of 

 the constant-pressure and constant-volume thermometers, and 

 for the expansion of the reservoir, have been more satisfactorily 

 determined. Since, however, it is necessary at this present 

 time to adopt some particular value as the basis of the 

 standard scale, it may be well to discuss the question here 

 with a view to assisting in the decision. 



On the Difference beticeen the Scales of the Constant-Pressure 



and Constant- Volume Gas- Thermometers. 



The results of Messrs. Harker and Chappuis were obtained 



with constant-volume nitrogen-thermometers of hard-glass 



* Proc. Roy. Soc. 1809. 



