32 PROF. CALLENDAR AND MR. MOSS ON EXPANSION OF MERCURY. 



The actual coefficient at any temperature is seldom required with a high order of 

 accuracy. It may be obtained from the tables with sufficient accuracy by taking the 

 difference of the volumes for a range of 5 C. on either side of the point where the 

 coefficient is required, and dividing by 10. E.y., to find the coefficient at 300 C., 



we have 



Volume at 305 C. = 1 '0569474, 

 295 C. = 1-0549986, 

 Difference/10 = Coefficient of expansion at 300 C. = 0'00019488. 



[Note added Fel>i->irii 13, 1911.. It should be observed that the expansion of 

 iiiiTcury is here expressed in terms of the scale of temperature, based on the platinum 

 resistance thermometer, proposed by CALLENDAR ('Phil. Mag.,' December, 1899, 

 p. 519) at the meeting of the British Association at Dover. This scale assumes the 

 formula given on p. 7 alxrve for reducing the readings of a platinum thermometer to 

 the gas-scale, and is equivalent to assuming the value 444'53 C. for the boiling-point 

 of sulphur. It was admitted that this value might require a correction between 

 + 0'3 C. and 0'5 C. to reduce it to the absolute scale, but, as this correction 

 depended on the extrapolation of experiments between C. and 100 C., it was 

 considered inadvisable to alter the existing standard scale of platinum thermometry 

 until further experiments had been made with helium and argon at high tempera- 

 tures. Many writers now adopt values ranging from 444 '8 to 445 '0 for the boiling- 

 point of sulphur. This may lead to some confusion unless a definite convention is 

 established. Until the correction to the absolute scale has been determined with 

 greater precision it would be preferable to retain the old scale.] 



