1890.] Determination of the Boiling Point of Sulphur, Sfc. 57 



found most suitable for standardising platinum thermometers by 

 means of it. 



The paper is divided into three parts. 



Part I contains a description of the method and apparatus em- 

 ployed in comparing the platinum thermometers used in this investi- 

 gation with the air thermometer at a temperature very near the 

 boiling point of sulphur. 



Part II contains the determination of the actual boiling point of 

 sulphur by means of the thermometers thus standardised, and a 

 description of the method and apparatus to be used in standardising 

 other platinum thermometers. A table is also given reduced from a 

 previous series of observations of other fixed points which may be 

 used for the same purpose. 



Part III contains a comparison of the platinum and air thermo- 

 meters between and 100, and shows that the ^-formula holds 

 accurately between those limits. 



The determination of the boiling point of sulphur was made by 

 means of three platinum thermometers, L, M 1? and M 2 , constructed 

 out of the wire used in the experiments of 1887, before referred to. 



Full descriptions of these thermometers are given in the paper. 

 They were furnished with double electrodes for measuring the resist- 

 ance of the connecting wires at each observation, their insulation 

 was carefully tested, and all due precautions were taken to guard 

 against thermal effects and other sources of error. 



Thermometers Mj and M 2 were standardised by direct comparison 

 with an air thermometer at the boiling point of sulphur. Full 

 particulars are given of the details of the observations and calcula- 

 tions, showing the limits of error of the experiments. 



The expansion of the glass forming the bulb of the air thermo- 

 meter was determined both by the method of linear expansion, and 

 also by using the bulb itself as a mercury weight thermometer. The 

 values found by the two methods agreed very closely. 



The small changes of the volume of the bulb were determined 

 from time to time during the progress of the experiments. The final 

 observations were not taken till the thermometer had reached a fairly 

 steady state. 



The limit of accuracy attainable with this air thermometer was 

 found to depend chiefly on that of the barometric readings. The 

 barometer used was therefore verified by a careful comparison with 

 the standard metre scale. 



The . iron-tube apparatus in which the platinum and air thermo- 

 meters were compared was so constructed as to be capable of being 

 laintained at a constant temperature by a steady flow of sulphur 

 vapour for any length of time. 



Observations were taken with it on two separate days. On each 



