Section III., 1913. [61] Trans. R.S.C. 



On the Temperature of the Mercury Arc 

 By Professor J. C. McLennan, University of Toronto. 



(Read, May 28, 1913.) 



In the course of some experiments recently carried out by the 

 writer on the fluorescence of iodine vapour under stimulation by the 

 light from the mercury arc, it became necessary to know the tempera- 

 ture to which the iodine vapour was submitted during the exposures. 

 As there appeared to be no information available on the temperatures 

 obtainable from such types of mercury arcs as those used in the 

 fluorescence experiments mentioned a few sets of observations 

 were made with the special form of tube used and an account of these 

 is given in the following note. 



The tube consisted of two parts and is shown in Fig. 1. The 

 outer portion was of glass and had the form of the ordinary Cooper 

 Hewitt lamp provided with an extension at one end. The inner 

 tube was of clear fused quartz with walls about 1 • 5 to 2 mm. in thick- 

 ness. It was inserted axially in the outer tube and sealed into the 

 latter with ordinary mastic wax. In studying the fluorescence of 

 vapours the quartz tube was provided with a crystalline quartz win- 

 dow sealed on with mastic to the projecting end, and exposuies 

 were made by directing the collimator of the spectroscope at this 

 window. 



In order to investigate the temperatures to which the vapours 

 were subjected by the mercury arc the crystalline quartz window was 

 removed and one junction of a nickel iron carefully insulated thermo- 

 couple was inserted in the inner tube. When the mercury arc was 

 established the other junction was kept at 0°C by melting ice, and 

 the electromotive forces of the thermocouple were measured by means 

 of a standard Siemens and Halske potentiometer. 



In studying the fluorescence of the vapours the tube in action 

 was of course maintained in a horizontal position, but in taking the 

 temperatures the tube was inclined at the angle indicated in the dia- 

 gram, so as to have the closed end of the quartz tube dipping into 

 the mercury at the negative terminal of the arc. 



Before commencing to take the temperatures of the arc the 

 thermocouple was calibrated by keeping one junction at the tempera- 

 ture of melting ice and the other, first at this temperature, and then 



