determining tlie Vapour-Density of Metallic Vapours. 549 



The thermometer was of the Callendar platinum-resistance 

 type *, with auxiliary compensating-leads and direct reading 



Fig. 2. 



I) 



Wheatstone bridge, and was capable of reading to 0°*005 ; 

 this particular instrument was one of those calibrated at the 

 Kew Observatory. 



In order to make an observation a tube (a, fig. 2) was 

 sealed to the larger tube (b), and the capillary-tube (c) drawn 

 down at (d) ; this being done, and both tubes and bulb 

 thoroughly dried, a small piece (05-07 gm.) of C.P. metallic 

 sodium was introduced into (a) , and the latter quickly sealed 

 off at (e) y as shown ; (c) was now connected to a Geissler- 

 pump, and the air drawn out to a residual pressure of 01- 

 0*2 mm., after which the bulb was filled with some inert gas 

 (H or N) and again pumped out and the capillary-tube 

 sealed off at (d) . The bulb thus prepared was now introduced 

 into the bath and the temperature raised to any required 

 point ; the apparatus was kept at the desired temperature for 

 fifteen or twenty minutes, thermometer-readings being taken 

 every tw r o minutes ; the cooling had to be done very slowly, as 

 the capsule containing the molten sodium was very liable to 

 crack ; and the inrushing air carried the metal into the bulb. 

 Upon removing the latter from the bath, the whole inner 

 surface showed a bright metallic coating of condensed sodium 

 vapour, varying in thickness with the temperature to which 

 the bulb had been subjected. To determine the amount of 



* E. H. Griffiths/ Nature,' Nov. 14, 1895. 



