208 On the Vapour Densities of Potassium and Sodium. [June 19, 



results. Zinc was next used, but with very slight success, owing, we 

 believe, to the formation of an alloy of zinc and iron, even although 

 the temperature was far above the boiling point of zinc. This was 

 shown by the wire gauze plug and capsule becoming very brittle, 

 so that the wire gauze might be rubbed to pieces between the fingers. 

 An approach to approximate results was once got with an apparatus 

 which had not been reduced by hydrogen, the coating of oxide of iron 

 and silica having protected the iron from the zinc. Mercury gave 

 good results in this apparatus. With potassium and sodium greater 

 difficulties stood in the way of exact results, as the correct weight of 

 the pieces inserted could not be ascertained, and both metals seemed 

 to have a tendency to behave in the same manner as zinc, viz., to form 

 an alloy with the iron ; because the vapour evolved after a few 

 minutes was absorbed completely in spite of all precautions to pre- 

 vent oxidation. Both of these sources of error tend to make the 

 numbers found for potassium and sodium too high. The mean error 

 due to the weighing of the metals was found by measuring the 

 hydrogen given off by their action on water, and the action of the 

 iron was partially overcome by raising the temperature as much as 

 possible, so as to induce rapid volatilization. 



After each experiment the metallic vapours were blown out by a 

 strong current of hydrogen, hy inserting (as before in reducing the 

 apparatus) the tube E, and passing the hydrogen in at D, refilling 

 with nitrogen, and proceeding as before. 



The following molecular weights are calculated by finding the 

 weight of metal in milligrams required to give 22*34 cub. centims. of 

 vapour at 0° C. + 760 millims. pressure. 



Cadmium. 



O'lOf) grm. g 



•ave 22-0 c.c. at 0° + 760 mm. . 



. mol. wt. 



=107-7 



0-103 







109-6 



0-106 



20-8 „ „ . 



55 



113-2 



o-ioo 



19-2 



55 



116-3 



0-105 



20-4 



55 



115-0 



0-102 



20-7 



55 



110-1 



0-102 



20-9 





109-0 



0-106 



211 



55 



114-0 



894-9 



Mean molecular weight Ill "8 6 



Mercury. 



0*198 grm. gave 22'8 c.c. at 0°C. + 760 mm. . ■ . mol. wt.=194 

 0-210 „ 23-8 „ „ „ 199-6 



