[ 384 ] 

 L. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 311.] 



March 6, 1873.— Sir George Biddell Airy, K.C.B., President, in 

 the Chair. 

 r PHE following communication was read : — 

 ■*- " On the Yapour-densitj of Potassium." — Preliminary Notice. 

 By James Dewar and William Dittmar. 



Since the elaborate experiments of Deville andTroost on the vapour- 

 densities of substances at high temperatures, little has been added 

 to chemical science in this field of research. Doubtless this is in 

 great part owing to the difficulty of any one student manipula- 

 ting the complex apparatus necessary for the execution of the 

 experiments. But the operations are greatly increased in difficulty 

 when we select bodies that are readily inflammable in air and attack 

 with facility glass and porcelain at the high temperatures to which 

 they are exposed. This is the reason why the molecular weights 

 of a most important class of elementary bodies, viz. the alkali- 

 metals (although these are volatile at moderate temperatures), 

 have remained to the present time undetermined. It was with the 

 view of adding something to our knowledge in this department, that 

 we recently undertook some experiments with potassium, the results 

 of which we now beg leave to lay before the Society. The special 

 difficulties we had to overcome are involved in the endeavour to 

 answer the following questions : — 



1. Is it possible to convert potassium into a gas of one atmo- 

 sphere's pressure at any of the constant temperatures we can at pre- 

 sent command ? 



2. Is it possible to generate pure potassium-vapour and to keep 

 it from getting oxidized ? 



3. Supposing a definite volume of such vapour to have been pro- 

 cured, how can its weight be ascertained ? 



After a succession of failures, which we shall not detail, we at 

 last succeeded in devising a workable process, which may be briefly 

 described as follows : — 



A cylindrical iron bottle of at least 200 cub. centims. capacity, 

 of a thickness in the body ensuring sufficient rigidity at even a 

 bright red heat, and provided with a well-ground inbent neck, 

 pierced with a canal of about 2 millims. diameter, is employed as 

 a generator and receptacle of the vapour. 



A mass of about 20 kilogrs. of zinc contained in a plumbago 

 crucible, which being placed in a forge-fire can be readily heated 

 up to the boiling-point, serves as a bath. 



The experiment begins by first deoxidizing the inside of the 

 receptacle at a red heat by means of a current of dry hydrogen, 

 which is continuously maintained until the bottle has cooled down 

 below redness. At this stage about 200 grins, of pure mercury 

 are introduced into the bottle, which is then inserted into the red- 

 hot zinc, without, however, covering the upper extremity of the 



