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Prof. J. Dewar and Mr. A. Scott. |_J une 19 > 



XVII. " On the Vapour Densities of Potassium and Sodium." 

 By Professor James Dewar, M.A., F.R.S., and Alexander 

 Scott, B.A. Received June 12, 1879. 



The ingenious method of determining the vapour densities of 

 volatile bodies invented by Professor Meyer* suggested the revision of 

 work done by one of us in connexion with Professor Dittmar,f on 

 the vapour density of potassium. The new method possesses several 

 advantages over that formerly employed, and seemed to be especially 

 fitted to simplify the difficulties of working with substances having 

 the chemical characters of the alkali metals. The fact that a constant 

 temperature need not be maintained unless for a short time, and that 

 all absolute determinations of temperature are unnecessary during the 

 course of the experiments, renders Professor Meyer's system the 

 simplest when the boiling point of the substance is above a white 

 heat. 



The apparatus used was simply a reproduction in wrought iron of 

 Professor Meyer's glass one, on a somewhat larger scale. The bottle 

 A had a capacity of 160 — 180 cub. centims. The tube B was about 

 2 feet long and in diameter about "5 inch. A and a considerable part 

 of B was coated with a glaze of borax and then fitted between two 

 crucibles as in figure, and the crucibles filled with fine sand as com- 

 pletely as possible. 



This apparatus kept a tolerably constant temperature for a con- 

 siderable time even when the furnace temperature varied considerably, 

 as it was only slowly affected, owing to the mass of the crucibles and 

 sand surrounding it. The chief difficulty at first was the diffusion of 

 the furnace gases, especially carbonic oxide, through the hot iron. 

 This, however, seems to be completely prevented by the borax glaze. 

 To ensure success, especially in the cases of potassium and sodium, it 

 was found to be essential to clean the apparatus by treatment with 

 dilute hydrochloric acid to remove the larger part of the oxide of 

 iron, then to fuse in it some caustic potash, and thus get rid of the 

 silica, &c, used in the welding in of the tube and bottom of A, after 

 boiling several times with water to dissolve out the potash, it was 

 again treated with dilute hydrochloric acid, and then thoroughly 

 washed and dried. The crucibles were arranged of such a height 

 that the iron was in no place exposed to the gases of the furnace. 

 The fuel used was gas carbon broken into pieces the size of walnuts, 

 this gave a very constant temperature, and the furnace fed with it 

 on more than one occasion fused the bottle A. 



* " Deut. Ckem. Gees. Ber.," vol. xi 3 2253. 

 f " Proc. Roy. Soc," vol. xxi. 



