1890.] Experiments on Vapow-density. 45 



Figs. 7 and 8 represent these values of , n, and in, respectively, 

 the curves marked 1 being those obtained with tin at 650 ; those 

 marked 2 with tin at 800 ; and those marked 3 with silver at 800 ; 

 the value of x being abscissae, and those of m and n ordinates. 



We have much pleasure in acknowledging the assistance afforded 

 us by Mr. T. M. Wyatt in carrying out a considerable portion of the 

 analytical results above detailed. 



IL " Experiments on Vapour-density. 1 ' By E. P. PERMAN, B.Sc., 

 Clothworkers' Exhibitioner at University College, London. 

 Communicated by Professor RAMSAY, F.R.S. Received 

 April 17, 1890. 



The Vapour-density of Bromine. 



The main purpose of my work on this subject was to discover if 

 bromine had any tendency to dissociate at low pressures, i.e., down to 

 about 15 mm. of mercury, and at moderate temperatures. 



The subject was suggested by Professor Ramsay, to whom I am 

 greatly indebted for constant advice and assistance in carrying out 

 the work. The method adopted for determining the vapour-densities 

 was a modification of the Dumas method, from which it differed in 

 three essential points : (1) The globe was not immersed in a bath, 

 but had a vapour-jacket. (2) The weight of the vapour in the globe 

 was not found by direct weighing, but by running in an absorbent 

 liquid, and estimating it volumetrically. (3) A series of vapour- 

 density determinations at different pressures was made with the same 

 quantity of vapour by lowering the pressure, absorbing the vapour 

 drawn off and estimating its quantity. 



As to previous work on this subject, Jahn has shown ('Wien, 

 Akad. Sitzber.,' vol. 85, 2. Abth., 1882, p. 778) that the vapour-density 

 becomes normal at about 230 ; Meier and Ziiblin (' Dedtsch. Chem. 

 Ges. Berichte,' vol. 13, 1880, p. 405) and Crafts ('Comptes Rendns,' 

 vol. 90, 1880, p. 183) have shown that partial dissociation takes place at 

 very high temperatures; and Professor J. J. Thomson states that 

 " vapour-density determinations showed that bromine vapour is dis- 

 sociated if it is heated for a long time at a low pressure, even though 

 the temperature is not very high " (' Roy. Soc. Proc.,' vol. 42, 1887, 

 p. 345). His chief results are : 



