48 



Mr. E. P. Perman. 



[May 8, 



he stopcock E turned very cautiously, letting out the bromine 

 vapour little by little ; when it ceased to come over, the globe and 

 its contents were left for two or three minutes in order to regain the 

 heat lost by the adiabatic expansion ; it was necessary to repeat this 

 process till the bubble in the bulb-tube Gr remained undisturbed on 

 quickly opening and closing the stopcock E. The pressure recorded 

 by the gauge was then carefully read, the stopcock L was closed, the 

 absorption-tube removed, and its contents washed into a stoppered 

 bottle. The whole process was repeated at successively lower 

 pressures till the lowest desired was reached ; a good water-pump will 

 reduce the pressure to the vapour-pressure of water at the temperature 

 of the water passing through the pump. 



The residual bromine in the globe was estimated by running in 

 potassium iodide solution, and titrating with a standard solution of 

 sodium thiosulphate. The successive quantities of bromine collected 

 in the absorption-tube were also estimated in the same way. The 

 bhiosulphate solution was standardised with the specially prepared 

 bromine, and also with pure iodine ; the results agreed within 0'2 per 

 cent., and the mean value was used. 



By adding together the residual quantity of bromine and the 

 quantities removed, the weight of bromine in the globe at each 

 pressure was found. The capacity of the globe being accurately 

 known, and also the temperature of the jacket from the tables of 

 Professors Ramsay and Young (' Chem. Soc. Journ.,' vol. 47, 1885, 

 p. 640), the data requisite for the determination of a series of 

 vapour-densities at different pressures were available. 



Results. After some practice in conducting the experiment, 

 satisfactory results were obtained ; they are tabulated below : 



Series I. Temperature, 77'8 (Alcohol) ; Capacity of Globe, 446'3 c.c. 



Some bromine escaped absorption, causing the first vapour-densities 

 to be too low. 



