232 Proceedings of Royal Society of Edinburgh. [sess. 
surface of a metal in contact with a solution. In the paper on 
the Aluminium Anode (loc. cit . ) it is suggested that the permeability 
of the surface film of aluminium hydroxide by Cl' and imperme- 
ability by S0 4 " is the cause of the differences observed between 
hydrochloric acid and sulphuric acid ; and if this he so, differ- 
ences of permeability by Cl', Br', and I' are to he expected. 
As it seemed probable that similar differences might manifest 
themselves in other physical properties, we decided to determine 
the relative viscosity of solutions of chloride, bromide, and iodide 
under various conditions of temperature and concentration. The 
viscosity of solutions of potassium chloride has been determined 
many times at one temperature (17° or 25° C.) and one concen- 
tration (usually 1 n). Sprung* determined the viscosity of 
potassium chloride, bromide, and iodide over a considerable range 
of temperature (5° C. to 50° C.), but at only two concentrations of 
chloride, and the other solutions were not at comparable concen- 
trations. Wagner f also made determinations of viscosity of 
hydrochloric acid at various concentrations and temperatures. 
Their results are referred to later on. 
Experimental. 
The potassium chloride and bromide were purified by repeated 
precipitation from hot aqueous solution by addition of ethyl 
alcohol : the iodide was twice recrystallised from water. The 
hydrobromic acid was made by the direct union of hydrogen and 
bromine in contact with hot platinised tile, the gas absorbed in 
water, and the solution redistilled ; no rubber or cork joints were 
used in the apparatus, so that the bromine and acid never came in 
contact with organic matter. The most concentrated solutions of 
the salts were made up by weight, and the others were prepared 
from them by dilution ; the concentration of each solution was 
further checked by titration with silver nitrate. The concen- 
trations of the acid solutions were ascertained by titration with 
barium hydroxide solution. 
The densities were determined by means of an Ostwald-Sprengel 
pyknometer. The viscosity apparatus used is the form figured 
Pogg. Ann., 159, p. 1 (1876). 
t Wied. Ann., 18, p. 259 (1883). 
