1907-8.] On the “Negative” Viscosity of Aqueous Solutions. 463 
Temperature 25° C. 
Mols. per litre. 
•5 
■25 
T25 
Absolute Yiscositv. 
K 3 Fe(CN) 6 . K 4 Fe(CN) 6 . 
•00988 
•00932 
•00910 
•01120 
•00996 
•00944 
The differences are similar at 15° C. At T6° C. the viscosity of the 
ferricyanide solutions was negative. These differences cannot be explained 
by differences in degree of ionisation of ferrocyanide and ferricyanide, for 
freezing-point determinations made some time ago, but not yet published, 
prove that the degree of ionisation of the two salts is about the same at the 
same molecular concentrations. 
All the more complete investigations on the viscosity of solutions bear 
out the conclusion that, in general, concentration- viscosity curves pass 
through a minimum, although, owing to limitations of solubility, it may not 
always be possible to obtain a minimum at all temperatures. It is not, 
however, at all impossible that the size of ions may have a direct influence 
on the absolute value of the viscosity, and thus be an important factor in 
determining whether this minimum value for the solutions is less than the 
viscosity of the pure solvent at the same temperature. In order definitely 
to test this point, we have determined the viscosity of aqueous solutions of 
certain organic salts whose cations must have widely different ionic 
volumes. The five salts chosen were tetra-methyl ammonium iodide, tetra- 
ethyl ammonium chloride and bromide, tetra-propyl ammonium chloride 
and iodide. These substances were used on account of their stability, and 
because they are typical electrolytes in every way comparable with salts of 
the alkalies or of ammonium. They were purchased from Kahlbaum, with 
the exception of tetra-propyl ammonium chloride, which was made from 
the corresponding iodide by treatment with freshly precipitated silver 
oxide, and exact neutralisation of the filtrate with hydrochloric acid. As 
some of the salts are deliquescent, all the solutions were made up approxi- 
mately by weighing, and the actual concentrations found by titration with 
standard silver nitrate. 
In order to have the same ratio of molecules of solvent to one molecule 
of solute, all the concentrations are expressed in gm. molecules of solute to 
1000 gm. of water. 
The densities were determined by means of an Ostwald-Sprengel 
pycnometer. The viscosities were determined by the Ostwald-Poiseuille 
transpiration method, the apparatus and mode of working being the same 
as that employed in our previous work, and fully described in the paper by 
