470 
Proceedings of the Royal Society of Edinburgh. [Sess. 
It must be remembered that these figures are not strictly comparable, 
for the ammonium iodide concentration is in mols. per litre, the others 
in mols. per kilogram of solvent ; also the difference in viscosity between 
tetra-ethyl ammonium bromide and iodide is no doubt very slight, and 
would reduce the difference between it and the methyl compound. 
In conclusion, attention is directed to a very curious density relation 
which has been found to exist in the case of the tetra-propyl ammonium 
chloride solutions. On reference to the tables it will be noticed that the 
density of the more concentrated solutions is less than that of the more 
dilute ones ; the difference is quite marked, and holds at all the con- 
centrations examined, and at both temperatures, viz. 25° C. and 35° C. 
(see fig. 3). The decrease of density with increase of concentration is 
more marked at the higher temperature. Tetra-propyl ammonium iodide 
does not show this peculiarity, nor do any of the other substances under 
the conditions of examination. But it will be noticed that the density of 
the tetra-ethyl ammonium chloride increases very slightly with increasing 
concentration, and more especially at 35° C. In comparison with 
ammonium chloride the differences are very striking, 
25° C. 
Concentration. 
Density (Water at 4° = 1). 
NH 4 C1. 
(C 2 H 5 ) 4 NC1. 
(C 3 H 7 ) 4 NC1. 
•2733 
•9952 
•2914 
•9971 
•4517 
1*0071 
•5893 
•9976 
•6177 
•99433 
•7884 
•99425 
•9185 
1-0138 
1-148 
•9994 
1-402 
1-0204 
Here, too, the values for ammonium chloride are taken from Getman’s 
tables, but the concentrations have been recalculated into the same units as 
the others. It will also be noticed that the actual densities of the solutions 
of the substituted ammonium salts are very small, being even less dense 
than water ( d 25 0 /4°= : 0 , 99707) in the case of the tetra-propyl ammonium 
chloride. Whether the two are directly connected or not, the abnormally 
low densities of these solutions are in marked contrast with their viscosities, 
which are abnormally large. It is intended to fully investigate the density 
