144 Proceedings of the Royal Society of Edinburgh. [Sess. 
Among the changes described by J. Gibson as being subject to this 
influence, that of the colour of cobalt chloride * seemed to give opportunity 
for a test which would be roughly quantitative. Acids of various concen- 
tration were made up and saturated with salt at room temperature in clear 
glass bottles, each containing 60 c.c. of acid. To each, 0T5 c.c. of a saturated 
solution of cobalt chloride was added and the mixture was shaken. By 
analysis of the solution it was found that an acid of 19*8 per cent, would, 
after saturation with salt, just tinge the pink colour of the cobalt salt with 
a faint shade of purple. An acid of 21*2 per cent, was just able, after 
saturation, to change the colour to blue green. Since J. Gibson (loc. cit.) 
found that in solutions containing no salt the first purple change took 
place at a concentration of 18*2 per cent., it is apparent that the critical 
point, so far as the behaviour to cobalt chloride is concerned, has shifted 
through an interval of 1*6 per cent. 
The point of maximum conductivity in the acid medium has, however, 
been moved over a range of 21*9 — 19*1 = 2*8 per cent. The difference 
between this and the other figure is rather large. Difficulty of determining 
the maxima accurately may account for the difference to a small extent, 
but in any case the fact that the movement of the critical point is in the 
same direction as the movement of the point of maximum conductivity 
is significant. 
On the other hand, it is difficult to see how, in view of these results, 
the colour change in cobalt chloride can be due to dehydration by the acid. 
A solution containing 18*2 per cent, of hydrogen chloride is capable of 
producing the colour change in its first stage. If dehydration were the 
explanation, saturating this solution with salt might reasonably be expected 
to increase the dehydrating action, and to render the colour change more 
marked. After saturation with salt, however, the acid of 18*2 per cent, 
has no effect at all on the colour. In order to produce an effect, a more 
concentrated acid must be employed. 
Summary. 
(1) Determinations have been made, at 18” C., of: — {a) the specific 
electrical conductivity, (h) the percentage composition by weight, of a 
series of mixtures made by saturating, with sodium chloride, solutions of 
hydrochloric acid containing from 16 to 27 per cent, of hydrogen chloride. 
(2) Within this range of concentration the salt-saturated acid has lower 
specific conductivity than the aqueous solution of hydrogen chloride alone, 
from which it may be supposed to be derived. Of the salt-saturated acid 
* Trans., loc. cit., p. 125. 
