1914-15.] Conductivity of Salt- saturated Hydrochloric Acid. 189 
is most clearly marked in the region of that point. Taking hydrochloric 
acid as an example, it is notable that “ maximal ” {circa 20 per cent.) acid 
is the least concentrated solution which has a marked odour and is not 
hygroscopic. It is also curious that the constant boiling mixture has at 
atmospheric pressure a concentration of 20 ’2 per cent. 
In view of these considerations, any observations on the attainment of 
maximum conductivity in solutions of strong electrolytes are of interest. 
The measurements to be described were made at the suggestion of the 
late Professor John Gibson, to ascertain where the maximum would occur 
in salt-saturated hydrochloric acid. 
The thermostat and electrical apparatus used were those described by 
J. and G. E. Gibson in these Proceedings.^ With these it was found 
possible to maintain the temperature constant to C. and to measure 
the conductivity with an accuracy much higher than the immediate 
purpose required. 
The hydrochloric acid and sodium chloride were Merck’s guaranteed 
reagents. The acid solutions were jipproximately adjusted to the strength 
required by means of a hydrometer and were put into stoppered bottles 
with a large excess of the pure salt. The stoppers were tied down and 
sealed with paraffin wax. After remaining for several days fixed to the 
revolving wheel of a thermostat kept at 18°, they were transferred to a 
submerged wire grating in the large thermostat already mentioned. 
The measurement of the conductivity of such solutions as these presents 
considerable difficulties. With the open form of cell, closed only by lids, 
or with a dipping electrode in a test-tube, any volatile constituent escapes 
— in this case the hydrogen chloride — and constant readings cannot be 
obtained. Also, in transferring the solution from the containing vessel to 
the cell, a pipette is usually necessary, and in the pipette a saturated 
solution is very apt to deposit the dissolved substance. Even if this does 
not occur, change of concentration from the same cause or by evaporation 
will probably occur while the cell is being washed out with the solution. 
After several attempts to secure concordant readings with some of the 
ordinary types of cell, a pattern was devised (fig. 1) which was found free 
from their defects and is otherwise considerably more convenient in use. 
The cell consists of two electrode compartments connected, in the case 
of the pattern used for these highly conducting solutions, by three inches 
of glass tube of one-eighth inch bore. The electrodes are sealed into narrow 
glass tubes c and c by means of blue enamel glass. Thick platinum wire, 
* Proc. Roy. Soc. Edin., xxx, p. 254. 
