124 PROFESSOR JOHN GIBSON ON 
In this case water is neither abstracted from nor added to the aldehyde. The 
assumption of a tendency towards increased conductivity suggests that there should be 
an increase in specific conductivity accompanying the reaction. How can this be so ? 
Aldehyde is a non-electrolyte, and, when added toa solution of hydrochloric acid, 
it lowers the specific conductivity of the acid solution. This lowering of the specific 
conductivity is greater the greater the number of molecules of the non-electrolyte 
which are added. When molecules of aldehyde unite to form molecules of aldol, the 
number of molecules of non-electrolyte is halved, and the specific conductivity must, 
therefore, be increased. Further, so long as the reaction does not involve dehydration, 
the possible gain in specific conductivity thus brought about will probably be at a maxi- 
mum when the acid has the concentration corresponding to maximum specific conductivity. 
In order to demonstrate that a rise of specific conductivity does actually accompany 
this reaction, the following experiment was tried :— 
The specific conductivity of a mixture of aldehyde and maximum conductivity 
hydrochloric acid (18 per cent. to 19 per cent.) was measured from time to time, the 
mixture being kept in a thermostat at 18° C. During the three days subsequent 
to the preparation of the mixed solution its conductivity rose steadily, and by 
the third day had risen 12°5 per cent., or from 10°K,,=203 to 10°K,, = 228. 
When aldol is treated with a solution of hydrochloric acid somewhat stronger than 
that having maximum specific conductivity, that is, with a slightly ultramaximal 
solution, it loses water and is converted into crotonic aldehyde. 
On the other hand, crotonic aldehyde in presence of a much more dilute hydro- 
chloric acid takes up water, re-forming aldol. 
The aldol condensation, the dehydration of aldol, and the hydrolysis of crotonic 
aldehyde all take place in hydrochloric acid solutions, but the conditions in each case 
would appear to be such that an increase of specific conductivity accompanies the 
progress of the reaction. 
Thus the tendency towards increased specific conductivity may show itself 
otherwise than by favouring dehydration or hydrolysis. The effect of adding 
moleeules of a non-electrolyte to a conducting solution is in general to decrease 
the specific conductivity of the solution. This lowering of the specific conductivity 
depends primarily on the number of molecules of non-electrolyte added, so that a not 
inconsiderable increase in the specific conductivity of the solution may result from the 
polymerisation and consequent decrease in the number of such molecules. The 
polymerisation of molecules of the non-electrolyte present in a conducting solution may 
be regarded as favoured by the gain in conductivity which results from the removal or 
decrease in the number of such molecules. 
Solutions which can be obtained by adding non-electrolytes, or relatively weak 
electrolytes, to solutions of strong electrolytes, may be conveniently distinguished 
by the prefix “sub.” A solution obtained by adding a non-electrolyte to an ultra- 
maximal solution may, on dilution, gain in conductivity, and may be converted into a 
