MAXIMUM SPECIFIC ELECTRICAL CONDUCTIVITY IN CHEMISTRY. 131 
concentration of hydroxyl ions. A solution containing a concentration of hydrogen 
ions such as is found in hydrochloric acid of maximum specific conductivity (curea 18 
per cent. HCl), along with a concentration of hydroxy] ions such as is found in aqueous 
caustic potash of maximum specific conductivity (cuca 27°5 per cent. KOH), would be a far 
better conductor than any known solution, as these two ions have greater mobility than 
any others; but the strong affinity which determines their immediate association and 
the formation of water brings about a solution having a specific conductivity lower than 
that of either solution taken separately. Thus, on mixing the best conducting acid 
solution known with an equivalent quantity, that is, with a nearly equal volume, of the 
best conducting alkaline solution known, 7.e. maximal caustic potash, the tendency for 
the hydrogen ions of the acid and the hydroxy] ions of the alkali to associate overcomes 
and masks the tendency towards an increase in specific conductivity, so that only the 
much less mobile potassium and chlorine ions are left as the chief carriers of electricity 
in the neutral and less highly conducting solution of potassium chloride. 
The hypothesis is applicable to homogeneous, 2.e. single-phase systems. It cannot 
even be formulated for heterogeneous systems, since the term ‘specific conductivity ” 
applied to a heterogeneous system has no meaning. There are, however, many cases 
where it is possible to apply the hypothesis usefully, and to predict the course of events 
correctly, although the system is, or becomes, heterogeneous. ‘Thus, in cases where a_ 
rearrangement resulting in alteration of specific conductivity brings about the separa- 
tion of a non-electrolyte froma highly conducting solution, the system no doubt becomes 
heterogeneous, but its heterogeneity may be disregarded whenever the actual change of 
| conductivity would not have been materially affected had the non-electrolyte remained 
in supersaturated solution. Heterogeneity confuses the issue only when marked changes 
in conductivity are the direct result of the appearance of the new phase or phases. 
The separation of the non-electrolyte sulphur, in the action of hydrogen sulphide on a 
solution of iodine in hydriodie acid, is a case where heterogeneity may be disregarded. 
The precipitation of barium sulphate and silver chloride, on mixing equivalent 
solutions of silver sulphate and barium chloride, is a case where the hypothesis is 
clearly not applicable. In this and similar cases of the double decomposition of salts 
the removal of electrolytes from the solutions necessarily implies a lowering of its 
conductivity, while at the same time the system becomes heterogeneous. 
APPLICATION OF THE HyporHEsiIs TO PLANT CHEMISTRY. 
This investigation was originally undertaken with the special object of throwing 
further light upon the chemistry of plant metabolism. With the exception of the first- 
cited instances of photochemical action, the examples and reactions discussed so far 
| have all been reactions in which strong mineral acids play an essential part. This is 
not a matter of choice, the reason being that the tendency towards increased con- 
: | ductivity depends on the value for y=Kyax,—K, which is necessarily small when 
