36 Proceedings of Royal Society of Edinburgh. [sess. 
to the ionisation. Only so long as a marked increase of conductivity 
results from the dilution of the acid brought about by the reaction 
is the velocity of reaction considerable. 
As soon as further dilution, although it may increase ionisation, 
ceases to involve marked increase in conductivity, the velocity of 
these reactions rapidly diminishes. In other words, the velocity 
of the reaction is proportional to the rate of increase in the number 
of carriers of electricity per unit volume. 
One class of reactions may he referred to here, because of their 
general importance, and because they seem to present special 
difficulties from the point of view taken in this paper. 
When metals combine directly with oxygen, metallic con- 
ductivity is lost, and the oxides formed are themselves poor con- 
ductors, and are frequently insoluble in water. 
A closer consideration of the facts shows, however, that solid 
metals do not readily combine with pure oxygen. Even metallic 
potassium throughout a wide range of temperature does not com- 
bine directly with pure dry oxygen, and may, in fact, he distilled 
unchanged in it. The means taken to induce combination, namely, 
to apply heat, tends in the first place to diminish the metallic con- 
ductivity, and secondly to increase the vapour pressure of the 
metal. It seems more than probable that the chemical union takes 
place between gaseous oxygen and gaseous , not solid, metal. 
But metallic vapours are dielectrics. 
The following is a preliminary list of reactions which the author 
has been led to consider tentatively as characterised by increase of 
electrolytic conductivity : — 
(a) Photo-chemical reactions generally. 
(b) Reactions characteristic of concentrated solutions of acids 
and bases. 
( c ) Reversible actions generally. 
(< d ) Many reactions employed in the synthesis of organic sub- 
stances such as : 
Etherification. 
Esterification. 
Saponification. 
Hydrolytic actions. 
The reduction of nitro bodies by alcoholic potash. 
