24 
Proceedings of Royal Society of Edinburgh. [sess. 
law of unimolecular actions. He was therefore forced to conclude 
that in these cases the action takes place in two stages, viz. — 
I. AsH 3 = As + 3H , 
TT / 4 As = As 4 , 
l 2H = H 2 , 
and that the velocity of the first stage alone determines the 
velocity of the whole reaction. How that this may he so, the 
actions of the second stage must proceed at a rate immeasurably 
greater than the rate of the first stage. An analogy may serve 
to make this point clear. The time occupied in the transmission 
of a telegraphic message depends both on the rate of transmission 
along the conducting wire and on the rate of the messenger . who 
delivers the telegram; hut it is obviously this last, slower rate 
that is of really practical importance in determining the total time 
of transmission. When we measure, then, the rate of a chemical 
change which proceeds in two stages, that stage which proceeds 
most slowly plays the principal part in determining the rate of the 
whole reaction, being only more or less modified by the other more 
rapid changes. If these are indefinitely faster than the slow 
change, their modifying influence will he so slight as to be 
negligible, and the really complex action will appear to obey the 
formula of a simple pure action within the limits of the experi- 
mental error. 
According to the hypothesis of electrolytic dissociation all re- 
actions which take place between acids, bases, and salts in aqueous 
solutions at finite concentrations must he complex reactions of the 
kind considered above ; for in these circumstances the dissociation 
is never complete. If the action takes place between ions, the 
undissociated portion must at each instant partially dissociate in 
order to restore the dissociation equilibrium ; for the same reason, if 
the reaction involves directly the undissociated molecule, the ions 
must reunite progressively as the reaction goes on. We know 
that the processes of electrolytic dissociation and reassociation are 
practically instantaneous, on the evidence of our ordinary chemical 
experience and of the rate of change in the resistance of an 
electrolytic solution with varying conditions of dilution, etc. They 
exert, therefore, little or no modifying influence on the rate of 
