372 J. W. Langley—Chemical Affinity. 
The laws developed by Deville and his successors in this 
field show us, that after the point is reached at which decompo- 
sition commences, the further breaking up is determined by 
the pressure of the evolved products of the reaction, so that the 
ermanence of the body depends on the magnitude of two 
variables, pressure and temperature, either of which may be 
varied at will through a wide range. Deville thus gives usa 
fundamentally new mental tool with which to attack the prob- 
lem of affinity by showing the close parallelism between 
chemical decomposition and the ordinary evaporation of a 
liquid at its point of maximum tension. 
The electrical method of dissecting chemical forces has been 
followed less actively than the thermal one. Besides the 
well-known experimental contributions of Davy, Becquerel 
and Faraday, many other more recent workers have studied 
the chemical changes of the battery and the electrolytic cell. 
Among these may be mentioned Joule’s researches on the heat 
absorbed during electrolysis, and especially the work of C. 
Adler Wright on the ‘Determination of Affinity as Electro- 
motive force” in the Philosophical Magazine for 1880, 1881 
and 1882. 
The general outeome of these researches is that the products 
of electrolysis are so numerous and so varied by the results © 
secondary actions that it is very doubtful whether the electro- 
motive force measured is that due solely to the union of those 
atoms which are indicated by the principal equation of the 
reaction. The method of time, or speed of chemical reactions, 
has a history as old as that of its two associates, but the story 
is much less eventful, for very little work has been done 10 
this field. Wenzel held that the affinity of metals for a com- 
mon solvent, such as nitric acid, was inversely as the time 
necessary to dissolve them; the attacked surfaces being equal 
and invariable. He experimented on small cylinders covered 
with wax except on one of their bases.” 
The most notable work in this field has been done by Glad- 
stone and Tribe” by ascertaining the rate at which a metallic 
plate could precipitate another metal from a solution; DY 
Berthelot, Menschutkin and others who have studied the time 
necessary for etherification. The rate of inversion of cane 
sugar has been investigated by Urech, and in this country by 
R. B. Warder. The above are only three out of many subjects 
which have been studied in regard to their time rate. An 
index to the literature of this subject has been made by one of 
the members of this section, Professor R. B. Warder, and may 
be found in the proceedings of last year.” 
To these general methods for studying the problem of chem- 
ical dynamics should be added the investigation of the action 
of mass by Gladstone, in his well-known color work on the: 
