488 JOHN JOHNSTON AND PAUL NIGGLI 
if there were a definite connection between heat of reaction and 
rate of reaction, the existence of endothermic compounds would 
appear to be highly improbable, if not impossible.* 
The rate of reaction varies enormously with the temperature, 
in general doubling for a rise of 10°. A rise of roo” therefore causes 
a reaction to go about one thousand times faster, while its speed is 
increased about a million times when the temperature is raised 
about 200°. In accordance with this we find that the rate of trans- 
formation of one crystal form into another tends to be greater the 
higher the temperature at which the transformation takes place. 
There is very little direct evidence with regard to the effect of pres- 
sure on the rate of reaction (as distinguished from its effect on the 
position of equilibrium); such as there is indicates that it has little, 
if any, effect. In any case it is a safe assertion that the effect of 
pressure on the rate is absolutely negligible as compared with the 
enormous acceleration produced by change of temperature. 
The rate of a reaction is quite generally much affected by the 
presence of certain specific substances; thus, palladium black 
brings about the union of oxygen with hydrogen around too”. 
Such substances affect only the rate at which equilibrium is estab- 
lished, but are usually understood to be without influence on the 
position of equilibrium; they are commonly grouped together, for 
convenience, as catalysis—a term used to conceal our present lack 
of knowledge of the mode of action of the majority of them. In 
some cases it is known that the catalytic agent acts merely as a 
solvent, hence producing its characteristic effect; as examples, we 
t There is a relation between the explosibility of such metastable gaseous mixtures 
and the heat change accompanying their reaction. 
2 There is of course in principle absolutely no difference between a reaction and a 
transformation, the latter including the processes of fusion, sublimation, and vapori- 
zation as well as the change from one crystal form to another; all are amenable to the 
same thermodynamical reasoning. The only differences are in the number of com- 
ponents, which cause corresponding differences in the equations expressing the rela- 
tions quantitatively, the qualitative relations being deducible from the phase rule. 
3 Doelter (Handbuch Mineralchemie, I [1912], 604) estimates the influence of 
pressure on speed of crystallization to be great for the reason that crystallization is 
hastened by shaking or knocking; this inference, however, is incorrect, because he 
fails to distinguish between uniform and non-uniform pressure, attributing to the 
former effects which could only be produced by the latter. 
