A.—MATHEMATICSTAND} PHYSICS, 29 
of an effect of light in changing the composition of matter, whether the 
parts affected are atoms, groups of atoms, ions, or electrons.) For 
example, the approximation about boiling points known as Trouton’s rule 
is a fairly obvious deduction from the photochemical standpoint. The 
photochemical point of view has recently been put very strongly by 
Perrin, who would make it the entire motif of all chemical reaction, as 
well as of radioactivity and changes of state. In view of the rather minor 
part it seems to play in thermionic emission, where one would a priori 
have expected light to be especially effective, this is probably claiming 
too much for it, but the chemical evidence contains one item which is 
certainly difficult to comprehend from the kinetic standpoint... The 
speed of chemical decomposition of certain gases is independent of 
their volume, showing that the decomposition is not due to molecular 
collisions. The speed does, however, increase very rapidly with rising 
temperature. What the increased temperature can do except increase 
the number and intensity of the collisions, factors which the indepen- 
dence of volume at constant temperature show to be without. effect, 
and increase the amount of. radiation received by the molecules, is 
not too obvious. It seems, however, that, according to calculations 
by Langmuir,* the radiation theory does not get us out of this difficulty ; 
for, just as in the ordinary photoelectric case, there is nothing like 
enough radiation to account for the observed effects. It seems that 
in the case of these mono-molecular reactions the phenomena ‘cannot 
be accounted for either by simple collisions, or by radiation, or by a 
mixture of both, and it is necessary to fall back on the internal structure 
of the decomposing molecule. This is complex enough to afford material 
sufficient to cover the possibilities; but, from the standpoint of the 
temperature energy relations of its’ parts, it cannot at present be 
regarded as much more than a field for speculation. 
Contact Electricity. 
A controversy about the nature of the contact potential difference 
between two metals, similar to that to which I have referred in connec- 
tion with thermionic emission, has existed for over a century. In 
1792 Volta wrote: ‘The metals . . . can by themselves, and of their 
own proper yirtue, excite and dislodge the electric fluid from its state 
of rest." The contrary position that the electrical manifestations are 
inseparably connected with chemical action was developed a few years 
later by, Fabroni. Since that time electrical investigators have been 
fairly evenly divided between these two opposing camps. Among the 
supporters of the intrinsic or contact view of the type of Volta we 
may recall Davy, Helmholtz, and Kelvin. On the other side we have 
_to place Maxwell, Lodge, and Ostwald. In 1862 we find Lord Kelvin ? 
writing: ‘ For nearly two years [ have felt quite sure that the proper 
explanation of voltaic action in the common voltaic arrangement. is 
very near Volta’s, which fell into discredit because Volta or his 
followers neglected the principle of the conservation of force.’ On the 
cther hand, in 1896 we find Ostwald“ referring to Volta’s views as 
2 Journ. Am. Chem. Soc., vol. xlii., p. 2190 (1920). 
* Papers on Electrostatics and Magnetism, p. 318. 
4 Elektrochemie, Ihre Geschichte und Lehre, p. 65, Leipzig (1896). 
