286 



SCIENCE 



[N. S. Vol. LIV. No. 1396 



obtain the necessary energy. One answer is 

 that the electrons have it already in the in- 

 terior of the substance by virtue of their 

 energy of thermal agitation. But thermal 

 agitations now appear less simple than they 

 used to be regarded, and in any event they 

 do not exhaust the possibilities. 



We know that when light of short enough 

 wave-lengths falls on matter it causes the ejec- 

 tion of electrons from it — the so-called photo- 

 electric effect. Since the formula for the 

 radiation emitted by a body at any given tem- 

 perature contains every wave-length without 

 limitation, there must be some emission of 

 electrons from an incandescent body as the 

 result of the photoelectric effect of its own 

 luminosity. Two questions obviously put them- 

 selves. Will this photoelectric emission caused 

 by the whole spectrum of the hot body vary 

 as the temperature of the incandescent body 

 is raised in the way which is known to char- 

 acterize thermionic emission? A straightfor- 

 ward thermodynamic calculation shows that 

 this is to be expected from the theoretical 

 standpoint, and the anticipation has been con- 

 firmed by the esperiments of Professor W. 

 Wilson. Thus the autophotoelectric emission 

 has the correct behavior to account for the 

 thermionic emission. The other question is: 

 Is it large enough ? This is a question of fact. 

 I have considered the data very carefully. 

 There is a little uncertainty in some of the 

 items, but when every allowance is made there 

 seems no escape from the conclusion that the 

 photoelectric efEeet of the whole spectrum is 

 far too small to account for thermionic emis- 

 sion. 



This question is an important one, apart 

 from the particular case of thermionic emis- 

 sion. The same dilemma is met with when 

 we seek for the actual modus operandi of evap- 

 oration, chemical action, and a number of 

 other phenomena. These, so far as we know, 

 might be fundamentally either kinetic or 

 photochemical or a mixture of both. In my 

 judgment the last alternative is the most prob- 

 able. (I am using the term photochemical 

 here in the wide sense 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 approxi- 

 mation about boiling points known as Trou- 

 ton's rule is a fairly obvious deduction from 

 the photochemical standpoint. The photo- 

 chemical 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 compre- 

 hend 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 col- 

 lisions. 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, fac- 

 tors which the independence of volume at con- 

 stant temperature shows 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 ob- 

 served effects. It seems that in the case of 

 these mono-molecular reactions the phenomena 

 can not 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 mole- 

 cule. This is complex enough to afford ma- 

 terial sufficient to cover the possibilities; but, 

 from the standpoint of the temperature energy 

 relations of its parts, it can not at present be 

 regarded as much more than a field for specu- 

 lation. 



CONTACT ELECTRICITY 



A controversy about the nature of the con- 

 tact potential difference between two metals, 



'Journ. Am. Chem. Soc, Vol. XLII., p. 2190 

 (1920). 



