MODERN CONCEPTS IN PHYSICS — LANQMUIR 231 



electrons act as tliou^h they had a temperature of perhaps 50,000°, 

 whereas the atoms have motions corresponding to far lower tempera- 

 tures. Strictly speakinj;, neitlier the electrons nor atoms have well- 

 defined temperatures, for the conditions are far removed from 

 equilibrium. 



In much of the recent discussion of tlie radiation hypotliesis of 

 chemical reactions, chemists have been discussing meaningless ([ues- 

 tions usually without realizing it. At first it was proposed that the 

 radiation is absorbed by the reacting gas to form excited molecules 

 in accordance with Einstein's photochemical law. When this is 

 found not to be in accortl with experiment, tlie concept of radiation 

 is alti'red repeatedly as new experimental facts are found so as to 

 make the modified tlieory continue to lit the facts. After this process 

 lias been carried on sufficiently, it no longer has any meaning to ask 

 whether the reaction is caused by radiation or whether the radiation 

 liypothosis is true. 



In the studies of the properties of liquids, questions of the degree 

 of ionization and of association and in some cases of internal pres- 

 sures have been discussions of questions without meaning. A great 

 deal of such discussion might be simplified or even avoided entirely 

 if chemists would agree in defining these concepts in terms of 

 operations. 



Theories of valence within recent years have been afflicted with the 

 same difficulties. As long as chemists deal with the ordinary valence 

 rules of organic ciiemistry, they are dealing with concepts of valence 

 which are actually defined in terms of operations; that is, the organic 

 chemists know how to conduct experiments to prove that the valence 

 (jf nitrogen in dimethylaniline is 3. The types of operations needed 

 to establish the valence of magnesium in magnesium chloride are in 

 many ways quite different, and they are still different if we consider 

 the case of so-called quin(}uevalent nitrogen in ammonium chloride or 

 heptavalent chlorine in perchloric acid. I believe that the chemist 

 has much to learn from the physicist in regard to the proper method 

 of attacking such problems as these. 



The electrochemist has been troubled in locating the source of 

 electromotive force in cells. The physicist has similar difficulty in 

 finding the origin of the contact potential between metals. Funda- 

 mentally it must be recognized that unless or until there are methods 

 by which these quantities can be measured, questions involving them 

 have no meaning. 



A practical example of the meaninglessness of some questions in- 

 volving electric potential has recently arisen in the numerous pro- 

 posals that have been made to construct a speed indicator for 

 airplanes which will give the speed with respect to the earth s surface 



