362 SCIENCE PROGRESS 



Fog is always preceded by mist. Banks of mist are met with 

 by aviators, which banks are not visible from the ground, and 

 in them the air is not saturated. These occur even in strong 

 winds, and are always associated with an inversion of tempera- 

 ture. There are two important kinds of ground-fog. In one 

 there is a rise of temperature immediately above the ground ; 

 this kind occurs both in strong and light winds in the summer, 

 and has no definite upper boundary, passing into mist and then 

 clear air. The second kind occurs in anticyclones in winter ; 

 here there is a definite upper boundary, and the temperature 

 falls with height until a warm stratum is encountered at the top. 

 These fogs generally extend over a wide area and are thickest 

 in the centre of the area of high pressure, forming an umbrella- 

 shaped surface co-extensive with the high pressure. When the 

 latter becomes larger the area of fog also expands. 



PHYSICS. By J. Rice, M.A., University of Liverpool. 



Contact Potentials, Photoelectric, and Thermionic Emission and 

 Electrochemical Action. — The existence of contact potentials be- 

 tween different metals of the order of a volt is a matter which has 

 given rise to a great deal of controversy during the past century. 

 The whole question has been raised again during the past few 

 years by the researches of Richardson, Millikan, Langmuir, and 

 their co-workers on thermionic and photo-electric effects, the 

 most recent contribution being a paper of Millikan's to the 

 September number of the Physical Review. As is well known, 

 contact potentials were first observed by Volta, who found that 

 the metals could be arranged in a series so that each was posi- 

 tive with respect to those preceding it, but that of course in a 

 complete circuit of metals the P.D.s would balance and there 

 would be no E.M.F. In a circuit containing an electrolyte 

 Volta considered that the effect of the liquid was to equalise 

 the potentials of the metal surfaces in contact with it, and so 

 to permit the contact potential at the junction of the metals 

 to function as an E.M.F. However, a few years later, Ritter 

 observing that chemical change always accompanied the action 

 of the voltaic pile, drew the conclusion that the chemical 

 change should be regarded as the cause of the electrical pheno- 

 mena. The dispute was rather an indefinite one at the time, 

 since clear conceptions of the energy relations involved had not 

 as yet made their appearance in scientific literature. However, 

 each point of view was ably defended by its champions ; contact 

 theorists such as Kohlrausch, Hankel, Lord Kelvin, Ayrton, 

 Perry, and Clifton obtained in various ways evidence for the 

 existence of potential differences established between metals 

 or between a metal and a liquid by contact and then separation, 

 and succeeded in measuring several such quantities and de- 



