DEVELOPMENTS IN ELECTEOMAGNETISM — BLOCH. 233 



their properties, are due to the motion of free electrons. We may, 

 indeed, derive thus the law of Wiedemann and Franz. Electrons 

 should therefore play an important part in the specific heats of the 

 metals. But, according to the theory of quanta, the specific heat is 

 uniquely related to the uncharged atomic resonators (Einstein). 

 This accounts for the behavior of the specific heats at low tempera- 

 tures. But the quanta theory has nothing to offer as to the thermal 

 and electrical conductivities. The discordance is, however, decisive. 

 It is perhaps premature to try to reconcile matters until measures of 

 the thermal conductivities at low temperatures have been made, com- 

 parable with the excellent ones on the electrical conductivities made 

 by Kamerlingh Onnes ^ at the temperature of liquid air and hydrogen. 



IV. THE MAGNETON. 



The electron seems to have definitely become one of our physical 

 properties. P. Weiss ^ has for several years, and with increasing 

 success, tried to introduce an element of magnetism, the magneton, 

 bringing to bear upon it an imposing mass of experimental results. 



He started from the theoi-y of dia- and para-magnetism built by 

 Langevin.^ In that theory diamagnetism is explained by the de- 

 formation of the intra-atomic electronic trajectories under the influ- 

 ence of an exterior electric field paramagnetism results fi^om the 

 existence of a molecular magnetic moment of certain substances. 

 Weiss has elaborated this theory so as to include ferromagnetism by 

 means of a supplementary hypothesis, that of molecular magnetic 

 fields proportional to the magnetizing force. This idea of an electric 

 field is not new. Through it Kitz* developed his beautiful theory 

 of the structure of the series of certain spectrum lines and the Zeeman 

 effect. It led Weiss to formulae which are well substantiated by ex- 

 periment not only in the legitimate field of electromagnetism (the 

 variation of the Curie constant with the temperature), but also as 

 to the specific heats of ferromagnetic bodies. It was while looking 

 for such precise experimental confirmation that Weiss was led to 

 the theory of the magneton. 



The measure of the absolute value of the atomic magnetic moments 

 of iron and nickel at the temperature of liquid hydrogen, made in 

 collaboration with Kamerlingh Onnes, led at the start to numbers 

 12,360 and 3,370, which divided, respectively, by 11 and 3 lead prac- 

 tically to the same quotient, 1,123.5. For cobalt the corresponding 

 number was later found to be very close to 9X1,123.5. For the 



1 With regard to all these questions which we can not stop to more than sketch, see the 

 lecture which we delivered before the Societe de physique in December, 1911, upon the 

 electron theory of metals and also the book which we havo several times cited on the 

 Theory of Radiation. 



- Weiss, .Journal de physique, pp. 900, 905, 1911. 



^ Langevin, Annales de chimie et de physique, vol. .''(, p. 70, 1905. 



* Ritz, Annalen der Physik, vol. 25, p. 660, 1908. 



