October 14, 1921] 



SCIENCE 



341 



distinguish themselves by their energetic para- 

 magnetic action in the magnetic field. 



The foregoing may be summed up by the 

 folio-wing : 



TABLE I. 



Diamaguetic substances, /t less than unity, Tc nega- 

 tive and does not vary with H. 



Paramagnetic substances, /t small but greater than 

 unity, fc positive and does not vary with H. 



Ferromagnetic substance, /* greater than unity and 

 varies with S, Tc positive and a complicated 

 function of ff and T. 



This is practically the state of knowledge in 

 which Faraday, Pliicker, Becquerel and others 

 left this field of knowledge fifty years ago. 



B. Modern Theories of Dia-, Para- and 

 Ferromagnetism. — The electron theory forms 

 the basis of the modern theories of magnetism 

 which took their rise from an extensive in- 

 vestigation made on the magnetic properties 

 of bodies by Professor Curie,- whose name 

 is mainly associated with the discovery of 

 radium. Yet in this field, which we are 

 discussing, Curie's name must always stand 

 forth as one of the pioneers. 



Based largely on Curie's work Langevin^ 

 has built up a theory of dia- and paramagnet- 

 ism which has been extended to ferromagnet- 

 ism by Weiss.* These theories have been 

 of value in that they have led to new experi- 

 mental evidences concerning the behavior of 

 substances magnetically, so that in our discus- 

 sion these three names, naturally, will receive 

 more attention than others, although the con- 

 tributions of others are exceedingly important. 

 Among others to be mentioned are Honda, K. 

 Onnes, Dewar and Fleming, Oosterhuis, 

 Pascal, Oxley, Kunz and Owen. 



In a long and careful series of investiga- 

 tions. Curie observed the behavior of various 

 substances when placed in a non-uniform mag- 

 netic field, in which the observations were ex- 

 tended over a wide range of field intensities 

 and temperatures. Figures illustrating the 



2 Curie, Ann. de Chim. et de Phys., 5, 289, 1895. 



3 Langevin, Ann. de Chem. et de Phys., 4, 70, 

 1905; Jour, de Phys., 4, 678, 1905. 



4 Weiss, Jour, de Phys., 6, 661, 1907; Comp. 

 Send., 152; 79, 187, 367, 688, 1911. 



apparatus used will be found in the original 

 articles. The range of field strengths was 

 from about 25 to 1,500 c.g.s. units and of the 

 temperature from about 23° C. to 1350° C. 

 His results are generally expressed in terms 

 of mass susceptibility where fc is positive 

 when the substance moves toward more in- 

 tense field strengths and negative when op- 

 positely drawn. Curie examined a series of 

 substances in each of the three groups, dia-, 

 para- and ferromagnetic materials. 



1. Diamagnetic Substances. — Rock salt, 

 quartz, water, KCl, K,SO„ KNOg, S, Se, I, 

 Br, Te, P, Bi, and Sb were the substances 

 studied. Special attention was paid to water 

 in order to determine Tc absolutely as a 

 standard of reference. Bismuth showed 

 remarkable properties as it passed through its 

 melting point. In every case Tc was indepen- 

 dent of H and with the exception of tliree 

 all gave a value of h independent of tempera- 

 ture and of physical state. 



2. Paramagnetic Substances. — Air, palla- 

 dium, FeSO^ in aqueous solution, oxygen, 

 glass and porcelain were the subjects investi- 

 gated. Glass and porcelain were studied 

 because they were used as the material for the 

 container in which to test gaseous and other 

 forms of materials. The other four paramag- 

 netic substances were found to have a sus- 

 ceptibility independent of field strength and 

 satisfied the condition that h varies as 1/T. 

 Beside the work on FeSO^ in water Curie 

 tried also the magnetic salts of Co, Mn and 

 KiSO.,. The first two fitted in with the 

 general law but NiSO^ showed too rapid a 

 change in its susceptibility for the inverse 

 temperature law. The second law of Curie 

 that h varies as 1/T may be expressed by say- 

 ing that hT = a const, which has become 

 known as Curie's constant. 



3. Ferromagnetic Substances. — Curie in- 

 vestigated nickel, soft iron, magnetite and 

 cast iron. He paid particular attention to 

 soft iron, studying the variation of I with T 

 when H was maintained constant and again 

 the variation of / with H when T was kept 

 constant. For a certain range of temperature 

 above the critical temperature of magnetic 



