PARAMAGNETIC & DIAMAGNETIC SUBSTANCES 291 



the temperature rose. Bismuth was submitted to special examina- 

 tion, and it was found that between 20 C. and the melting-point 

 273 C. the value of x was given by 



10 6 X = 1-35[1-0-00115(*-20)]. 



At 273 C. 10 6 X fell from 0-957 in the solid to 0'38 in the 

 liquid state, and then remained constant as the liquid was raised 

 to 400 C. 



The value of 10' x for water was 0-79. 



An easy calculation shows that C Uriels relative values of K for 

 air, water, and bismuth at 20 agree closely with those found by 

 Becquerel and Faraday ; Curie's values, reduced to Faraday's scale, 

 being 3-74, -96 '3, and -1830 respectively. 



Certain paramagnetic substances were next investigated, espe- 

 cially as to the effect of rise of temperature. G. Wiedemann* had 

 already shown that for solutions of certain salts, over the range 

 from 15C. to 80 C. , the value of x agreed nearly with Xt = Xo (l - a t) 

 and he found a = 0'00325 the same for all. Plessnerf also found 

 a constant temperature coefficient for other solutions, obtaining 

 a = 0-00355. These values, so near to 0'00367, suggest that x 

 varies inversely as the absolute temperature. Curie re-examined 

 the results of Wiedemann and Plessner and showed that they 

 agreed very nearly with X T = constant, the law which he had 

 already obtained for oxygen. He showed also that Plessner's 

 measurements with the salts in the solid state agreed with the 

 law. He then showed that the product was constant for proto- 

 sulphate of iron over a range from 12 C. to 108 C., and for 

 palladium over the range from 22 to 1370. 



Curie's law. These results may be summed up in what is 

 known as Curie's law, which states that for a paramagnetic body 

 the product of the specific coefficient of magnetisation x and the 

 absolute temperature T is constant, each body having its own 

 constant. 



Glass and porcelain, if paramagnetic at ordinary temperatures, 

 became diamagnetic at high temperatures, and if diamagnetic to 

 begin with showed stronger diamagnetism at higher temperatures. 

 In each case the diamagnetism appears to approach a limit. This 

 is easily explained if we suppose that there are two constituents 

 present, one paramagnetic, the other diamagnetic. We shall then 

 have 



A 

 X = Fp- i3 



A 



~ representing the paramagnetism of one constituent varying 



inversely as the absolute temperature, and therefore diminishing 



* Pogff. Ann., cxxvi. (1865), p. 1. 

 f Wied. Ann., xxxix. (1890), p. 38. 



