18 Mr. H. Tornlinson on the Sudden Loss of the 



The conductivity of metals decreases with rising tempera- 

 ture ; consequently the velocity of light must also decrease 

 with rising temperature, and the index of refraction must 

 increase. It is true that Sissingh was not able to observe a 

 change in the reflexion-constant of iron with change of tem- 

 perature*^ but the experiments cannot be considered decisive ; 

 the indices of refraction must be determined directly by 

 measuring the prismatic deviation at different temperatures. 

 I do not consider the experimental difficulties of such an 

 investigation to be insurmountable. It is known also that the 

 electric resistance of metals varies differently in a magnetic 

 field. Hence the index of refraction of a metal should be 

 affected by magnetization. Should this change prove too 

 small to be measured, yet under suitable conditions the aeolo- 

 tropism, which the metals suffer upon magnetization in respect 

 of their conductivity, might be rendered evident by optical 

 means by the double refraction of light produced. 



II. A Theory concerning the Sudden Loss of the Magnetic 

 Properties of Lron and Nickel at certain Temperatures. 

 By Herbert Tomlinson, B.AJ 



THAT very high temperatures destroy both the magnetic 

 susceptibility of iron and its power of retaining mag- 

 netism has been known since the days of Queen Elizabeth. 

 Thus Gilbert found that a loadstone and a piece of iron equally 

 lost their power of affecting the magnetic needle when very 

 hot ; he also noticed that whilst the magnetic property re- 

 turned to the iron after it had cooled a little, the magnetic 

 virtue of the loadstone was altogether destroyed. Similar 

 results were obtained by Brugmans, Boyle, Cavallo, Barlow 

 and Bonnycastle, Christie, Ritchie, Erman, Scoresby, See- 

 beck, and others |. Faraday found that a steel magnet lost 

 its permanent magnetism rather suddenly at a temperature a 

 little under the boiling-point of almond oil (the best bitter- 

 almond oil boils at 180° C.) ; it behaved like soft iron till it 

 was raised to an orange-red heat, and then lost its magnetic 

 susceptibility and became indifferent. The temperature at 

 which the retentive power for permanent magnetism was lost 

 appeared to vary in steel with the hardness and structure ; in 

 fragments of loadstone it was very high ; they retained their 



* Sissingh, "Mesures de la polarisation elliptique de la lumiere," 

 Archives Neerlandaises, t. xx. 



t Communicated by the Physical Society : read April 28, 1888. 

 X See Professor Chrystal's Article on Magnetism, Enc. Brit. 



